HCIP-Datacom-Core Technology V1.0 Training Material PDF

Introduction to Network Devices  Foreword ⚫ As the current network develops rapidly, people can easily learn the latest information in the world and communicate with friends and colleagues at any time. This greatly enriches people's communication and life. The future will...

Introduction to Network Devices  Foreword ⚫ As the current network develops rapidly, people can easily learn the latest information in the world and communicate with friends and colleagues at any time. This greatly enriches people's communication and life. The future will hold a fully connected and intelligent world. ⚫ Network construction requires the support of network devices, physical connections, and multiple protocols between devices. Routers and switches are the most commonly used network devices for network interconnection. ⚫ There are various types of low-end, mid-range, and high-end routers and switches on networks of different levels. How do these devices work and process data packets? ⚫ This course uses routers and switches as examples to describe the working mechanism and packet forwarding process of network devices. 2 Huawei Confidential  Objectives ⚫ On completion of this course, you will be able to:  Describe the logical structure of network devices.  Describe each functional module of network devices.  Describe the forwarding process of network devices. 3 Huawei Confidential  Contents 1. Overview of Network Device Framework 2. Packet Processing by Network Devices 4 Huawei Confidential Network Devices ⚫ The network infrastructure consists of switches, Layer 3 forwarding routers, and firewalls. These devices receive and send data. How does a device forward data received from an interface to another interface? ⚫ What are components of a network device? How do these components work collaboratively?...... Layer 2 forwarding How does a switch forward packets from one interface to another interface? 5 Huawei Confidential Hardware Modules of Modular Devices ⚫ The following uses the S12700E-8 as an example to describe the architecture of a typical network device. MPU  Main Processing Unit (MPU): is responsible for control plane and management plane of the entire system. LPU  Switch Fabric Unit (SFU): is responsible for the data plane of the entire system. The data plane provides high-speed non-blocking SFU data channels for data switching between service modules.  Line Processing Unit (LPU): provides data forwarding and optical LPU and electrical interfaces with different rates.  SFUs and LPUs have their own management chips, which work with MPUs to form the control and management plane of the entire device. Typical panel of a network device 6 Huawei Confidential MPU ⚫ The MPU provides the control plane and management plane for the entire system.  The control plane provides functions such as protocol processing, service processing, route calculation, forwarding control, service scheduling, traffic statistics collection, and system security.  The management plane is responsible for system status monitoring, environment monitoring, log and alarm processing, system software loading, and system upgrade. MPU 7 Huawei Confidential SFU ⚫ The SFU provides the data plane for the entire system. LPUs and MPUs communicate with each other through SFUs. SFU 8 Huawei Confidential LPU ⚫ The LPU provides access interfaces of different types (optical and electrical interfaces) and different rates, and forwards data through the distributed data plane. LPU 9 Huawei Confidential Fixed Device ⚫ Different from a modular device, service modules of fixed devices are not independent hardware modules. Instead, they are integrated in one chassis. Main control module Switching module Access interface 10 Huawei Confidential Logical Diagram of Module Connections LPU LPU MPU Standby MPU LPU1 LPU5 LPU SFU LPU2 LPU6 SFU LPU3 LPU7 LPU SFU LPU4 LPU8 ⚫ Modules of a modular device are categorized as different boards. The cards communicate with each other through internal connections of the modular device. ⚫ The fixed integrates the modules, and the modules communicate with each other through internal connections. ⚫ LPUs are connected to each other through SFUs, and communication data between LPUs is forwarded through SFUs. 11 Huawei Confidential Logical Architecture of Network Devices ⚫ Network devices can be logically divided into three planes: data plane, control and management plane, and monitoring plane. 12 Huawei Confidential Control Plane ⚫ The control plane of a device consists of the MPU and the management unit of the LPU. ⚫ The control and management plane provides control and management functions for the system and is the core of the entire system. The control plane provides functions such as protocol processing, service processing, route calculation, forwarding control, service scheduling, traffic statistics collection, and system security. The control plane of a switch is used to control and manage the running of all network protocols. The control plane provides various network information and forwarding entries required by the data plane for data processing and forwarding. Main control unit MPU Management unit LPU Control plane Route Forwarding Service... System calculation control scheduling security 13 Huawei Confidential Forwarding Plane ⚫ The forwarding plane consists of SFUs and LPUs. ⚫ An LPU has a forwarding plane engine (FPE), which is essentially a switching chip that implements switching between interfaces on the LPU. ⚫ The data plane is responsible for high-speed processing and non-blocking switching of data packets. It encapsulates or decapsulates packets, forwards IPv4/IPv6/MPLS packets, performs QoS and scheduling, completes inner high-speed switching, and collects statistics. Forwarding plane Service Encapsulation Decapsulation LPU scheduling... LPU Packet Internal SFU forwarding QoS switching LPU LPU 14 Huawei Confidential Monitoring Plane ⚫ The monitoring plane consists of the monitoring units of MPUs and LPUs. Some modular switches have independent centralized monitoring units (CMUs). ⚫ The monitoring plane monitors the ambient environment to ensure the secure and stable operation of the system. It detects voltage levels, controls system power-on and power-off, monitors the temperature, and controls fan modules. If a unit fails, the monitoring plane isolates the faulty unit promptly so that the other units remain unaffected. Monitoring unit MPU Monitoring unit LPU CMU Monitoring plane Voltage Temperature... Power supply Fan control detection detection control 15 Huawei Confidential  Contents 1. Overview of Network Device Framework 2. Packet Processing by Network Devices 16 Huawei Confidential  Uplink and Downlink Packet Forwarding ⚫ With the SFU as the center, packet forwarding involves uplink and downlink processing. Uplink SFU Uplink LPU LPU Downlink Downlink 17 Huawei Confidential Packets processed by a network device are classified into service packets and protocol packets. The device only forwards service packets from one interface to another interface based on forwarding entries. After receiving protocol packets (such as ARP, OSPF, and BGP packets), the device sends the packets to the control plane for processing. For example, the ARP packets are sent to the control plane for processing. After determining whether to respond to the ARP packets, the device determines whether to learn the source MAC address and source IP address in the ARP packets.  Processing of Service Packets ⚫ After service packets enter the uplink LPU from an interface, they are sent to the SFU through the internal bus of the modular switch. The SFU sends the service packets to the downlink LPU for processing and then sends them out from the interface. Uplink LPU Service packet Receive Query entries Fragment optical/electrical Parse packets and forward packets signals packets LPU PFE SFU Send Obtain Perform egress Reassemble optical/electrical encapsulation processing packets signals information LPU PFE Downlink LPU 18 Huawei Confidential PFE: Packet Forwarding Engine Service packets: packets during interaction between services and applications Fragmentation: Before packets are sent to the SFU, they are sliced with a fixed length based on a certain granularity. Reassembly: Fragmented packets sent from the SFU are reassembled.  Determining the Egress of Packets (1) ⚫ When a packet enters an LPU, the device determines the outbound interface of the packet based on the forwarding entry (such as the IP routing table and MAC address table). For a modular switch, the downlink LPU needs to be determined. ⚫ When the packet reaches the SFU, the outbound interface and downlink LPU are specified. Therefore, the forwarding entry is queried on the uplink LPU. Forwarding entry MPU's CPU MPU Service packet LPU LPU's CPU 1 LPU's CPU LPU Both the MPU and LPU have CPUs and SFU provide the control plane function. Are LPU's CPU LPU forwarding entries stored and queried on the MPU or LPU? LPU's CPU LPU 19 Huawei Confidential For Layer 2 forwarding, the MAC address table is queried. For Layer 3 forwarding, the Layer 3 routing table is queried.  Determining the Egress of Packets (2) ⚫ Forwarding entries are stored on the MPU. After packets enter the LPU, the LPU queries entries from the MPU. ⚫ The LPU needs to communicate with the MPU when packets are forwarded each time. The forwarding efficiency is low and the packet delay increases. For a high-rate LPU, the forwarding rate decreases Forwarding greatly. entry MPU's CPU MPU Store forwarding entries Query the forwarding entry 2 Service 1 packet 3 LPU's CPU LPU LPU's CPU LPU SFU LPU's CPU LPU LPU's CPU LPU 5 4 20 Huawei Confidential For Layer 2 forwarding, the MAC address table is queried. For Layer 3 forwarding, the Layer 3 routing table is queried.  Determining the Egress of Packets (3) ⚫ Forwarding entries are stored on the LPU. After packets enter the LPU, the LPU directly queries the packets, improving the packet forwarding efficiency. ⚫ Forwarding entries are stored on all LPUs, causing high resource usage on the control plane. MPU's CPU MPU Deliver forwarding entries Service 1 packet 2 LPU's CPU LPU LPU's CPU LPU Forwarding entry SFU Store LPU's CPU LPU forwarding entries LPU's CPU LPU 4 3 21 Huawei Confidential For Layer 2 forwarding, the MAC address table is queried. For Layer 3 forwarding, the Layer 3 routing table is queried. Forwarding Information ⚫ Service packets of high-end devices are not processed by the CPU of the MPU, and forwarding information is queried by the LPU. ⚫ The forwarding information on the LPU does not exist in forwarding entries (such as the IP routing table and MAC address table) of the MPU. After generating forwarding entries, the MPU generates corresponding forwarding information and delivers the information to the LPU. IP routing table Generation MPU's CPU MPU FIB table FIB table FIB table FIB table LPU LPU LPU The IP routing table is used as an example. After the routing table is generated, the MPU generates the FIB table based on the routing table and delivers the FIB table to the LPU. The LPU then forwards packets based on the FIB table. 22 Huawei Confidential Hardware-based Forwarding ⚫ The PFE, usually an NP or ASIC chip, is responsible for packet forwarding on an LPU. The LPU directly forwards packets without the involvement of the MPU. ⚫ The forwarding plane and control plane of a high-end modular switch are distributed to different components. The MPU on the control plane runs forwarding-related protocols and maintains forwarding entries. The LPU on the forwarding plane can independently complete forwarding based on forwarding information delivered by the control plane. When the MPU on the control plane is heavily loaded, the forwarding plane is not affected. This working mechanism is called forwarding-control decoupling. Service Uplink LPU packet … … PFE SFU Downlink LPU FIB table Service packets are forwarded by the LPU and SFU independently, without the participation of the MPU. 23 Huawei Confidential  Processing of Protocol Packets ⚫ Protocol packets received by a device, such as OSPF packets, IS-IS packets, BGP packets, ARP packets, STP BPDUs, and ICMP Request packets, need to be processed by the control plane of the device. That is, these packets are sent to the CPU of the MPU for processing. Uplink LPU Protocol packet Receive Search entries Perform ingress Fragment optical/electrical and forward processing packets signals packets LPU PFE SFU Send Obtain Perform egress Reassemble optical/electrical encapsulation processing packets signals information LPU PFE LPU's CPU Downlink LPU MPU's CPU MPU 24 Huawei Confidential After receiving protocol packets, the CPU of the MPU processes the packets. If the CPU needs to respond to the packets, the control board constructs the protocol packets. For example, after receiving ARP Request and ICMP Echo Request packets sent to the CPU, the MPU constructs the ARP Reply and ICMP Echo Reply packets. The CPU processing capability of the MPU is limited. If too many protocol packets are sent to the CPU of the MPU, the CPU is busy and cannot respond to the protocol packets in a timely manner. Therefore, the rate at which various protocol packets are sent to the CPU of the MPU is limited by default. When the forwarding engine PFE (NP or ASIC chip) parses packets, the protocol field in the Layer 2 frame header can be used to determine whether the packets need to be sent to the local CPU for processing. (such as ARP, RARP, IS-IS, LLDP, LACP, and PPP control packets) or the destination address is a specified reserved multicast IP address. (As defined in the standard, multicast addresses 224.0. 0.1 to 224.0. 0.255 are used by routing protocols.) The protocol packets of the do not need to be forwarded in the upstream direction by looking up the table.  After the packets are forwarded by querying the table in the upstream direction, the SFU obtains the information about the destination interface board and the outbound interface of the packets. In this way, the SFU can exchange data to the corresponding downstream board according to the information about the destination interface, and the downstream board sends the packets according to the outbound interface information. For the protocol packets (such as BGP) that can be identified by packet parsing, which are forwarded in the upstream direction without querying the table. What are the destination interface board and outbound interface information? The answer is that the board number is the same as that of the inbound interface board, and the outbound interface is the CPU. That is, the flow chart in the preceding figure applies to this type of protocol packets. The uplink LPU and the downlink LPU are actually the same LPU. For protocol packets that can be identified based on the Layer 2 frame header or destination IP address, the upstream LPU directly sends the packets to the CPU. Processing of Protocol Packets Sent by the Device ⚫ Protocol packets sent by the device, such as OSPF packets, IS-IS packets, BGP packets, ARP packets, STP BPDUs, and ICMP packets, are constructed by the CPU of the MPU and then sent by the LPU. MPU MPU's CPU Protocol packet Uplink LPU LPU's CPU Receive Perform ingress Fragment optical/electrical Forward packets processing packets signals LPU PFE SFU Send Obtain Perform egress Reassemble optical/electrical encapsulation processing packets signals information LPU PFE Downlink LPU 26 Huawei Confidential  Quiz 1. (Single) Which of the following modules on a modular switch is responsible for running routing protocols and generating and maintaining routing tables? ( ) A. LPU B. SFU C. MPU D. SPU 2. (Essay) When forwarding service packets, does a high-end modular switch query forwarding entries from the MPU? 27 Huawei Confidential 1. C 2. No. The high-end modular switch delivers forwarding information to the LPU, and the LPU directly forwards packets without querying forwarding entries from the MPU.  Summary ⚫ Each network device has an independent control plane, forwarding plane, and monitoring plane. The control plane is responsible for protocol processing, route calculation, and service scheduling, and the forwarding plane is responsible for data forwarding to implement packet exchange between service modules. The monitoring plane monitors the ambient environment to ensure the secure and stable operation of the system. ⚫ On a high-end modular switch, each plane has different boards installed. LPUs and SFUs implement functions of the forwarding plane, MPUs implement functions of the control plane, and CMUs implement functions of the monitoring plane. ⚫ High-end modular switches use hardware-based forwarding. Packets are directly forwarded by LPUs, without participation of the control plane. Therefore, the packet forwarding efficiency is high. 28 Huawei Confidential Thank you. 把数字世界带入每个人、每个家庭、 每个组织，构建万物互联的智能世界。 Bring digital to every person, home, and organization for a fully connected, intelligent world. Copyright© 2025 Huawei Technologies Co., Ltd. All Rights Reserved. The information in this document may contain predictive statements including, without limitation, statements regarding the future financial and operating results, future product portfolio, new technology, etc. There are a number of factors that could cause actual results and developments to differ materially from those expressed or implied in the predictive statements. Therefore, such information is provided for reference purpose only and constitutes neither an offer nor an acceptance. Huawei may change the information at any time without notice. IP Routing Basics  Foreword ⚫ When a router receives an IP packet, the router searches the Forwarding Information Base (FIB) table according to the destination IP address of the packet. After finding the most matching routing entry, the router forwards the packet according to the outbound interface or next hop in the routing entry. ⚫ A router can obtain three types of routes: dynamic routes such as the Open Shortest Path First (OSPF), static routes, and direct routes. A network may have the three types of routes. How are routers that use different types of routes are reachable? ⚫ This course describes the basics of IP routing, including the routing forwarding, routing table, and route import. 2 Huawei Confidential  Objectives ⚫ On completion of this course, you will be able to:  Identify the IP routing table and FIB table.  Analyze the route-based forwarding process.  Describe the principles of route import.  Understand application scenarios of route import. 3 Huawei Confidential  Contents 1. IP Routing Basics 2. Advanced Applications of IP Routing 4 Huawei Confidential Overview of IP Routing IP Packet GE0/0/0 PC1 PC2 192.168.11.1/24 192.168.21.1/24 ⚫ When receiving an IP packet, a router matches the destination address of the IP packet with a routing entry.  If the IP packet matches a routing entry, the router forwards the packet according to the outbound interface or next hop in the routing entry.  If the IP packet does not match any routing entry, the router does not have routing information to guide packet forwarding. In this case, the router discards the packet. 5 Huawei Confidential  RIB and FIB ⚫ A network device that provides the routing function maintains two important data tables: Routing Information Base (RIB) and Forwarding Information Base (FIB). Direct route Static route OSPF process 1 … IS-IS process 1 Direct routing Static routing OSPF process 1 IS-IS process 1 … table table RIB table RIB table A router maintains a local core RIB table Route selection and RIB tables of routing protocols. Control plane RIB Download The router downloads the optimal Data plane IP packet route from the local core routing table FIB to the FIB table. The forwarding chip of the router forwards packets according to the FIB table. 6 Huawei Confidential RIB table: ▫ A RIB table can be considered to be located on the control plane of a router. Actually, a RIB table does not directly guide data forwarding. When a router queries routes, it does not query the destination address of a packet in the RIB table. Instead, it queries the FIB table to guide data forwarding. The router downloads the optimal route from the RIB table to the FIB table. If related entries in the RIB table change, the FIB table is synchronized immediately. ▫ Because the two tables are consistent and the RIB table is easy to read, the RIB table (routing table) is used in most cases to describe the data forwarding process of a router. Actually, the router queries the FIB table, and the RIB table at the control layer provides only routing information. FIB table: ▫ The FIB table is located on the data plane of a router and is also called the forwarding table. Each forwarding entry specifies the outbound interface and next-hop IP address for reaching a destination. Note: ▫ Huawei routers and Layer 3 switches provide the routing function. This course uses routers as an example. ▫ Both OSPF and Intermediate System to Intermediate System (IS-IS) use the Shortest Path First (SPF) algorithm to calculate routes based on link state information. For details about OSPF and IS-IS, see the following courses. ▫ Routing process: A router supports multiple OSPF and IS-IS processes. Different processes can be assigned based on service types, and they are independent of each other. An OSPF process ID takes effect on the local device, and does not affect packet exchange between the local route and other routers. Packets can be exchanged between routers with different process IDs.  RIB Table ⚫ Each router has a RIB table. RIB tables fall into the local core routing table and protocol routing tables. Protocol Routing Table Local Core Routing Table A protocol routing table stores routing information discovered Each router stores a local core routing table. A router by the protocol. preferentially selects the same entries in the routing table of OSPF is used as an example. each protocol to obtain the local core routing table, and then delivers the local core routes to the FIB table to guide packet forwarding. Public routing table : OSPF Routing Tables: Public Destinations : 1 Routes : 1 Destinations : 10 Routes : 10 Destination/Mask Proto Pre Cost NextHop Interface OSPF routing table status : 0.0.0.0/0 Static 60 0 10.0.2.2 GigabitEthernet0/0/1 Destinations : 1 Routes : 1 10.3.3.3/32 OSPF 10 1 10.0.1.1 GigabitEthernet0/0/0 10.4.4.4/32 EBGP 255 0 10.0.2.2 GigabitEthernet0/0/1 Destination/Mask Proto Pre Cost NextHop Interface 10.0.1.0/24 Direct 0 0 10.0.1.2 GigabitEthernet0/0/0 10.3.3.3/32 OSPF 10 1 10.0.1.1 GigabitEthernet0/0/0 10.0.1.2/32 Direct 0 0 127.0.0.1 GigabitEthernet0/0/0 10.0.3.0/24 EBGP 255 0 10.0.2.2 GigabitEthernet0/0/1 OSPF routing table status : 127.0.0.0/8 Direct 0 0 127.0.0.1 InLoopBack0 Destinations : 0 Routes : 0 127.0.0.1/32 Direct 0 0 127.0.0.1 InLoopBack0 The optimal route in the local core routing table is selected based on the preference and metric of each routing protocol. 7 Huawei Confidential Key fields in a routing table: ▫ Destination: indicates the destination address of a route. It identifies the destination IP address or destination network segment of IP packets. ▫ Mask: indicates the subnet mask of the destination IP address. It is used with the destination address to identify the address of the network segment where the destination host or router is located. ▫ Proto (protocol): indicates the protocol through which routes are learned. ▫ Pre (Preference): indicates the routing protocol preference of the route. ▪ Routers define external and internal preferences. The external preference can be manually configured for each routing protocol, while the internal preference cannot be manually modified. ▪ During route selection, a router first compares the external preferences of routes. When the same external preference is set for different routing protocols, the router selects the optimal route based on the internal preference. ▫ Cost: indicates the cost of a route. ▫ NextHop: indicates the next hop to the destination network. It specifies the next-hop device to which packets are forwarded. ▫ Interface: indicates the outbound interface that forwards packets to the destination network. It specifies the local router interface from which packets are forwarded. The Preference value is used to compare the preferences of different routing protocols, while the Cost value is used to compare the preferences of different routes of the same routing protocol. Note: The routing table in the body is truncated.  Longest Match Principle for IP Route Query ⚫ When searching the FIB table, a router performs the "AND" operation on the destination address in a packet and the network mask of each entry in the FIB table. The router then compares the result of the "AND" operation with entries in the FIB table. ⚫ According to the comparison, the router selects the optimal route to forward packets according to the longest match. [Huawei] display fib 0 Route Flags: G - Gateway Route, H - Host Route, U - Up Route S - Static Route, D - Dynamic Route, B - Black Hole Route --------------------------------------------------------------------------------------------- FIB Table: If the destination IP Total number of Routes : 8 address of a data Destination/Mask Nexthop Flag TimeStamp Interface TunnelID packet is 10.3.3.3, 10.3.3.3/32 10.0.1.1 DGHU t GE0/0/0 0x0 10.4.4.4/32 10.0.2.2 DGHU t GE0/0/1 0x0 which interface does 10.0.1.2/32 127.0.0.1 HU t InLoop0 0x0 127.0.0.1/32 127.0.0.1 HU t InLoop0 0x0 the router forward the 127.0.0.0/8 127.0.0.1 U t InLoop0 0x0 0.0.0.0/0 10.0.2.2 GSU t GE0/0/1 0x0 data packet? 10.0.1.0/24 10.0.1.2 U t GE0/0/0 0x0 10.0.3.0/24 10.0.2.2 DGU t GE0/0/1 0x0 8 Huawei Confidential Each entry in the FIB table contains the physical or logical interface through which a packet is sent to a network segment or host to reach the next-hop router. An entry also indicates whether the packet can be sent to a destination host on a directly connected network. The display fib [ slot-id ] command is used to check information about the FIB table. ▫ slot-id: displays information about the FIB table with a specified slot ID. The value is an integer, and the value range depends on the device configuration. Fields in the FIB table: ▫ Total number of Routes: indicates the total number of routes in the routing table. ▫ Destination/Mask: indicates the destination address or mask length. ▫ Nexthop: indicates the next hop. ▫ Flag: indicates the current flag, which is the combination of G, H, U, S, D, and B. ▪ G (Gateway): indicates that the next hop is a gateway. ▪ H (Host): indicates that the next hop is a host. ▪ U (Up): indicates that the route status is Up. ▪ S (Static): indicates the static route. ▪ D (Dynamic): indicates the dynamic route. ▪ B (Blackhole): indicates the blackhole route, with the next hop as a null interface. ▫ Timestamp: indicates the lifetime of an entry, in seconds. ▫ Interface: indicates the outbound interface used to reach the destination. ▫ TunnelID: indicates the index of a forwarding entry. If the value is not 0, packets matching the entry are forwarded through a tunnel, for example, MPLS tunnel. If the value is 0, packets are not forwarded through a tunnel.  Route Types Direct Route Static Route Dynamic Route Direct routes are automatically Static routes are manually configured by Dynamic routes are learned by dynamic generated by devices and point to network administrators. routing protocols running on routers. local directly connected networks. 192.168.31.0/24 GE0/0/0 192.168.11.0/24 192.168.21.0/24 GE0/0/1 10.0.12.0/24 Dynamic routing GE0/0/1 protocol GE0/0/1 Example: OSPF Outbound Route Type Destination/Mask Interface Outbound Route Outbound Route Type Destination/Mask Destination/Mask Direct route 192.168.11.0/24 GE0/0/0 Interface Type Interface Direct route 10.0.12.0/24 GE0/0/1 Static route 192.168.21.0/24 GE0/0/1 OSPF 192.168.31.0/24 GE0/0/1 10 Huawei Confidential Direct routes are destined for the subnets to which directly connected interfaces belong. They are automatically generated by devices. Static routes are manually configured by network administrators. Dynamic routes are learned by dynamic routing protocols, such as OSPF, IS-IS, and Border Gateway Protocol (BGP). ▫ The Border Gateway Protocol (BGP) is a distance vector routing protocol that allows devices in different ASs to communicate and select optimal routes. ▫ An AS is a group of IP networks that are controlled by one entity, typically an Internet service provider (ISP), and have the same routing policy. Dynamic Routing Protocols ⚫ Dynamic routing protocols are classified into the following types based on application scopes:  Interior Gateway Protocols (IGPs): run inside an autonomous system (AS), including the OSPF and IS-IS.  Exterior Gateway Protocols (EGPs): run between ASs, including the Border Gateway Protocol (BGP). ASBR1 ASBR3 192.168.11.0/24 OSPF BGP IS-IS 192.168.21.0/24 192.168.12.0/24 192.168.22.0/24 192.168.13.0/24 192.168.23.0/24...... ASBR2 ASBR4 Company A's network Company B's network 11 Huawei Confidential Route Recursion ⚫ Routes can be used to forward traffic only when they have directly connected next hops. However, a static or BGP route may contain an indirect next hop. Therefore, in a process referred to as route recursion, a device needs to search for a directly connected next hop for the route. GE0/0/0 GE0/0/1 10.0.12.2/24 10.0.23.3/24 192.168.21.0/24 GE0/0/0 GE0/0/1 R1 10.0.12.1/24 R2 10.0.23.2/24 R3 ip route-static 192.168.21.0 24 10.0.23.3 Obtain the directly connected next hop through recursion Recursion In this example, a route to 10.0.23.3 is added to R1 so that the ip route-static 10.0.23.0 24 10.0.12.2 route to 192.168.21.0/24 can be recursive. Destination Outbound Next Hop Network Interface 192.168.21.0/24 10.0.23.3 GE0/0/0 10.0.23.0/24 10.0.12.2 GE0/0/0 12 Huawei Confidential  Data Forwarding Process Destination/Mask Next Hop Outbound Interface 10.0.12.0/24 10.0.12.2 GE0/0/0 Destination 10.0.23.0/24 10.0.23.2 GE0/0/1 IP address: 192.168.11.0/24 10.0.12.1 GE0/0/0 192.168.21.1 192.168.21.0/24 10.0.23.3 GE0/0/1 R2's routing table 1 Gate 2 3 Gate 4 GE0/0/1 way GE0/0/0 GE0/0/0 GE0/0/1 GE0/0/0 way GE0/0/1 PC1 10.0.12.0/24 10.0.23.0/24 PC2 192.168.11.2 10.0.12.1 10.0.12.2 10.0.23.2 10.0.23.3 192.168.21.2 192.168.11.1/24 R1 R2 R3 192.168.21.1/24 R1's routing table R3's routing table Outbound Outbound Destination/Mask Next Hop Destination/Mask Next Hop Interface Interface 10.0.12.0/24 10.0.12.1 GE0/0/0 10.0.12.0/24 10.0.23.2 GE0/0/0 10.0.23.0/24 10.0.12.2 GE0/0/0 10.0.23.0/24 10.0.23.3 GE0/0/0 192.168.11.0/24 192.168.11.2 GE0/0/1 192.168.11.0/24 10.0.23.2 GE0/0/0 192.168.21.0/24 10.0.12.2 GE0/0/0 192.168.21.0/24 192.168.21.2 GE0/0/1 13 Huawei Confidential The process for PC1 to send a data packet to PC2 is as follows: 1. PC1 sends the packet to the gateway R1. 2. R1 searches the routing table for the next hop and outbound interface, and forwards the packet to R2. 3. R2 forwards the packet to R3 based on the routing table. 4. After receiving the packet, R3 looks up the routing table and finds that the destination IP address of the packet belongs to the network segment where the local interface resides. R3 then forwards the packet locally and finally sends the packet to the destination PC2.  Contents 1. IP Routing Basics 2. Advanced Applications of IP Routing 14 Huawei Confidential  Scenario Analysis of Advanced Applications of IP Routing (1) Company A R1 R3 Company B 192.168.11.0/24 OSPF OSPF/IS-IS IS-IS 192.168.21.0/24 192.168.12.0/24 192.168.22.0/24 192.168.13.0/24 192.168.23.0/24 …… …… R2 R4 ⚫ Scenario:  Assume that company A and company B have their own networks, which are managed and maintained independently. The networks of company A and company B run OSPF and IS-IS, respectively.  After the two companies are merged into one company, the original two networks must be integrated. To ensure that service traffic of the new company can be normally exchanged on the integrated network, the company requires interworking based on routing. 15 Huawei Confidential OSPF and IS-IS are two different dynamic routing protocols, so they cannot directly exchange routing information. In the figure, OSPF is deployed on the network of company A, and R1 and R2 are edge devices. IS-IS is deployed on the network of company B, and R3 and R4 are edge devices. OSPF or IS-IS can be deployed on the connected network segments of borders. For example, OSPF can be deployed on network segments between R1 and R3 and between R2 and R4. In this case, only R3 and R4 are border devices. Scenario Analysis of Advanced Applications of IP Routing (2) ⚫ Scenario: HQ BGP  On a large-scale enterprise network, a Backbone network single routing protocol cannot meet network requirements. In most cases, Branch multiple routing protocols coexist. IS-IS Region core  In addition, different routing protocols are designed and deployed in different network topologies considering the service logic or administrative management, making the routing Municipal OSPF/ hierarchy clear and controllable. company... Static routing LAN  In such a network environment, interworking based on routing also needs to be implemented. 16 Huawei Confidential  Basic Concept of Route Import R1 To implement interworking based on routing, perform the following operations: 1. Re-plan and modify network-wide routing protocols, which is complex. 192.168.11.0/24 OSPF IS-IS 192.168.21.0/24 2. Perform operations on the edge devices in OSPF 192.168.12.0/24 192.168.22.0/24 and IS-IS routing domains so that routing 192.168.13.0/24 192.168.23.0/24 information can be transmitted between OSPF...... and IS-IS. This solution does not change the R2 original topology and is easy to deploy. However, loops may occur. Routing information of two routing protocols is isolated from each other. ⚫ Route import refers to the process of advertising routing information from one routing protocol to another.  Route import allows routing information to be transmitted between different routing protocols.  When importing routes, you can deploy route control to flexibly control service traffic. 17 Huawei Confidential In the figure, OSPF and IS-IS networks have different network segments. Only R1 and R2 know all routing entries. Question: How do all devices obtain all routes?  Direction of Route Import ⚫ Route import is directional. If routing information is imported from routing protocol A to routing protocol B, routing protocol B can learn routing information about routing protocol A. However, routing protocol A does not know routing information about routing protocol B, unless route import from routing protocol B to routing protocol A is configured. Import OSPF routes ⚫ When importing routes, pay attention to the following points: into IS-IS  Route preference R1  Route injection  Route metric OSPF IS-IS 192.168.11.0/24 192.168.21.0/24 192.168.12.0/24 Import IS-IS route 192.168.22.0/24 192.168.13.0/24 into OSPF 192.168.23.0/24...... Question: Can two networks communicate with each other if only OSPF routes are imported to IS-IS? 18 Huawei Confidential During route import, focus on the route convergence time. This course does not describe the route convergence time. The implementation and configuration of route import will be described in other HCIP-Datacom certification courses.  Route Import: Route Preference 3 Import OSPF routes into IS-IS Scenario: 1. R1 imports the direct route 10.1.1.0/24 into OSPF. R2 Import 2 4 2. R3 learns the route (OSPF external route with the preference 1 direct route of 150) to the network segment 10.1.1.0/24 through OSPF. 3. R2 imports OSPF routes to the IS-IS process. 10.1.1.0/24 OSPF IS-IS 4. R3 also learns the route (the preference is 15) to the R1 5 R4 network segment 10.1.1.0/24 through IS-IS. 2 4 5. For R3, the IS-IS route has a higher priority than the OSPF R3 external route, so the IS-IS route from R4 is preferred. R3 accesses the network segment 10.1.1.0/24 through the path R3->R4->R2->R1, which is the second optimal OSPF route Destination/Mask Proto Pre IS-IS route 10.1.1.0/24 ISIS 15 path. Access traffic 19 Huawei Confidential Route preferences defined by Huawei: ▫ Direct: 0 ▫ OSPF: 10 ▫ IS-IS: 5 ▫ Static: 60 ▫ OSPF ASE: 150 ▫ OSPF NSSA: 150 ▫ IBGP: 255 ▫ EBGP: 255 Note: The route preferences may vary with vendors. Route Import: Route Injection 3 Import OSPF route into IS-IS Scenario: 1. R1 imports the direct route 10.1.1.0/24 to OSPF. R2 Import 2 4 2. The direct route 10.1.1.0/24 is advertised in the entire 1 direct route OSPF domain. 6 3. R2 imports OSPF routes to the IS-IS process. 10.1.1.0/24 OSPF IS-IS 6 4. The direct route 10.1.1.0/24 is advertised in the entire R1 R4 IS-IS domain. 2 4 5. R3 imports IS-IS routes to the OSPF process. R3 6. The direct route 10.1.1.0/24 is advertised in the entire OSPF domain again, resulting in route injection. 5 Import IS-IS route into OSPF OSPF route IS-IS route 20 Huawei Confidential Route Import: Route Metric 1 Import OSPF route into IS-IS Scenario: 1. OSPF routes are imported into IS-IS. R1 2. IS-IS routes are imported into OSPF. 192.168.11.0/24 cost=100 192.168.21.0/24 cost=10 Different routing protocols define different route 192.168.12.0/24 cost=200 192.168.22.0/24 cost=10 metrics. How are metrics of imported routes defined OSPF IS-IS when routes are imported between routing protocols? What are the metrics? 2 Import IS-IS route into OSPF 21 Huawei Confidential Route Import Scenario ⚫ Route import involves the following scenarios:  Route import between dynamic routing protocols  Importing direct routes to a dynamic routing protocol  Importing static routes to a dynamic routing protocol 1. Import OSPF routes into IS-IS 2. Import direct route 3. Import 192.168.11.0/24 static route Directly connected OSPF IS-IS 192.168.21.0/24 Static route 22 Huawei Confidential  Basic Configuration Commands for Route Import 1. Configure OSPF to import external routes. [Huawei-ospf-100] import-route { bgp | direct | static | isis [ process-id-isis ] | ospf [ process-id-ospf ]} In the OSPF view, import BGP routes, direct routes, static routes, IS-IS routes, and routes of other OSPF processes. 23 Huawei Confidential If a device on an OSPF network needs to access a device on the network running a non-OSPF protocol, the OSPF device needs to import routes from the non-OSPF protocol into the OSPF network.  Case 1: Importing Direct Routes to OSPF Import direct route import-route direct 10.0.12.1 10.0.23.2 192.168.11.0/24 Directly 10.0.12.2 10.0.23.3 connected R1 R2 R3 OSPF R1's routing table R3's routing table Destination/Mask Protocol Next Hop Destination/Mask Protocol Next Hop 192.168.11.0/24 Direct 192.168.11.1 192.168.11.0/24 O_ASE 10.0.23.2 10.0.12.0/24 Direct 10.0.12.1 10.0.12.0/24 OSPF 10.0.23.2 10.0.23.0/24 OSPF 10.0.12.2 10.0.23.0/24 Direct 10.0.23.3 ⚫ You can run the import-route direct command to import all direct routes in the routing table to a dynamic routing protocol. ⚫ The imported routes are advertised as OSPF external routes on the entire OSPF network. 24 Huawei Confidential To enable a device configured with a dynamic routing protocol to advertise the routes of its directly connected interface to a dynamic routing protocol, enable the dynamic routing protocol on the interface. In addition, direct routes can be imported to dynamic routing protocols. In the figure: ▫ OSPF is deployed on R1, R2, and R3. R1 has a direct network segment 192.168.11.0/24. To enable R2 and R3 to generate a route to 192.168.11.0/24, import the direct route to OSPF on R1. Note: On an OSPF network, if the protocol field in the routing table is displayed as O_ASE, the route is an OSPF external route.  Case 2: Importing Static Routes to OSPF Import static route import-route static ip route-static 192.168.11.0 24 10.0.12.1 10.0.12.1 10.0.23.2 192.168.11.0/24 10.0.12.2 10.0.23.3 R1 R2 R3 OSPF not supported OSPF R2's routing table R3's routing table Destination/Mask Protocol Next Hop Destination/Mask Protocol Next Hop 192.168.11.0/24 Static 10.0.12.1 192.168.11.0/24 O_ASE 10.0.23.2 10.0.12.0/24 Direct 10.0.12.2 10.0.23.0/24 Direct 10.0.23.3 10.0.23.0/24 Direct 10.0.23.2 ⚫ You can run the import-route static command to import all static routes in the routing table to a dynamic routing protocol. ⚫ The imported routes are advertised as OSPF external routes on the entire OSPF network. 25 Huawei Confidential For dynamic routing protocols, static routes are considered as external routes and are not detected by dynamic routing protocols. To enable all devices in a dynamic routing protocol domain to learn a static route, import the static route to the dynamic routing protocol. In the figure: ▫ R2 and R3 run OSPF, but R1 does not support OSPF. Add a static route pointing to network segment 192.168.11.0/24 and import the static route to OSPF on R2 so that both R2 and R3 can generate a route to 192.168.11.0/24.  Case 3: Importing IS-IS Routes to OSPF Import IS-IS routes import-route isis 1 10.0.12.1 10.0.23.2 192.168.11.0/24 10.0.12.2 10.0.23.3 R1 R2 R3 IS-IS OSPF R2's routing table R3's routing table Destination/Mask Protocol Next Hop Destination/Mask Protocol Next Hop 192.168.11.0/24 IS-IS 10.0.12.1 192.168.11.0/24 O_ASE 10.0.23.2 10.0.12.0/24 Direct 10.0.12.2 10.0.12.0/24 O_ASE 10.0.23.2 10.0.23.0/24 Direct 10.0.23.2 10.0.23.0/24 Direct 10.0.23.3 ⚫ You can run the import-route isis 1 command to import all IS-IS routes in the routing table to a dynamic routing protocol. ⚫ The imported routes are advertised as OSPF external routes on the entire OSPF network. 26 Huawei Confidential The typical scenario is to import routes from one dynamic routing protocol to another. In the figure: ▫ IS-IS runs on R1 and R2, and OSPF runs on R2 and R3. The routes maintained by the two protocols are isolated. Therefore, R1 has all routes on the IS-IS network but cannot access the OSPF network. R3 has all routes on the OSPF network but cannot access the IS-IS network. You can configure R2 to import IS-IS routes to OSPF.  Quiz 1. (Single) If a router has the following four FIB entries, which route is used by the router to forward the IP packet with the destination IP address 10.0.1.1? ( ) A. 0.0.0.0/0 B. 10.0.0.0.0/16 C. 10.0.1.0/24 D. 10.0.2.0/24 27 Huawei Confidential 1. C  Quiz 2. (Multiple) Which of the following statements about route import are false? ( ) A. In the OSPF process of a router, you can run the import-route command to import routes from other OSPF processes. B. In the OSPF process of a router, you can run the import-route command to import static routes of the routing table. C. On a router, to import routes from routing protocol X to routing protocol Y, you need to run the import-route command in the view of routing protocol X. D. After the import-route command is run on a router and the direct route of GE0/0/1 is imported to OSPF, OSPF is activated on the interface and the interface periodically sends Hello packets. 28 Huawei Confidential 2. CD  Summary ⚫ Different routing protocols have different working mechanisms, so multiple routes may be generated to the same destination network segment. A router selects the optimal route based on the priority of the routing protocol and the route cost, and adds the optimal route to the FIB table. The router forwards data according to the FIB table. ⚫ On a large-scale network running multiple routing protocols, routes are advertised between routing protocols by importing routes. A large number of routes may be imported, and some low-performance devices cannot support the imported routes. Therefore, route control is required to implement on-demand route distribution. ⚫ OSPF, IS-IS, and route control listed in this course will be illustrated in subsequent courses. 29 Huawei Confidential Thank you. 把数字世界带入每个人、每个家庭、 每个组织，构建万物互联的智能世界。 Bring digital to every person, home, and organization for a fully connected, intelligent world. Copyright© 2025 Huawei Technologies Co., Ltd. All Rights Reserved. The information in this document may contain predictive statements including, without limitation, statements regarding the future financial and operating results, future product portfolio, new technology, etc. There are a number of factors that could cause actual results and developments to differ materially from those expressed or implied in the predictive statements. Therefore, such information is provided for reference purpose only and constitutes neither an offer nor an acceptance. Huawei may change the information at any time without notice. OSPF Basics  Foreword ⚫ Routers forward data packets based on routing tables. Routing entries can be manually configured or generated using dynamic routing protocols. ⚫ Compared with dynamic routes, static routes use less bandwidth and do not utilize CPU resources for route calculation and update analysis. Static routes alone can implement interworking for simple networks. If a network fault occurs or the topology changes, static routes cannot be automatically updated and must be manually reconfigured to adapt to the network change. ⚫ Compared with static routes, dynamic routing protocols have higher scalability and better adaptability. ⚫ The Open Shortest Path First (OSPF), as an Interior Gateway Protocol (IGP), is widely used because it features high scalability and fast convergence. ⚫ This course describes basic OSPF concepts, OSPF adjacency establishment, and basic OSPF configurations. 2 Huawei Confidential  Objectives ⚫ On completion of this course, you will be able to:  Describe the overall process of OSPF route calculation.  Clarify functions of the DR and BDR.  Describe OSPF packets and their functions.  Configure basic OSPF functions.  Distinguish the OSPF neighbor relationship and adjacency. 3 Huawei Confidential  Contents 1. Introduction to Dynamic Routing Protocols 2. Overview of OSPF 3. OSPF Working Mechanism 4. Basic OSPF Configurations 4 Huawei Confidential  Classification of Dynamic Routing Protocols By ASs Interior Gateway Protocols (IGPs) Exterior Gateway Protocols (EGPs) RIP OSPF IS-IS BGP By working mechanisms and algorithms Distance Vector Routing Protocols Link-State Routing Protocols RIP OSPF IS-IS 5 Huawei Confidential BGP uses the path-vector algorithm, which is a modified version of the distance- vector algorithm. Distance-Vector Routing Protocol ⚫ A router running a distance-vector routing protocol periodically floods its routing table. Through route exchange, each router learns routes from neighboring routers, loads the routes to its routing table, and then advertises the routes to other neighboring routers. ⚫ All routers on a network do not know the network topology. They only know the direction to a destination network segment and the cost. Routing Routing Routing Table Table Table 10.0.3.3 R1 R2 R3 To reach the device at 10.0.3.3, pass through R2. 6 Huawei Confidential  Link State Routing Protocol: LSA Flooding ⚫ A link-state routing protocol advertises the link state but not routing information. ⚫ Routers running link-state routing protocols establish neighbor relationships and then exchange Link State Advertisements (LSAs). Advertise LSAs to describe link status LSA LSA information. R2 An LSA describes the status of a router interface, such as the cost of the interface and the connected object. R1 R3 LSA LSA R4 OSPF 7 Huawei Confidential Each router generates a link state advertisement (LSA) that describes the status of its directly connected interface, including the interface cost and the relationship between the router and its neighboring router. Link State Routing Protocol: LSDB Maintenance ⚫ Each router generates LSAs and adds the received LSAs to its own link state database (LSDB). Routers parse the LSAs stored in their LSDBs to obtain the network topology. LSDB Routers use LSDBs to store LSAs. An LSDB usually stores various types LSA LSA of LSAs, and each type of LSA R2 describes different information. LSDB LSDB R1 R3 LSDB LSA LSA R4 OSPF 8 Huawei Confidential  Link State Routing Protocol: SPF Calculation ⚫ Each router uses the Shortest Path First (SPF) algorithm to calculate routes based on the LSDB. Each router calculates a loop-free tree with itself as the root and the shortest path. With the tree, the router knows the optimal paths to all network segments. LSDB Each router calculates a loop-free tree with itself as the root and the shortest path. R2 LSDB LSDB 2 R1 R3 LSDB 3 1 R4 4 OSPF 9 Huawei Confidential SPF is a core algorithm of OSPF. It is used to select optimal routes on a complex network. Link State Routing Protocol: Routing Table Generation ⚫ A router installs the calculated optimal path to its routing table. Routing Based on the SPF calculation result, each LSDB router installs routes to the routing table. table Routing R2 Routing LSDB table LSDB Routing table Table R1 R3 R4 Routing LSDB table OSPF 10 Huawei Confidential  Summary of Link State Routing Protocols Establish a neighbor LSDB Exchange link LSDB relationship state information R1 R2 R1 R2 R3 1 2 R3 LSDB Calculate Calculate 3 4 Routing table Routing table the path the path Generate routing entries R1 R2 R1 R2 1 2 Calculate Routing table R3 R3 the path 3 11 Huawei Confidential A link state routing protocol involves four steps: ▫ Step 1: Establish a neighbor relationship between neighboring routers. ▫ Step 2: Exchange link state information and synchronize LSDBs between neighbors. ▫ Step 3: Calculate the optimal path. ▫ Step 4: Generate routing entries according to the shortest path tree and load the routing entries to the routing table.  Contents 1. Introduction to Dynamic Routing Protocols 2. Overview of OSPF 3. OSPF Working Mechanism 4. Basic OSPF Configurations 12 Huawei Confidential Overview of OSPF ⚫ OSPF, defined by the Internet Engineering Task Force (IETF), is an IGP based on the link state. OSPF version 2 (OSPFv2), defined in RFC 2328, is intended for IPv4, and OSPF version 3 (OSPFv3)), defined in RFC 2740, is intended for IPv6. ⚫ OSPF has the following advantages:  Uses the accumulated link cost as the reference value for route selection based on the SPF algorithm.  Transmits and receives some protocol packets in multicast mode.  Supports area partition  Supports load balancing among equal-cost routes.  Supports packet authentication. 13 Huawei Confidential  OSPF Application Scenarios ⚫ OSPF is usually deployed on large-scale enterprise networks to ensure reachable routes between buildings. Core layer The core and aggregation layers are deployed in Area 0  the OSPF backbone area.  The access and aggregation layers are deployed in Aggregation layer the OSPF non-backbone area. Area 1 Area N Access layer … 14 Huawei Confidential Access layer: uses transmission media such as optical fibers, twisted pairs, coaxial cables, and wireless access technologies to connect to users and allocate services and bandwidth. The access layer allows terminal users to connect to the network. Therefore, access switches have low costs and high port density. Aggregation layer: provides policy-based connections for the access layer, such as address combination, protocol filtering, routing service, and authentication management. Network segments are divided to implement isolation, preventing network faults from spreading and affecting the core layer. The aggregation layer also provides interconnection between virtual networks at the access layer, controls and restricts access from the access layer to the core layer, and ensures security and stability of the core layer. Core layer: implements optimized transmission between backbone networks. The core layer focuses on redundancy, reliability, and high-speed transmission.  Router ID Area Metric Basic OSPF Concepts: Router ID ⚫ A router ID is a 32-bit integer that uniquely identifies an OSPF router in an AS. ⚫ The rules for selecting a router ID are as follows:  The router ID of an OSPF router is manually configured (recommended).  If the router ID is not manually configured, a router uses the largest IP address of a loopback interface as the router ID.  If no loopback interface is configured, the router uses the largest IP address of a physical interface as the router ID. Router ID 10.0.1.1 Router ID 10.0.2.2 R1 Area 0 R2 I'm 10.0.1.1 Router ID 10.0.3.3 R3 15 Huawei Confidential To change a specified router ID, you must restart the OSPF process. In practice, it is recommended that router IDs of OSPF routers be specified manually. First, plan a private network segment such as 192.168.1.0/24 for OSPF router ID selection. Before starting the OSPF process, create a loopback interface on each OSPF router, and use a private IP address with a 32-bit mask as the IP address of the loopback interface. This private IP address is then used as the router's router ID. If there is no special requirement, this loopback interface address does not need to be advertised to the OSPF network.  Router ID Area Metric Basic OSPF Concepts: Area ⚫ Each OSPF area is regarded as a logical group and identified by an area ID. ⚫ An OSPF area ID is a 32-bit non-negative integer in dotted decimal notation (the format is the same as that of an IPv4 address), for example, area 0.0.0.1. For simplicity, an OSPF area ID is also expressed in decimal notation. R1 Area 0 R2 R3 16 Huawei Confidential For example, area 0.0.0.1 is equivalent to area 1, area 0.0.0.255 is equivalent to area 255, and area 0.0.1.0 is equivalent to area 256. Devices of many network vendors support the two area ID configuration and representation modes.  Router ID Area Metric Basic OSPF Concepts: Metric ⚫ OSPF uses the cost as the route metric. Each OSPF-enabled interface maintains an interface cost. The default interface cost is 100 Mbit/s divided by interface bandwidth. The value 100 Mbit/s is the default reference value specified by OSPF and is configurable. ⚫ OSPF uses the accumulated cost, that is, the total cost of the outbound interfaces of all routers that the traffic passes from the source network to the destination network. Cost of the OSPF Interface Accumulated Cost of the OSPF Path Serial interface (1.544 Mbit/s) 10.0.1.1/32 Default cost = 64 FE interface GE interface Default cost = 1 Default cost = 1 Cost = 1 Cost = 64 R1 R2 R3 Different OSPF interfaces have different costs because of In the routing table of R3, the cost of the OSPF route to network their different bandwidths. segment 10.0.1.1/32 is 1 plus 64, that is, 65. 17 Huawei Confidential In practice, you are advised to manually set the cost based on the interface bandwidth instead of changing the OSPF reference bandwidth.  Router ID Area Metric Basic OSPF Concepts: Example for Changing the Metric Area 0 Area 0 10.0.1.1/32 10.0.1.1/32 R1 Default cost of GE 0/0/0 R2 R1 GE 0/0/0 Cost 100 R2 Aggregation layer Default cost of GE 0/0/1 Default cost of GE 0/0/1 GE 0/0/1 Cost 10 GE 0/0/1 Cost 10 Area 1 Area 1 Default cost of GE 0/0/0 GE 0/0/0 Cost 10 Access layer R3 R4 R3 R4 Aggregation layer [R4]display ip routing-table 10.0.1.1 [R4]display ip routing-table 10.0.1.1 Summary Count : 2 Summary Count : 1 Destination/Mask Proto Cost NextHop Interface Destination/Mask Proto Cost NextHop Interface 10.0.1.1/32 OSPF 2 10.0.34.3 GigabitEthernet0/0/1 10.0.1.1/32 OSPF 20 10.0.34.3 GigabitEthernet0/0/0 OSPF 2 10.0.24.2 GigabitEthernet0/0/0 By default, there are two paths from R4 to network segment In the figure, the cost of the device interface is changed to ensure that 10.0.1.1/32, and the data forwarding path is uncontrollable. traffic does not need to pass through R2 when the access router accesses R1. 18 Huawei Confidential During traffic path planning, it is recommended that the cost of the direct link at the aggregation layer be greater than the sum of costs of all links on the access ring. In this way, traffic can be directly transmitted to R1 or R2 through the access router. The preceding figure is used as an example. R1 and R2 are located at the aggregation layer of the enterprise network. The direct link between R1 and R2 belongs to area 0. R1 has a directly connected network segment 10.0.1.1/32 in area 0. ▫ By default, the route from R4 to 10.0.1.1/32 has two next hops. ▫ After the cost is changed, the route from R4 to 10.0.1.1 has only one next hop.  OSPF Neighbor Table LSDB OSPF Routing Table Three OSPF Tables: OSPF Neighbor Table ⚫ OSPF has three important tables: OSPF neighbor table, LSDB, and OSPF routing table. Pay attention to the following information about the OSPF neighbor table:  Before OSPF transmits link state information, OSPF neighbor relationships must be established.  OSPF neighbor relationships are established by exchanging Hello packets.  The OSPF neighbor table displays the status of the neighbor relationship between OSPF routers. You can run the display ospf peer command to view the status. display ospf peer OSPF Process 1 with Router ID 10.0.1.1 Neighbors [R1]display ospf peer Area 0.0.0.0 interface 10.0.12.1(GigabitEthernet1/0/0)'s neighbors Router ID: 10.0.2.2 Address: 10.0.12.2 GR State: Normal State: Full Mode:Nbr is Master Priority: 1 Router ID:10.0.1.1 Router ID:10.0.2.2 DR: 10.0.12.1 BDR: 10.0.12.2 MTU: 0 Dead timer due in 35 sec GE1/0/0 GE1/0/0 Retrans timer interval: 5 R1 10.0.12.1/30 10.0.12.2/30 R2 Neighbor is up for 00:00:05 Authentication Sequence: [ 0 ] 19 Huawei Confidential  OSPF Neighbor Table LSDB OSPF Routing Table Three OSPF Tables: LSDB ⚫ Pay attenti