CSNETWK_06_Routing_Forwarding_Summary.pdf

SUMMARY Routing and Forwarding NETWORK (IP) LAYER At the Network Layer, protocols facilitate the forwarding of Transport Layer Protocol Data Units (PDUs) between hosts. These PDUs are known as packets at the Network Layer, which overse...

SUMMARY Routing and Forwarding NETWORK (IP) LAYER At the Network Layer, protocols facilitate the forwarding of Transport Layer Protocol Data Units (PDUs) between hosts. These PDUs are known as packets at the Network Layer, which oversees the delivery of packets from host to host. At this layer, segments are transferred using the services provided by the Data Link Layer. The Network Layer protocol encapsulates these segments and adds its own protocol information to create a packet. Upon reaching its destination, this process is known as decapsulation or de- encapsulation. The Data Link Layer manages communication over a specific link (node-to-node communication), while the Transport Layer ensures logical end-to-end transport or communication between processes (process-to-process communication). SERVICES AND PROTOCOLS mobile network 1. The Network Layer at the sending host retrieves national or global ISP segments from its Transport Layer. 2. It encapsulates each segment into datagrams. application 3. These datagrams are then sent to the nearest router. transport network link 4. At the receiving host, the Network Layer accepts physical datagrams from its nearby router. network network link physical link physical 5. It extracts the Transport Layer segments from the received datagrams. network link network 6. Finally, the Network Layer delivers these segments up physical link physical network to the Transport Layer at the receiving host. link datacenter physical network In every host and router within the network, there is an important component of the Network Layer. Routers application with truncated protocol stacks—lacking upper layers such transport as Transport and Application Layers—do not implement network enterprise link Application- and Transport-Layer protocols. network physical TWO KEY NETWORK-LAYER FUNCTIONS Forwarding: This involves transferring a packet from an incoming link to an outgoing link within a single router. Data plane: This refers to the local, per-router actions of forwarding or moving a datagram from an input link to an output link at a router. Routing: Involves all routers in the network, whose collective interactions via routing protocols determine the paths that packets take from source to destination nodes. Control plane: This manages the network-wide process that provides an end-to-end view of routing packets from one edge of the network to the other. It involves coordinating and managing all devices interconnected on the Internet.  TWO KEY NETWORK-LAYER FUNCTIONS A packet arrives at a router with a header field value of Routing 0111. Algorithm control plane The router consults its data forwarding table and identifies plane that the output link interface for this packet is interface 2. The routing algorithm values in arriving packet header calculates and updates the 0111 1 values that are to be inserted 2 into the router's forwarding 3 tables. STATIC ROUTING Routing involves selecting and defining paths for IP packet traffic across an internetwork, from source to destination. Routers are responsible for transferring packets between different networks. Routers learn about remote networks through either dynamic routing protocols or manual configuration with static routes: Static Routing is the manual configuration of network routers. For full connectivity, a static route to every network must be configured on every router. Static routes to network destinations are unchangeable, making static routing not fault-tolerant. Any changes to the routing infrastructure require manual intervention. STATIC ROUTING PROS CONS No CPU cycles are used on route calculation Proper implementation requires complete and communication. knowledge of the network topology. There is no CPU or memory overhead. Configuration and maintenance requires administrator intervention, which is highly Static routing avoids bandwidth consumption time-consuming. since routers do not exchange route updates. Security is enhanced as static routes are not advertised over the network. Implementation is straightforward in small networks. The route to the destination remains constant. No dynamic routing algorithm is necessary. STATIC ROUTING Static routing has three primary uses: 1. Predictable Network Traffic: It is ideal for environments where network traffic patterns are consistent and predictable. 2. Simple Network Design: Static routes are beneficial for smaller networks with straightforward designs, particularly those with a single path to an external network. 3. Routing to Stub Networks: It efficiently handles routing to and from stub networks. A stub network is accessed via a single route, and the router typically connects to only one neighbor. STATIC ROUTING Two Types of Static Routes: 1. Standard Static Route: This type is used to connect to a specific remote network. It is commonly used for connecting to stub networks but can also be configured to connect to any network. 2. Default Static Route: This route applies to all packets and specifies the gateway IP address to which the router forwards IP packets that do not match any learned or static routes. A default static route is defined by the destination IPv4 address 0.0.0.0/0. Configuring a default static route establishes a Gateway of Last Resort. Default static routes are used: When no other routes in the routing table match the packet destination IP address. When a more specific match does not exist. When a router is connected to only one other router, a condition known as a stub router. STATIC ROUTING Configuring static routes requires the following parameters: Network address of the destination remote network Subnet mask of the destination remote network Next-hop router IP address is the IP address of the router OR; Exit interface which is the exit point interface on the router DYNAMIC ROUTING Dynamic Routing involves routers continuously exchanging network status updates and dynamically sharing routing information. Routing updates are transmitted by routing protocols. A routing protocol consists of processes, algorithms, and messages that enable routers to exchange routing information. It dynamically selects the best path available or adjusts to changes in the routing infrastructure, facilitating the exchange of routing information between routers. DYNAMIC ROUTING The purpose of dynamic routing protocols include: Discovering remote networks. Maintaining current routing information. Determining the best path to destination networks. Automatically finding an alternative best path if the current path becomes unavailable. DYNAMIC ROUTING Routing protocols enable routers to dynamically exchange information about remote networks and automatically update their routing tables with this information. These protocols determine the best path or route to each network, which is then added to the routing table. A key advantage of dynamic routing protocols is their ability to facilitate automatic information exchange among routers in response to changes in network topology. This capability allows routers to discover new networks and identify alternative paths in the event of a link failure to an existing network. DYNAMIC ROUTING PROS CONS Applicable to all topologies requiring Initial implementation can be more multiple routers. complex. Generally independent of network Less secure due to broadcast and size. multicast routing updates, requiring Automatically adjusts the network additional security configuration topology to reroute traffic as settings. necessary. Requires additional resources such as CPU, memory, and link bandwidth. DYNAMIC ROUTING Most routing protocols are classified into two main categories: Distance Vector or Link State. Distance Vector routing involves advertising routes with two main characteristics: 172.16.3.0/24 Distance: This indicates the distance to the R1 S0/0/0 R2 destination network and is measured using metrics such as hop count, cost, bandwidth, delay, etc. If hop R1: 1 7 2.1 6.3.0 /2 4 i s one hop aw ay count is used as the metric, each router through (distance) a n d c a n be reached through which a packet passes is counted as one hop. The R2 via S 0 / 0 / 0 interface (vector) route with the fewest hops is considered the best route. Vector: This specifies the direction (next-hop router or exit interface) to reach the destination network. DYNAMIC ROUTING A router using a distance vector protocol lacks knowledge of the entire path to a destination network or an actual map of the network topology. Instead, it broadcasts its entire routing table to directly connected neighbors. These protocols treat routers as signposts along the path to the destination network. The only information a router has about a remote network is the distance or metric to reach it and which path or interface to use. DYNAMIC ROUTING The following describes the characteristics, operations, and functionality of distance vector routing protocols: Distance vector routing protocols exchange updates with neighboring routers. Some distance vector protocols, like RIP, send periodic updates to all neighbors (e.g., RIP updates every 30 seconds). Neighbors are routers that share a link and are configured with the same routing protocol. A router using distance vector routing knows only the network addresses of its own interfaces and the remote network addresses reachable through its neighbors. Routers using distance vector routing protocols are not aware of the entire network topology. DYNAMIC ROUTING In contrast to distance vector routing protocols, a router configured with a link state routing protocol can create a complete view of the network topology by gathering information from R4 all other routers. Unlike distance vector routing protocols, where routers act as signposts along the path, in link state routing protocols, all routers use an identical map of the network. Link update fromR2R1 A link state router uses the gathered link state information to create a detailed topology map and to determine the best paths to all destination networks within the topology. Link update 172.16.3.0/24 froRm1 R1 R3 ROUTING PROTOCOL METRICS In some scenarios, a routing protocol may discover multiple routes to the same destination. To determine the best path among these routes, the routing protocol evaluates and distinguishes between them using routing metrics. Routing metrics are measurable values assigned by the routing protocol to different routes based on their perceived quality or efficiency. When multiple paths to the same remote network exist, routing metrics are used to calculate the overall 'cost' of each path from source to destination. Routing protocols select the best path by choosing the route with the lowest cost.  ROUTING PROTOCOL METRICS Routing protocols employ various metrics that are Both RIP and OSPF routing protocols are specific to each protocol. These metrics are not directly enabled on R1. comparable across different routing protocols, which means that two different protocols may choose RIP determines the best route based on the different paths to reach the same destination. For fewest number of hops. Therefore, when a packet instance, consider the scenario where PC1 needs to arrives at R1 destined for PC2, it would choose send a packet to PC2: to send it directly to R2, even though this link 172.16.1.0/24 may be slower than others. PC2 In contrast, OSPF selects the best route based on the highest available bandwidth. Thus, when a packet arrives at R1 for PC2, OSPF would direct OSPF RI it to R3, which then forwards it to R2 via a faster P R2 link. 56kbps In summary, RIP prioritizes the shortest hop PC1 OSP count path, while OSPF prioritizes faster links. R1 R3 F 172.16.3.0/24 ROUTING INFORMATION PROTOCOLS RIPv1 broadcasts updates to all its neighbors using the all-hosts IPv4 address 255.255.255.255. In contrast, RIPv2 uses multicast addresses to ensure that only relevant neighbors receive updates. Additionally, RIPv2 supports an authentication mechanism to secure routing table updates exchanged between neighbors. LINK STATE PROTOCOLS The following describes the characteristics, operations, and functionality of link state routing protocols: Link state routing protocols, also known as shortest path first protocols, are based on Dijkstra’s shortest path first (SPF) algorithm. This algorithm calculates the accumulated costs along each path from source to destination to determine the total route cost. Although link state routing protocols are often perceived as more complex than distance vector counterparts, their basic functionality and configuration are straightforward.  LINK STATE PROTOCOLS Shortest Path for host on R2 LAN to reach host on R3 LAN: This means the total cost for R2 to R1 (20) + R1 to R3(5) + R3 to LAN (2) = 27 the shortest path from R2 to send packets to the LAN attached to R3 is 27. Each router independently determines its own cost to each destination in the topology using the SPF (Shortest Path First) algorithm. In other words, each router calculates the shortest path based on its own perspective. LINK STATE UPDATE PROCESS Link State Updates (LSUs) are the packets used for OSPF routing updates. In link state routing protocols, a 'link' refers to an interface on a router. Information regarding the status of these links is referred to as 'link states'. LINK STATE UPDATE PROCESS All routers in an OSPF area will complete the following generic link state routing process to reach a state of convergence: 1. Each router learns about its own links, its own directly connected network. 2. Each router is responsible for meeting its neighbors on directly connected networks. 3. Each router builds an LSP containing the state of each directly connected link. 4. Each router floods the LSP to all neighbors, who then store all LSPs received in a database. 5. Each router uses the database to construct a complete map of the topology and computes the best path to each destination network. Eventually, all routers receive an LSP from every other link state router in the routing area. These LSPs are stored in the link state database.  SHORTEST PATH (FROM R1) 10.5.0.0/16 Destination Shortest Path Cost Using link state 2 10.5.0.0/16 R1 → R2 22 information from all 10.6.0.0/16 R1 → R3 7 other routers, R1 begins 10.2.0.0/16 R2 10.7.0.0/16 10.8.0.0/16 R1 → R3 R1 → R3→R4 15 17 constructing an SPF tree 10 10.9.0.0/16 20 10.9.0.0/16 R1→R2 30 of the network. The SPF 10.6.0.0/16 10.11.0.0/16 10.10.0.0/16 R1 → R3→R4 25 algorithm initially 2 5 2 10.11.0.0/16 R1 → R3→R4 →R5 27 examines each router's R1 10.3.0.0/16 R3 2 R5 Link State Packet (LSP) to 10.1.0.0/16 10 Each router within identify networks and 20 10 10.7.0.0/16 10.10.0.0/16 the OSPF routing their associated costs. It 10.4.0.0/16 area utilizes the link then computes the shortest paths to each R4 state database and network, resulting in the 2 SPF (Shortest Path creation of the SPF tree. 10.8.0.0/16 First) algorithm to construct the SPF tree.