Cisco OSPFv2 Configuration PDF

17/08/2022 Module 2: Single-Area OSPFv2 Configuration Enterprise Networking, Security, and Automation v7.0 (ENSA)...

17/08/2022 Module 2: Single-Area OSPFv2 Configuration Enterprise Networking, Security, and Automation v7.0 (ENSA) 1 Module Objectives Module Title: Single-Area OSPFv2 Configuration Module Objective: Implement single-area OSPFv2 in both point-to-point and broadcast multiaccess networks. Topic Title Topic Objective OSPF Router ID Configure an OSPFv2 router ID. Point-to-Point OSPF Networks Configure single-area OSPFv2 in a point-to-point network. Configure the OSPF interface priority to influence the Multiaccess OSPF Networks DR/BDR election in a multiaccess network. Implement modifications to change the operation of single- Modify Single-Area OSPFv2 area OSPFv2. Default Route Propagation Configure OSPF to propagate a default route. Verify Single-Area OSPFv2 Verify a single-area OSPFv2 implementation. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 2 2 1  17/08/2022 2.1 OSPF Router ID © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 3 3 OSPF Router ID OSPF Reference Topology The figure shows the topology used for configuring OSPFv2 in this module. The routers in the topology have a starting configuration, including interface addresses. There is currently no static routing or dynamic routing configured on any of the routers. All interfaces on R1, R2, and R3 (except the loopback 1 on R2) are within the OSPF backbone area. The ISP router is used as the gateway to the internet of the routing domain. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 4 4 2  17/08/2022 OSPF Router ID Router Configuration Mode for OSPF OSPFv2 is enabled using the router ospf process-id global configuration mode command. The process-id value represents a number between 1 and 65,535 and is selected by the network administrator. The process-id value is locally significant. It is considered best practice to use the same process-id on all OSPF routers. R1(config)# router ospf 10 R1(config-router)# ? area OSPF area parameters auto-cost Calculate OSPF interface cost according to bandwidth default-information Control distribution of default information distance Define an administrative distance exit Exit from routing protocol configuration mode log-adjacency-changes Log changes in adjacency state neighbor Specify a neighbor router network Enable routing on an IP network no Negate a command or set its defaults passive-interface Suppress routing updates on an interface redistribute Redistribute information from another routing protocol router-id router-id for this OSPF process R1(config-router)# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 5 5 OSPF Router ID Router IDs An OSPF router ID is a 32-bit value, represented as an IPv4 address. It is used to uniquely identify an OSPF router, and all OSPF packets include the router ID of the originating router. Every router requires a router ID to participate in an OSPF domain. It can be defined by an administrator or automatically assigned by the router. The router ID is used by an OSPF-enabled router to do the following: Participate in the synchronization of OSPF databases – During the Exchange State, the router with the highest router ID will send their database descriptor (DBD) packets first. Participate in the election of the designated router (DR) - In a multiaccess LAN environment, the router with the highest router ID is elected the DR. The routing device with the second highest router ID is elected the backup designated router (BDR). © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 6 6 3  17/08/2022 OSPF Router ID Router ID Order of Precedence Cisco routers derive the router ID based on one of three criteria, in the following preferential order: 1. The router ID is explicitly configured using the OSPF router-id rid router configuration mode command. This is the recommended method to assign a router ID. 2. The router chooses the highest IPv4 address of any of configured loopback interfaces. 3. The router chooses the highest active IPv4 address of any of its physical interfaces. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 7 7 OSPF Router ID Configure a Loopback Interface as the Router ID Instead of relying on physical interface, the router ID can be assigned to a loopback interface. Typically, the IPv4 address for this type of loopback interface should be configured using a 32-bit subnet mask (255.255.255.255). This effectively creates a host route. A 32-bit host route would not get advertised as a route to other OSPF routers. OSPF does not need to be enabled on an interface for that interface to be chosen as the router ID. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 8 8 4  17/08/2022 OSPF Router ID Explicitly Configure a Router ID In our reference topology the router ID for each router is assigned as follows: R1 uses router ID 1.1.1.1 R2 uses router ID 2.2.2.2 R3 uses router ID 3.3.3.3 Use the router-id rid router configuration mode command to manually assign a router ID. In the example, the router ID 1.1.1.1 is assigned to R1. Use the show ip protocols command to verify the router ID. R1(config)# router ospf 10 R1(config-router)# router-id 1.1.1.1 R1(config-router)# end *May 23 19:33:42.689: %SYS-5-CONFIG_I: Configured from console by console R1# show ip protocols | include Router ID Router ID 1.1.1.1 R1# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 9 9 OSPF Router ID Modify a Router ID After a router selects a router ID, an active OSPF router does not allow the router ID to be changed until the router is reloaded or the OSPF process is reset. Clearing the OSPF process is the preferred method to reset the router ID. R1# show ip protocols | include Router ID Router ID 10.10.1.1 R1# conf t Enter configuration commands, one per line. End with CNTL/Z. R1(config)# router ospf 10 R1(config-router)# router-id 1.1.1.1 % OSPF: Reload or use "clear ip ospf process" command, for this to take effect R1(config-router)# end R1# clear ip ospf process Reset ALL OSPF processes? [no]: y *Jun 6 01:09:46.975: %OSPF-5-ADJCHG: Process 10, Nbr 3.3.3.3 on GigabitEthernet0/0/1 from FULL to DOWN, Neighbor Down: Interface down or detached *Jun 6 01:09:46.981: %OSPF-5-ADJCHG: Process 10, Nbr 3.3.3.3 on GigabitEthernet0/0/1 from LOADING to FULL, Loading Done * R1# show ip protocols | include Router ID Router ID 1.1.1.1 R1# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 10 10 5  17/08/2022 2.2 Point-to-Point OSPF Networks © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 11 11 Point-to-Point OSPF Networks The network Command Syntax You can specify the interfaces that belong to a point-to-point network by configuring the network command. You can also configure OSPF directly on the interface with the ip ospf command. The basic syntax for the network command is as follows: Router(config-router)# network network-address wildcard-mask area area-id The network-address wildcard-mask syntax is used to enable OSPF on interfaces. Any interfaces on a router that match this part of the command are enabled to send and receive OSPF packets. The area area-id syntax refers to the OSPF area. When configuring single-area OSPFv2, the network command must be configured with the same area-id value on all routers. Although any area ID can be used, it is good practice to use an area ID of 0 with single-area OSPFv2. This convention makes it easier if the network is later altered to support multiarea OSPFv2. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 12 12 6  17/08/2022 Point-to-Point OSPF Networks The Wildcard Mask The wildcard mask is typically the inverse of the subnet mask configured on that interface. The easiest method for calculating a wildcard mask is to subtract the network subnet mask from 255.255.255.255, as shown for /24 and /26 subnet masks in the figure. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 13 13 Point-to-Point OSPF Networks Configure OSPF Using the network Command Within routing configuration mode, there are two ways to identify the interfaces that will participate in the OSPFv2 routing process. In the first example, the wildcard mask identifies the interface based on the network addresses. Any active interface that is configured with an IPv4 address belonging to that network will participate in the OSPFv2 routing process. Note: Some IOS versions allow the subnet mask to be entered instead of the wildcard mask. The IOS then converts the subnet mask to the wildcard mask format. R1(config)# router ospf 10 R1(config-router)# network 10.10.1.0 0.0.0.255 area 0 R1(config-router)# network 10.1.1.4 0.0.0.3 area 0 R1(config-router)# network 10.1.1.12 0.0.0.3 area 0 R1(config-router)# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 14 14 7  17/08/2022 Point-to-Point OSPF Networks Configure OSPF Using the network Command (Cont.) As an alternative, OSPFv2 can be enabled by specifying the exact interface IPv4 address using a quad zero wildcard mask. Entering network 10.1.1.5 0.0.0.0 area 0 on R1 tells the router to enable interface Gigabit Ethernet 0/0/0 for the routing process. The advantage of specifying the interface is that the wildcard mask calculation is not necessary. Notice that in all cases, the area argument specifies area 0. R1(config)# router ospf 10 R1(config-router)# network 10.10.1.1 0.0.0.0 area 0 R1(config-router)# network 10.1.1.5 0.0.0.0 area 0 R1(config-router)# network 10.1.1.14 0.0.0.0 area 0 R1(config-router)# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 15 15 Point-to-Point OSPF Networks Configure OSPF Using the ip ospf Command To configure OSPF directly on the interface, use the ip ospf interface configuration mode command. The syntax is as follows: Router(config-if)# ip ospf process-id area area-id Remove the network commands using the no form of the command. Then go to each interface and configure the ip ospf command R1(config)# router ospf 10 R1(config-router)# no network 10.10.1.1 0.0.0.0 area 0 R1(config-router)# no network 10.1.1.5 0.0.0.0 area 0 R1(config-router)# no network 10.1.1.14 0.0.0.0 area 0 R1(config-router)# interface GigabitEthernet 0/0/0 R1(config-if)# ip ospf 10 area 0 R1(config-if)# interface GigabitEthernet 0/0/1 R1(config-if)# ip ospf 10 area 0 R1(config-if)# interface Loopback 0 R1(config-if)# ip ospf 10 area 0 R1(config-if)# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 16 16 8  17/08/2022 Point-to-Point OSPF Networks Passive Interface By default, OSPF messages are forwarded out all OSPF-enabled interfaces. However, these messages only need to be sent out interfaces that are connecting to other OSPF- enabled routers. Sending out unneeded messages on a LAN affects the network in three ways: Inefficient Use of Bandwidth - Available bandwidth is consumed transporting unnecessary messages. Inefficient Use of Resources - All devices on the LAN must process and eventually discard the message. Increased Security Risk - Without additional OSPF security configurations, OSPF messages can be intercepted with packet sniffing software. Routing updates can be modified and sent back to the router, corrupting the routing table with false metrics that misdirect traffic. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 17 17 Point-to-Point OSPF Networks Configure Passive Interfaces Use the passive- interface router configuration mode command to prevent the transmission of routing messages through a router interface, but still allow that network to be advertised to other routers. The show ip protocols command is then used to verify that the interface is listed as passive. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 18 18 9  17/08/2022 Point-to-Point OSPF Networks OSPF Point-to-Point Networks By default, Cisco routers elect a DR and BDR on Ethernet interfaces, even if there is only one other device on the link. You can verify this with the show ip ospf interface command. The DR/ BDR election process is unnecessary as there can only be two routers on the point-to-point network between R1 and R2. Notice in the output that the router has designated the network type as BROADCAST. R1# show ip ospf interface GigabitEthernet 0/0/0 GigabitEthernet0/0/0 is up, line protocol is up Internet Address 10.1.1.5/30, Area 0, Attached via Interface Enable Process ID 10, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1 Topology-MTID Cost Disabled Shutdown Topology Name 0 1 no no Base Enabled by interface config, including secondary ip addresses Transmit Delay is 1 sec, State BDR, Priority 1 Designated Router (ID) 2.2.2.2, Interface address 10.1.1.6 Backup Designated router (ID) 1.1.1.1, Interface address 10.1.1.5 Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 oob-resync timeout 40 © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 19 19 Point-to-Point OSPF Networks OSPF Point-to-Point Networks (Cont.) To change this to a point-to-point network, use the interface configuration command ip ospf network point-to-point on all interfaces where you want to disable the DR/BDR election process. R1(config)# interface GigabitEthernet 0/0/0 R1(config-if)# ip ospf network point-to-point *Jun 6 00:44:05.208: %OSPF-5-ADJCHG: Process 10, Nbr 2.2.2.2 on GigabitEthernet0/0/0 from FULL to DOWN, Neighbor Down: Interface down or detached *Jun 6 00:44:05.211: %OSPF-5-ADJCHG: Process 10, Nbr 2.2.2.2 on GigabitEthernet0/0/0 from LOADING to FULL, Loading Done R1(config-if)# end R1# show ip ospf interface GigabitEthernet 0/0/0 GigabitEthernet0/0/0 is up, line protocol is up Internet Address 10.1.1.5/30, Area 0, Attached via Interface Enable Process ID 10, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 1 Topology-MTID Cost Disabled Shutdown Topology Name © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 20 20 10  17/08/2022 Point-to-Point OSPF Networks Loopbacks and Point-to-Point Networks Use loopbacks to provide additional interfaces for a variety of purposes. By default, loopback interfaces are advertised as /32 host routes. To simulate a real LAN, the loopback interface can be configured as a point-to-point network to advertise the full network. What R2 sees when R1 advertises the loopback interface as-is: R2# show ip route | include 10.10.1 O 10.10.1.1/32 [110/2] via 10.1.1.5, 00:03:05, GigabitEthernet0/0/0 Configuration change at R1: R1(config-if)# interface Loopback 0 R1(config-if)# ip ospf network point-to-point Result at R2: R2# show ip route | include 10.10.1 O 10.10.1.0/24 [110/2] via 10.1.1.5, 00:03:05, GigabitEthernet0/0/0 © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 21 21 Point-to-Point OSPF Networks Packet Tracer - Point-to-Point Single-Area OSPFv2 Configuration In this Packet Tracer activity, you will do the following: Explicitly configure router IDs. Configure the network command on R1 using wildcard mask based on the subnet mask. Configure the network command on R2 using a quad-zero wildcard mask. Configure the ip ospf interface command on R3. Configure passive interfaces. Verify OSPF operation using the show ip protocols and show ip route commands. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 22 22 11  17/08/2022 2.3 Multiaccess OSPF Networks © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 23 23 Multiaccess OSPF Networks OPSF Network Types Another type of network that uses OSPF is the multiaccess OSPF network. Multiaccess OSPF networks are unique in that one router controls the distribution of LSAs. The router that is elected for this role should be determined by the network administrator through proper configuration. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 24 24 12  17/08/2022 Multiaccess OSPF Networks OPSF Designated Router In multiaccess networks, OSPF elects a DR and BDR. The DR is responsible for collecting and distributing LSAs sent and received. The DR uses the multicast IPv4 address 224.0.0.5 which is meant for all OSPF routers. A BDR is also elected in case the DR fails. The BDR listens passively and maintains a relationship with all the routers. If the DR stops producing Hello packets, the BDR promotes itself and assumes the role of DR. All other routers become a DROTHER (a router that is neither the DR nor the BDR). DROTHERs use the multiaccess address 224.0.0.6 (all designated routers) to send OSPF packets to the DR and BDR. Only the DR and BDR listen for 224.0.0.6. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 25 25 Multiaccess OSPF Networks OPSF Multiaccess Reference Topology In the multiaccess topology shown in the figure, there are three routers interconnected over a common Ethernet multiaccess network, 192.168.1.0/24. Because the routers are connected over a common multiaccess network, OSPF has automatically elected a DR and BDR. R3 has been elected as the DR because its router ID is 3.3.3.3, which is the highest in this network. R2 is the BDR because it has the second highest router ID in the network. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 26 26 13  17/08/2022 Multiaccess OSPF Networks Verify OSPF Router Roles To verify the roles of the OSPFv2 router, use the show ip ospf interface command. The output generated by R1 confirms that the following: R1 is not the DR or BDR, but is a DROTHER with a default priority of 1. (Line 7) The DR is R3 with router ID 3.3.3.3 at IPv4 address 192.168.1.3, while the BDR is R2 with router ID 2.2.2.2 at IPv4 address 192.168.1.2. (Lines 8 and 9) R1 has two adjacencies: one with the BDR and one with the DR. (Lines 20-22) R1# show ip ospf interface GigabitEthernet 0/0/0 GigabitEthernet0/0/0 is up, line protocol is up Internet Address 192.168.1.1/24, Area 0, Attached via Interface Enable Process ID 10, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1 (output omitted) Transmit Delay is 1 sec, State DROTHER, Priority 1 Designated Router (ID) 3.3.3.3, Interface address 192.168.1.3 Backup Designated router (ID) 2.2.2.2, Interface address 192.168.1.2 (output omitted) Neighbor Count is 2, Adjacent neighbor count is 2 Adjacent with neighbor 2.2.2.2 (Backup Designated Router) Adjacent with neighbor 3.3.3.3 (Designated Router) Suppress hello for 0 neighbor(s) R1# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 27 27 Multiaccess OSPF Networks Verify OSPF Router Roles (Cont.) The output generated by R2 confirms that: R2 is the BDR with a default priority of 1. (Line 7) The DR is R3 with router ID 3.3.3.3 at IPv4 address 192.168.1.3, while the BDR is R2 with router ID 2.2.2.2 at IPv4 address 192.168.1.2. (Lines 8 and 9) R2 has two adjacencies; one with a neighbor with router ID 1.1.1.1 (R1) and the other with the DR. (Lines 20-22) R2# show ip ospf interface GigabitEthernet 0/0/0 GigabitEthernet0/0/0 is up, line protocol is up Internet Address 192.168.1.2/24, Area 0, Attached via Interface Enable Process ID 10, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1 (output omitted) Transmit Delay is 1 sec, State BDR, Priority 1 Designated Router (ID) 3.3.3.3, Interface address 192.168.1.3 Backup Designated Router (ID) 2.2.2.2, Interface address 192.168.1.2 (output omitted) Neighbor Count is 2, Adjacent neighbor count is 2 Adjacent with neighbor 1.1.1.1 Adjacent with neighbor 3.3.3.3 (Designated Router) Suppress hello for 0 neighbor(s) R2# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 28 28 14  17/08/2022 Multiaccess OSPF Networks Verify OSPF Router Roles (Cont.) The output generated by R3 confirms that: R3 is the DR with a default priority of 1. (Line 7) The DR is R3 with router ID 3.3.3.3 at IPv4 address 192.168.1.3, while the BDR is R2 with router ID 2.2.2.2 at IPv4 address 192.168.1.2. (Lines 8 and 9) R3 has two adjacencies: one with a neighbor with router ID 1.1.1.1 (R1) and the other with the BDR. (Lines 20-22) R3# show ip ospf interface GigabitEthernet 0/0/0 GigabitEthernet0/0/0 is up, line protocol is up Internet Address 192.168.1.3/24, Area 0, Attached via Interface Enable Process ID 10, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1 (output omitted) Transmit Delay is 1 sec, State DR, Priority 1 Designated Router (ID) 3.3.3.3, Interface address 192.168.1.3 Backup Designated Router (ID) 2.2.2.2, Interface address 192.168.1.2 (output omitted) Neighbor Count is 2, Adjacent neighbor count is 2 Adjacent with neighbor 1.1.1.1 Adjacent with neighbor 2.2.2.2 (Backup Designated Router) Suppress hello for 0 neighbor(s) R3# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 29 29 Multiaccess OSPF Networks Verify DR/BDR Adjacencies To verify the OSPFv2 adjacencies, use the show ip ospf neighbor command. The state of neighbors in multiaccess networks can be as follows: FULL/DROTHER - This is a DR or BDR router that is fully adjacent with a non-DR or BDR router. These two neighbors can exchange Hello packets, updates, queries, replies, and acknowledgments. FULL/DR - The router is fully adjacent with the indicated DR neighbor. These two neighbors can exchange Hello packets, updates, queries, replies, and acknowledgments. FULL/BDR - The router is fully adjacent with the indicated BDR neighbor. These two neighbors can exchange Hello packets, updates, queries, replies, and acknowledgments. 2-WAY/DROTHER - The non-DR or BDR router has a neighbor relationship with another non-DR or BDR router. These two neighbors exchange Hello packets. The normal state for an OSPF router is usually FULL. If a router is stuck in another state, it is an indication that there are problems in forming adjacencies. The only exception to this is the 2-WAY state, which is normal in a multiaccess broadcast network. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 30 30 15  17/08/2022 Multiaccess OSPF Networks Verify DR/BDR Adjacencies (Cont.) The output generated by R2 confirms that R2 has adjacencies with the following routers: R1 with router ID 1.1.1.1 is in a Full state and R1 is neither the DR nor BDR. R3 with router ID 3.3.3.3 is in a Full state and the role of R3 is DR. R2# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 1.1.1.1 1 FULL/DROTHER 00:00:31 192.168.1.1 GigabitEthernet0/0/0 3.3.3.3 1 FULL/DR 00:00:34 192.168.1.3 GigabitEthernet0/0/0 R2# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 31 31 Multiaccess OSPF Networks Default DR/BDR Election Process The OSPF DR and BDR election is based on the following criteria, in sequential order: 1. The routers in the network elect the router with the highest interface priority as the DR. The router with the second highest interface priority is becomes the BDR. The priority can be configured to be any number between 0 – 255. If the interface priority value is set to 0, that interface cannot be elected as DR nor BDR. The default priority of multiaccess broadcast interfaces is 1. 2. If the interface priorities are equal, then the router with the highest router ID is elected the DR. The router with the second highest router ID is the BDR. The election process takes place when the first router with an OSPF-enabled interface is active on the network. If all of the routers on the network have not finished booting, it is possible that a router with a lower router ID becomes the DR. The addition of a new router does not initiate a new election process. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 32 32 16  17/08/2022 Multiaccess OSPF Networks DR Failure and Recovery After the DR is elected, it remains the DR until one of the following events occurs: The DR fails. The OSPF process on the DR fails or is stopped. The multiaccess interface on the DR fails or is shutdown. If the DR fails, the BDR is automatically promoted to DR. This is the case even if another DROTHER with a higher priority or router ID is added to the network after the initial DR/BDR election. However, after a BDR is promoted to DR, a new BDR election occurs and the DROTHER with the highest priority or router ID is elected as the new BDR. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 33 33 Multiaccess OSPF Networks The ip ospf priority Command If the interface priorities are equal on all routers, the router with the highest router ID is elected the DR. Instead of relying on the router ID, it is better to control the election by setting interface priorities. This also allows a router to be the DR in one network and a DROTHER in another. To set the priority of an interface, use the command ip ospf priority value, where value is 0 to 255. A value of 0 does not become a DR or a BDR. A value of 1 to 255 on the interface makes it more likely that the router becomes the DR or the BDR. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 34 34 17  17/08/2022 Multiaccess OSPF Networks Configure OSPF Priority The example shows the commands being used to change the R1 G0/0/0 interface priority from 1 to 255 and then reset the OSPF process. R1(config)# interface GigabitEthernet 0/0/0 R1(config-if)# ip ospf priority 255 R1(config-if)# end R1# clear ip ospf process Reset ALL OSPF processes? [no]: y R1# *Jun 5 03:47:41.563: %OSPF-5-ADJCHG: Process 10, Nbr 2.2.2.2 on GigabitEthernet0/0/0 from FULL to DOWN, Neighbor Down: Interface down or detached © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 35 35 Multiaccess OSPF Networks Packet Tracer - Determine the DR and BDR In this activity, you will complete the following: Examine DR and BDR roles and watch the roles change when there is a change in the network. Modify the priority to control the roles and force a new election. Verify routers are filling the desired roles © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 36 36 18  17/08/2022 2.4 Modify Single-Area OSPFv2 © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 37 37 Modify Single-Area OSPFv2 Cisco OSPF Cost Metric Routing protocols use a metric to determine the best path of a packet across a network. OSPF uses cost as a metric. A lower cost indicates a better path. The Cisco cost of an interface is inversely proportional to the bandwidth of the interface. Therefore, a higher bandwidth indicates a lower cost. The formula used to calculate the OSPF cost is: Cost = reference bandwidth / interface bandwidth The default reference bandwidth is 108 (100,000,000); therefore, the formula is: Cost = 100,000,000 bps / interface bandwidth in bps Because the OSPF cost value must be an integer, FastEthernet, Gigabit Ethernet, and 10 GigE interfaces share the same cost. To correct this situation, you can: Adjust the reference bandwidth with the auto-cost reference-bandwidth command on each OSPF router. Manually set the OSPF cost value with the ip ospf cost command on necessary interfaces. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 38 38 19  17/08/2022 Modify Single-Area OSPFv2 Cisco OSPF Cost Metric (Cont.) Refer to the table for a breakdown of the cost calculation © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 39 39 Modify Single-Area OSPFv2 Adjust the Reference Bandwidth The cost value must be an integer. If something less than an integer is calculated, OSPF rounds up to the nearest integer. Therefore, the OSPF cost assigned to a Gigabit Ethernet interface with the default reference bandwidth of 100,000,000 bps would equal 1, because the nearest integer for 0.1 is 0 instead of 1. Cost = 100,000,000 bps / 1,000,000,000 = 1 For this reason, all interfaces faster than Fast Ethernet will have the same cost value of 1 as a Fast Ethernet interface. To assist OSPF in making the correct path determination, the reference bandwidth must be changed to a higher value to accommodate networks with links faster than 100 Mbps. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 40 40 20  17/08/2022 Modify Single-Area OSPFv2 Adjust the Reference Bandwidth (Cont.) Changing the reference bandwidth does not actually affect the bandwidth capacity on the link; rather, it simply affects the calculation used to determine the metric. To adjust the reference bandwidth, use the auto-cost reference-bandwidth Mbps router configuration command. This command must be configured on every router in the OSPF domain. Notice in the command that the value is expressed in Mbps; therefore, to adjust the costs for Gigabit Ethernet, use the command auto-cost reference-bandwidth 1000. For 10 Gigabit Ethernet, use the command auto-cost reference-bandwidth 10000. To return to the default reference bandwidth, use the auto-cost reference-bandwidth 100 command. Another option is to change the cost on one specific interface using the ip ospf cost cost command. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 41 41 Modify Single-Area OSPFv2 Adjust the Reference Bandwidth (Cont.) Whichever method is used, it is important to apply the configuration to all routers in the OSPF routing domain. The table shows the OSPF cost if the reference bandwidth is adjusted to accommodate 10 Gigabit Ethernet links. The reference bandwidth should be adjusted anytime there are links faster than FastEthernet (100 Mbps). Use the show ip ospf interface command to verify the current OSPFv2 cost assigned to the interface. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 42 42 21  17/08/2022 Modify Single-Area OSPFv2 OSPF Accumulates Cost The cost of an OSPF route is the accumulated value from one router to the destination network. Assuming the auto-cost reference-bandwidth 10000 command has been configured on all three routers, the cost of the links between each router is now 10. The loopback interfaces have a default cost of 1. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 43 43 Modify Single-Area OSPFv2 OSPF Accumulates Cost (Cont.) You can calculate the cost for each router to reach each network. For example, the total cost for R1 to reach the 10.10.2.0/24 network is 11. This is because the link to R2 cost = 10 and the loopback default cost = 1. 10 + 1 = 11. You can verify this with the show ip route command. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 44 44 22  17/08/2022 Modify Single-Area OSPFv2 OSPF Accumulates Cost (Cont.) Verifying the accumulated cost for the path to the 10.10.2.0/24 network: R1# show ip route | include 10.10.2.0 O 10.10.2.0/24 [110/11] via 10.1.1.6, 01:05:02, GigabitEthernet0/0/0 R1# show ip route 10.10.2.0 Routing entry for 10.10.2.0/24 Known via "ospf 10", distance 110, metric 11, type intra area Last update from 10.1.1.6 on GigabitEthernet0/0/0, 01:05:13 ago Routing Descriptor Blocks: * 10.1.1.6, from 2.2.2.2, 01:05:13 ago, via GigabitEthernet0/0/0 Route metric is 11, traffic share count is 1 R1# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 45 45 Modify Single-Area OSPFv2 Manually Set OSPF Cost Value Reasons to manually set the cost value include: The Administrator may want to influence path selection within OSPF, causing different paths to be selected than what normally would given default costs and cost accumulation. Connections to equipment from other vendors who use a different formula to calculate OSPF cost. To change the cost value reported by the local OSPF router to other OSPF routers, use the interface configuration command ip ospf cost value. R1(config)# interface g0/0/1 R1(config-if)# ip ospf cost 30 R1(config-if)# interface lo0 R1(config-if)# ip ospf cost 10 R1(config-if)# end R1# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 46 46 23  17/08/2022 Modify Single-Area OSPFv2 Test Failover to Backup Route What happens if the link between R1 and R2 goes down? You can simulate that by shutting down the Gigabit Ethernet 0/0/0 interface and verifying the routing table is updated to use R3 as the next-hop router. Notice that R1 can now reach the 10.1.1.4/30 network through R3 with a cost value of 50. R1# show ip route ospf | begin 10 10.0.0.0/8 is variably subnetted, 8 subnets, 3 masks O 10.1.1.4/30 [110/50] via 10.1.1.13, 00:00:14, GigabitEthernet0/0/1 O 10.1.1.8/30 [110/40] via 10.1.1.13, 00:00:14, GigabitEthernet0/0/1 O 10.10.2.0/24 [110/50] via 10.1.1.13, 00:00:14, GigabitEthernet0/0/1 O 10.10.3.0/24 [110/40] via 10.1.1.13, 00:00:14, GigabitEthernet0/0/1 R1# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 47 47 Modify Single-Area OSPFv2 Hello Packet Intervals OSPFv2 Hello packets are transmitted to multicast address 224.0.0.5 (all OSPF routers) every 10 seconds. This is the default timer value on multiaccess and point-to- point networks. Note: Hello packets are not sent on interfaces set to passive by the passive-interface command. The Dead interval is the period that the router waits to receive a Hello packet before declaring the neighbor down. If the Dead interval expires before the routers receive a Hello packet, OSPF removes that neighbor from its link-state database (LSDB). The router floods the LSDB with information about the down neighbor out all OSPF- enabled interfaces. Cisco uses a default of 4 times the Hello interval. This is 40 seconds on multiaccess and point-to-point networks. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 48 48 24  17/08/2022 Modify Single-Area OSPFv2 Verify Hello and Dead Intervals The OSPF Hello and Dead intervals are configurable on a per-interface basis. The OSPF intervals must match or a neighbor adjacency does not occur. To verify the currently configured OSPFv2 interface intervals, use the show ip ospf interface command. The Gigabit Ethernet 0/0/0 Hello and Dead intervals are set to the default 10 seconds and 40 seconds respectively. R1# show ip ospf interface g0/0/0 GigabitEthernet0/0/0 is up, line protocol is up Internet Address 10.1.1.5/30, Area 0, Attached via Interface Enable Process ID 10, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 10 Topology-MTID Cost Disabled Shutdown Topology Name 0 10 no no Base Enabled by interface config, including secondary ip addresses Transmit Delay is 1 sec, State POINT_TO_POINT Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 oob-resync timeout 40 (output omitted) © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 49 49 Modify Single-Area OSPFv2 Verify Hello and Dead Intervals (Cont.) Use the show ip ospf neighbor command to see the Dead Time counting down from 40 seconds. By default, this value is refreshed every 10 seconds when R1 receives a Hello from the neighbor. R1# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 3.3.3.3 0 FULL/ - 00:00:35 10.1.1.13 GigabitEthernet0/0/1 2.2.2.2 0 FULL/ - 00:00:31 10.1.1.6 GigabitEthernet0/0/0 R1# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 50 50 25  17/08/2022 Modify Single-Area OSPFv2 Modify OSPFv2 Intervals It may be desirable to change the OSPF timers so that routers detect network failures in less time. Doing this increases traffic, but sometimes the need for quick convergence is more important than the extra traffic it creates. Note: The default Hello and Dead intervals are based on best practices and should only be altered in rare situations. OSPFv2 Hello and Dead intervals can be modified manually using the following interface configuration mode commands: Router(config-if)# ip ospf hello-interval seconds Router(config-if)# ip ospf dead-interval seconds Use the no ip ospf hello-interval and no ip ospf dead-interval commands to reset the intervals to their default. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 51 51 Modify Single-Area OSPFv2 Modify OSPFv2 Intervals (Cont.) In the example, the Hello interval for the link between R1 and R2 is changed to 5 seconds. The Cisco IOS automatically modifies the Dead interval to four times the Hello interval. However, you can document the new Dead interval in the configuration by manually setting it to 20 seconds, as shown. When the Dead Timer on R1 expires, R1 and R2 lose adjacency. R1 and R2 must be configured with the same Hello interval. Use the show ip ospf neighbor command on R1 to verify the neighbor adjacencies. R1(config)# interface g0/0/0 R1(config-if)# ip ospf hello-interval 5 R1(config-if)# ip ospf dead-interval 20 R1(config-if)# *Jun 7 04:56:07.571: %OSPF-5-ADJCHG: Process 10, Nbr 2.2.2.2 on GigabitEthernet0/0/0 from FULL to DOWN, Neighbor Down: Dead timer expired R1(config-if)# end R1# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 3.3.3.3 0 FULL/ - 00:00:37 10.1.1.13 GigabitEthernet0/0/1 R1# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 52 52 26  17/08/2022 Modify Single-Area OSPFv2 Packet Tracer - Modify Single-Area OSPFv2 In this Packet Tracer activity, you will complete the following: Adjust the reference bandwidth to account for gigabit and faster speeds Modify the OSPF cost value Modify the OSPF Hello timers Verify the modifications are accurately reflected in the routers. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 53 53 2.5 Default Route Propagation © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 54 54 27  17/08/2022 Default Route Propagation Propagate a Default Static Route in OSPFv2 To propagate a default route, the edge router must be configured with the following: A default static route using the ip route 0.0.0.0 0.0.0.0 [next-hop-address | exit-intf] command. The default-information originate router configuration command. This instructs R2 to be the source of the default route information and propagate the default static route in OSPF updates. In the example, R2 is configured with a loopback to simulate a connection to the internet. A default route is configured and propagated to all other OSPF routers in the routing domain. Note: When configuring static routes, best practice is to use the next-hop IP address. However, when simulating a connection to the internet, there is no next-hop IP address. Therefore, we use the exit-intf argument. R2(config)# interface lo1 R2(config-if)# ip address 64.100.0.1 255.255.255.252 R2(config-if)# exit R2(config)# ip route 0.0.0.0 0.0.0.0 loopback 1 %Default route without gateway, if not a point-to-point interface, may impact performance R2(config)# router ospf 10 R2(config-router)# default-information originate R2(config-router)# end R2# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 55 55 Default Route Propagation Verify the Propagated Default Route You can verify the default route settings on R2 using the show ip route command. You can also verify that R1 and R3 received a default route. Notice that the route source on R1 is O*E2, signifying that it was learned using OSPFv2. The asterisk identifies this as a good candidate for the default route. The E2 designation identifies that it is an external route. The meaning of E1 and E2 is beyond the scope of this module. R2# show ip route | begin Gateway Gateway of last resort is 0.0.0.0 to network 0.0.0.0 S* 0.0.0.0/0 is directly connected, Loopback1 10.0.0.0/8 is variably subnetted, 9 subnets, 3 masks (output omitted) R1# show ip route | begin Gateway Gateway of last resort is 10.1.1.6 to network 0.0.0.0 O*E2 0.0.0.0/0 [110/1] via 10.1.1.6, 00:11:08, GigabitEthernet0/0/0 10.0.0.0/8 is variably subnetted, 9 subnets, 3 masks (output omitted) © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 56 56 28  17/08/2022 Default Route Propagation Packet Tracer - Propagate a Default Route in OSPFv2 In this Packet Tracer, you will complete the following: Propagate a Default Route Part 2: Verify Connectivity © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 57 57 2.6 Verify Single-Area OSPFv2 © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 58 58 29  17/08/2022 Verify Single-Area OSPFv2 Verify OSPF Neighbors After configuring single-area OSPFv2, you will need to verify your configurations. The following two commands are particularly useful for verifying routing: show ip interface brief - This verifies that the desired interfaces are active with correct IP addressing. show ip route- This verifies that the routing table contains all the expected routes. Additional commands for determining that OSPF is operating as expected include the following: show ip ospf neighbor show ip protocols show ip ospf show ip ospf interface © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 59 59 Verify Single-Area OSPFv2 Verify OSPF Neighbors (Cont.) Use the show ip ospf neighbor command to verify that the router has formed an adjacency with its neighboring routers. If the router ID of the neighboring router is not displayed, or if it does not show as being in a state of FULL, the two routers have not formed an OSPFv2 adjacency. Note: A non-DR or BDR router that has a neighbor relationship with another non-DR or BDR router will display a two-way adjacency instead of full. The following command output displays the neighbor table of R1. R1# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 3.3.3.3 0 FULL/ - 00:00:35 10.1.1.13 GigabitEthernet0/0/1 2.2.2.2 0 FULL/ - 00:00:31 10.1.1.6 GigabitEthernet0/0/0 R1# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 60 60 30  17/08/2022 Verify Single-Area OSPFv2 Verify OSPF Neighbors (Cont.) Two routers may not form an OSPFv2 adjacency if the following occurs: The subnet masks do not match, causing the routers to be on separate networks. The OSPFv2 Hello or Dead Timers do not match. The OSPFv2 Network Types do not match. There is a missing or incorrect OSPFv2 network command. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 61 61 Verify Single-Area OSPFv2 Verify OSPF Protocol Settings The show ip protocols R1# show ip protocols command is a quick way to *** IP Routing is NSF aware *** (output omitted) verify vital OSPF Routing Protocol is "ospf 10" configuration information, as Outgoing update filter list for all interfaces is not set shown in the command Incoming update filter list for all interfaces is not set Router ID 1.1.1.1 output. This includes the Number of areas in this router is 1. 1 normal 0 stub 0 nssa OSPFv2 process ID, the Maximum path: 4 router ID, interfaces Routing for Networks: Routing on Interfaces Configured Explicitly (Area 0): explicitly configured to Loopback0 advertise OSPF routes, the GigabitEthernet0/0/1 neighbors the router is GigabitEthernet0/0/0 Routing Information Sources: receiving updates from, and Gateway Distance Last Update the default administrative 3.3.3.3 110 00:09:30 distance, which is 110 for 2.2.2.2 110 00:09:58 Distance: (default is 110) OSPF. R1# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 62 62 31  17/08/2022 Verify Single-Area OSPFv2 Verify OSPF Process Information The show ip ospf command can also be R1# show ip ospf Routing Process "ospf 10" with ID 1.1.1.1 used to examine the Start time: 00:01:47.390, Time elapsed: 00:12:32.320 OSPFv2 process ID (output omitted) and router ID, as Cisco NSF helper support enabled Reference bandwidth unit is 10000 mbps shown in the Area BACKBONE(0) command output. Number of interfaces in this area is 3 This command Area has no authentication SPF algorithm last executed 00:11:31.231 ago displays the OSPFv2 SPF algorithm executed 4 times area information and Area ranges are the last time the SPF Number of LSA 3. Checksum Sum 0x00E77E Number of opaque link LSA 0. Checksum Sum 0x000000 algorithm was Number of DCbitless LSA 0 Number of indication LSA 0 executed. Number of DoNotAge LSA 0 Flood list length 0 R1# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 63 63 Verify Single-Area OSPFv2 Verify OSPF Interface Settings The show ip ospf interface command provides a detailed list for every OSPFv2-enabled interface. Specify an interface to display the settings of just that interface. This command shows the process ID, the local router ID, the type of network, OSPF cost, DR and BDR information on multiaccess links (not shown), and adjacent neighbors. R1# show ip ospf interface GigabitEthernet 0/0/0 GigabitEthernet0/0/0 is up, line protocol is up Internet Address 10.1.1.5/30, Area 0, Attached via Interface Enable Process ID 10, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 10 Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 2.2.2.2 Suppress hello for 0 neighbor(s) R1# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 64 64 32  17/08/2022 Verify Single-Area OSPFv2 Verify OSPF Interface Settings (Cont.) To get a quick summary of OSPFv2-enabled interfaces, use the show ip ospf interface brief command, as shown in the command output. This command is useful for seeing important information including: Interfaces are participating in OSPF Networks that are being advertised (IP Address/Mask) Cost of each link Network state Number of neighbors on each link R1# show ip ospf interface brief Interface PID Area IP Address/Mask Cost State Nbrs F/C Lo0 10 0 10.10.1.1/24 10 P2P 0/0 Gi0/0/1 10 0 10.1.1.14/30 30 P2P 1/1 Gi0/0/0 10 0 10.1.1.5/30 10 P2P 1/1 R1# © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 65 65 Verify Single-Area OSPFv2 Packet Tracer - Verify Single-Area OSPFv2 In this Packet Tracer, you will complete the following: Identify and verify the status of OSPF neighbors. Determine how the routes are being learned in the network. Explain how the neighbor state is determined. Examine the settings for the OSPF process ID. Add a new LAN into an existing OSPF network and verify connectivity. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 66 66 33  17/08/2022 2.7 Module Practice and Quiz © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 67 67 Module Practice and Quiz Packet Tracer - Single-Area OSPFv2 Configuration In this Packet Tracer, you will complete the following: Implement single-area OSPFv2 in both point-to-point and broadcast multiaccess networks. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 68 68 34  17/08/2022 Module Practice and Quiz Lab - Single-Area OSPFv2 Configuration In this lab, you will complete the following objectives: Build the network and configure basic device settings Configure and verify single-area OSPFv2 for basic operation Optimize and verify the single-area OSPFv2 configuration © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 69 69 Module Practice and Quiz What Did I Learn In This Module? OSPFv2 is enabled using the router ospf process-id global configuration mode command. The process-id value represents a number between 1 and 65,535 and is selected by the network administrator. An OSPF router ID is a 32-bit value, represented as an IPv4 address. The router ID is used by an OSPF-enabled router to synchronize OSPF databases and participate in the election of the DR and BDR. Cisco routers derive the router ID based on one of three criteria, in this order: 1) Router ID is explicitly configured using the OSPF router-id rid router configuration mode command, 2) the router chooses the highest IPv4 address of any of configured loopback interfaces or 3) the router chooses the highest active IPv4 address of any of its physical interfaces. The basic syntax for the network command is network network-address wildcard-mask area area- id. Any interfaces on a router that match the network address in the network command can send and receive OSPF packets. When configuring single-area OSPFv2, the network command must be configured with the same area-id value on all routers. The wildcard mask is typically the inverse of the subnet mask configured on that interface, but could also be a quad zero wildcard mask, which would specify the exact interface. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 70 70 35  17/08/2022 Module Practice and Quiz What Did I Learn In This Module? (Cont.) To configure OSPF directly on the interface, use the ip ospf interface configuration mode command. The syntax is ip ospf process-id area area-id. Use the passive-interface router configuration mode command to stop transmitting routing messages through a router interface, but still allow that network to be advertised to other routers. The DR/ BDR election process is unnecessary as there can only be two routers on the point-to- point network between R1 and R2. Use the interface configuration command ip ospf network point-to-point on all interfaces where you want to disable the DR/BDR election process. By default, loopback interfaces are advertised as /32 host routes. To simulate a real LAN, the Loopback 0 interface is configured as a point-to-point network. OSPF Network Types The DR is responsible for collecting and distributing LSAs. The DR uses the multicast IPv4 address 224.0.0.5 which is meant for all OSPF routers. If the DR stops producing Hello packets, the BDR promotes itself and assumes the role of DR. All other routers become a DROTHER. DROTHERs use the multiaccess address 224.0.0.6 (all designated routers) to send OSPF packets to the DR and BDR. Only the DR and BDR listen for 224.0.0.6. To verify the roles of the OSPFv2 router, use the show ip ospf interface command. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 71 71 Module Practice and Quiz What Did I Learn In This Module? (Cont.) To verify the OSPFv2 adjacencies, use the show ip ospf neighbor command. The state of neighbors in multiaccess networks can be: FULL/DROTHER, FULL/DR. FULL/BDR, or 2- WAY/DROTHER. The OSPF DR and BDR election decision is based on the router with the highest interface priority as the DR. The router with the second highest interface priority is elected as the BDR. If the interface priorities are equal, then the router with the highest router ID is elected the DR. The router with the second highest router ID is the BDR. The interface priority can be configured to be any number between 0 – 255. If the interface priority value is set to 0, that interface cannot be elected as DR nor BDR. The default priority of multiaccess broadcast interfaces is 1. OSPF DR and BDR elections are not pre-emptive. If the DR fails, the BDR is automatically promoted to DR. To set the priority of an interface, use the command ip ospf priority value, where value is 0 to 255. If the value is 0, the router will not become a DR or BDR. If the value is 1 to 255, then the router with the higher priority value will more likely become the DR or BDR on the interface. OSPF uses cost as a metric. A lower cost indicates a better path than a higher cost. The formula used to calculate the OSPF cost is: Cost = reference bandwidth / interface bandwidth. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 72 72 36  17/08/2022 Module Practice and Quiz What Did I Learn In This Module? (Cont.) Because the OSPF cost value must be an integer, FastEthernet, Gigabit Ethernet, and 10 GigE interfaces share the same cost. To correct this situation, you can adjust the reference bandwidth with the auto-cost reference-bandwidth command on each OSPF router, or manually set the OSPF cost value with the ip ospf cost command. The cost of an OSPF route is the accumulated value from one router to the destination network. OSPF cost values can be manipulated to influence the route chosen by OSPF. To change the cost value report by the local OSPF router to other OSPF routers, use the interface configuration command ip ospf cost value. If the Dead interval expires before the routers receive a Hello packet, OSPF removes that neighbor from its link-state database (LSDB). The router floods the LSDB with information about the down neighbor out all OSPF-enabled interfaces. Cisco uses a default of 4 times the Hello interval or 40 seconds on multiaccess and point-to-point networks. To verify the OSPFv2 interface intervals, use the show ip ospf interface command. OSPFv2 Hello and Dead intervals can be modified manually using the following interface configuration mode commands: ip ospf hello-interval and ip ospf dead-interval. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 73 73 Module Practice and Quiz What Did I Learn In This Module? (Cont.) In OSPF terminology, the router located between an OSPF routing domain and a non-OSPF network is called the ASBR. To propagate a default route, the ASBR must be configured with a default static route using the ip route 0.0.0.0 0.0.0.0 [next-hop-address | exit-intf] command, and the default-information originate router configuration command. Verify the default route settings on the ASBR using the show ip route command. Additional commands for determining that OSPF is operating as expected include: show ip ospf neighbor, show ip protocols, show ip ospf, and show ip ospf interface. Use the show ip ospf neighbor command to verify that the router has formed an adjacency with its neighboring routers. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 74 74 37  17/08/2022 Module 2: Single-Area OSPFv2 Configuration New Terms and Commands router ospf process-id clear ip ospf process router ID metric router-id rid cost show ip protocols auto-cost reference-bandwidth Mbps show ip ospf ip ospf cost value network network-address wildcard-mask area area-id ip ospf hello-interval value ip ospf process-id area area-id ip ospf dead-interval value passive interface default-information originate passive-interface intf-id show ip ospf interface intf-id ip ospf network point-to-point host route designated router (DR) backup designated router (BDR) DROTHER show ip ospf neighbor ip ospf priority © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 75 75 76 38