8-3 neighbor ip-address peer-group-name default-originate

8-3 neighbor { ip-address peer- group - name } default-originate

Syntax Description:

• ip-address Neighbor's IP address.

• peer-group-name Name of the peer group. See section 8-19.

Purpose: Every router should have a default route that is used to forward packets to networks that are not in the local IP routing table. One method for ensuring that every router has a default route is to configure a static route on every router to establish the default route. Another method is to create one default route and advertise this route to the BGP neighbors. The router owning the default route can advertise it through BGP using the default-originate form of the neighbor command. Using this form is not recommended, because the router always advertises the default route, even if the router does not have a default route or if the network to the default route is down.

Cisco IOS Software Release: 11.0. Extended access lists are permitted in Release 12.0.

Configuration Example 1: Single Default Route

Figure 8-4 shows an autonomous system that has a connection to the Internet from Router B. The network directly connected to the Internet is to be used as the default route for the autonomous system. A static default route could be used on every router in the AS, but this is not the preferred method. These static routes require a high degree of maintenance. If the default route on Router B changes, every static route on every router in the AS needs to be changed. The preferred method is to dynamically propagate the default route attached to Router B throughout the AS. The following configuration contains the necessary instructions to enable Router B to propagate the default route.

Figure 8-4. Single Default Route Advertisement

 Router A  router bgp 1   neighbor 10.1.1.2 remote-as 1   ___________________________________________________________________________  Router B  router bgp 1   neighbor 10.1.1.1 remote-as 1    neighbor 10.1.1.1 default-originate    !   ip route 0.0.0.0 0.0.0.0 serial 0  

Verification

As always, verify that Routers A and B have established a BGP connection before configuring the default route. After configuring the default route advertisement, check the BGP routing table on Router A to ensure that the default is being advertised using the show ip bgp command. The following output verifies that the route is being advertised by Router B:

 rtrA#  show ip bgp  BGP table version is 1, local router ID is 10.1.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete    Network          Next Hop            Metric LocPrf Weight Path  * i0.0.0.0          10.1.1.2                      100      0 i  

The preceding output verifies that Router A is receiving the default route advertisement from Router B. The next step in the verification process is to check the IP routing table of Router A to verify that the default route is being transferred via the show ip route command:

 rtrA#  show ip route  Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area        * - candidate default, U - per-user static route, o - ODR        P - periodic downloaded static route Gateway of last resort is not set 10.0.0.0/24 is subnetted, 1 subnets C       10.1.1.0 is directly connected, Serial0 

The default route has not been transferred to the IP routing table, as shown in the preceding output. The reason for this is a property of IBGP called synchronization (see section 12-1). IBGP will not install a route learned from another IBGP speaker unless the route was learned through an Interior Gateway Protocol (IGP). For this example, we will disable synchronization on Router A, as shown in the following configuration. Disable synchronization only if Router A has a route to the next hop advertised with the default route:

 Router A  router bgp 1   neighbor 10.1.1.2 remote-as 1   no synchronization  

With synchronization disabled, recheck the IP routing table on Router A:

 RtrA#  show ip route  Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area        * - candidate default, U - per-user static route, o - ODR        P - periodic downloaded static route Gateway of last resort is 10.1.1.2 to network 0.0.0.0 10.0.0.0/24 is subnetted, 1 subnets C       10.1.1.0 is directly connected, Serial0  B*   0.0.0.0/0 [200/0] via 10.1.1.2, 00:00:05  

The default route has been successfully installed in the IP routing table of Router A. This route will be installed regardless of whether Router B actually has a default route or if the network that is being used as the default route is functional. If the serial interface on Router B is shut down, Router B continues to advertise the default route. The next form of this instruction (see section 8-4) demonstrates how to conditionally advertise the default route based on the existence of a functional default network.

Configuration Example 2: Multiple Default Routes

In Figure 8-5, Router B is receiving a default route advertisement from both Routers A and C.

Figure 8-5. Multiple Default BGP Route Advertisements

 Router A  router bgp 1   neighbor 172.16.1.2 remote-as 2    neighbor 172.16.1.2 default-originate    !    ip route 0.0.0.0 0.0.0.0 serial 1    ___________________________________________________________________________  Router B  router bgp 2   neighbor 10.1.1.2 remote-as 3   neighbor 172.16.1.1 remote-as 1   !   ___________________________________________________________________________  Router C  router bgp 3   neighbor 10.1.1.1 remote-as 2    neighbor 10.1.1.1 default-originate    !    ip route 0.0.0.0 0.0.0.0 serial 1   

Verification

The default routes advertised to Router B can be examined by showing the BGP routing table:

 rtrB#  show ip bgp  BGP table version is 2, local router ID is 172.16.1.2 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete    Network          Next Hop            Metric LocPrf Weight Path *  0.0.0.0          172.16.1.1                             0 3 i *>                  10.0.0.2                               0 1 i 

Router B installs the "best" default route into the IP routing table. Which default route is the best? All things being equal, BGP selects the route advertised by the router that has the lowest router ID. In this case, Router C has the lower router ID, and the default route advertised by Router C is installed in the IP routing table, as shown. Because this example uses EBGP, we do not need to worry about synchronization.

 rtrB#  show ip route  Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR        T - traffic engineered route  Gateway of last resort is 10.1.1.2 to network 0.0.0.0  172.16.0.0/24 is subnetted, 1 subnets C       172.16.1.0 is directly connected, FastEthernet0/0      10.0.0.0/30 is subnetted, 1 subnets C       10.0.0.0 is directly connected, Serial3/0  B*   0.0.0.0/0 [20/0] via 10.1.1.2, 03:16:53  

Troubleshooting

1. Verify that the BGP neighbors are in the Established state using the show ip bgp neighbors command.

   If the neighbor relationship is not in the Established state, see section 8-23.

2. Verify that the originating router is advertising the default route using the show ip bgp command.

   If the default route is being advertised, go to Step 3. If the route is not being advertised, verify the configuration of the default-originate command on the originating router. Also ensure that no filters are blocking the default route.

3. Verify that the default route is in the IP routing table using the show ip route command.

   If the default route is not in the IP routing table, turn synchronization off if the next hop associated with the default route can be reached. This applies only to IBGP connections.