8-4 neighbor ip-address peer-group-name default-originate route-map route-map-name

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8-4 neighbor { ip-address peer- group - name } default-originate route-map route-map-name

Syntax Description:

  • ip-address Neighbor's IP address.

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

  • route-map-name Name of the route map.

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 route-map form of the neighbor command. Using this form is recommended, because the router advertises the default route only if the condition of the specified route map is satisfied. The condition that is typically used is whether or not the default network is up.

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

Configuration Example: Conditional Default Route Advertisement

Figure 8-6 shows an autonomous system that has a connection to the Internet through Router B. The network directly connected to the Internet, 10.1.2.0/30, 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 only if the default route exists.

Figure 8-6. Conditionally Advertising a Default Route

graphics/08fig06.gif

 Router A  router bgp 1   neighbor 10.1.1.2 remote-as 1   no sync   ___________________________________________________________________________  Router B  router bgp 1   neighbor 10.1.1.1 remote-as 1    neighbor 10.1.1.1 default-originate route-map exists    !    access-list 1 permit 10.1.2.0 0.0.0.3    !    route-map exists permit 10     match ip address 1   
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 default route is being advertised by Router B.

 rtrA#  show ip bgp  BGP table version is 3, local router ID is 172.17.1.1 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  *>i10.1.2.0/30      10.1.1.2                 0    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:

 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, * - candidate default        U - per-user static route, o - ODR, P - periodic downloaded static route        T - traffic engineered route  Gateway of last resort is 10.1.1.2 to network 0.0.0.0  10.0.0.0/30 is subnetted, 2 subnets B       10.1.2.0 [200/0] via 10.1.1.2 C       10.1.1.0 is directly connected, Serial0  B*   0.0.0.0/0 [200/0] via 10.1.1.2  

The default route has been successfully installed in the IP routing table of Router A. This route is installed only if network 10.1.2.0 is up. If the serial interface on Router B that is connected to network 10.1.2.0 is shut down, Router B discontinues advertising the default route.

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. Finally, check the syntax of the route map on the originating router.

  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.

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Cisco[r] BGP-4 Command and Configuration Handbook
Cisco BGP-4 Command and Configuration Handbook
ISBN: 1587055732
EAN: 2147483647
Year: 2001
Pages: 300

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