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8-4 neighbor { ip-address peer- group - name } default-originate route-map route-map-nameSyntax Description:
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 AdvertisementFigure 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
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 VerificationAs 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
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