BGP Backup with Static Routes Cont

Provider Configuration

/

Customer Customer Router Primary tShz

Customer Backup

11.2.3.0/24 Customer Network

y

Provider Provider Primary Router

X Provider ' Backup

AS 387

Service Provider Network

ip route 11.2.3.0 255.255.255.0 serial 0/0 tag 1000 250

router bgp 387 redistribute static route-map IntoBGP

ip route 11.2.3.0 255.255.255.0 serial 0/0 tag 1000 250

router bgp 387 redistribute static route-map IntoBGP

Caveat: The local BGP route Is always better than an IBGP route. The floating static route is inserted into the BGP table and should not be removed from there.

Caveat: The local BGP route Is always better than an IBGP route. The floating static route is inserted into the BGP table and should not be removed from there.

When floating static routes are configured on the provider edge routers, they are also redistributed into BGP. This configuration makes things a little bit more complicated.

The network operator configures a floating static route to the customer subnet 11.2.3.0/24. In the provider edge router, the floating static route is assigned the same tag value as the tag value being used in the primary router. The route-map IntoBGP is the same as in the primary router and provides the routes to the customer network with the same communities (the same QoS level and indication whether to explicitly announce route prefix to the rest of the Internet).

The floating static route is configured with an AD value of 250. This value is higher than any routing protocol. When the backup edge router of the ISP no longer receives any routing protocol information about the customer networks, the router will automatically install the floating static route and subsequently redistribute it into BGP.

Based on BGP route selection rules, the redistributed floating static route will always remain the preferred path if additional BGP configuration is not performed on the provider edge router. This preference means that regardless of whether the primary link comes back, the backup router selects the locally sourced route as the best route. Therefore, the backup router continues to announce a path toward the customer network. The backup link does not go back to the Idle state.

5-36 Configuring BGP on Cisco Routers (BGP) v3.2 © 2005, Cisco Systems, Inc.

In this example, the show ip bgp command is used in the backup edge router of the provider to display the information about the customer network 11.2.3.0/24. The primary link has come back, so the backup router now sees two alternate routes. The first route is the route that the router itself has redistributed into BGP using the floating static route. This route is locally sourced by this AS and has been assigned a weight value of 32768. The second route is the one that has been received by IBGP from the primary edge router. This AS also sources this route, but no weight value is assigned.

The BGP route selection algorithm selects the route with weight value 32768 as the best. As a result, the route that was received from the primary edge router is not a candidate to be installed in the routing table and never competes with the floating static route. The floating static route stays in the routing table, and redistribution of the route continues until the backup link goes down and the route becomes invalid.

© 2005, Cisco Systems, Inc. Customer-to-Provider Connectivity with BGP 5-37

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