BGP Backup with Static Routes

This example illustrates a case where the customer network has two connections to a single service provider. One connection between the customer network and the ISP is the primary connection, and the other connection is used for backup purposes only. If link-level procedures can detect link failures and a failure in the remote router, then static routing can be used instead of a dynamic routing protocol between the customer and provider networks.

As in the previous example, where no backup link is available, the primary edge router of the customer has a static default route toward the ISP and the primary edge router of the ISP has static routes toward the customer. The customer router redistributes the static default route into its IGP. The ISP router redistributes the static routes into BGP.

If the primary link goes down, the link-level procedures set the interface to the down state, causing the static routes pointing out through the interface to be invalid, and removing the routes from the routing table. When the interface changes back to the up state, the static route will reappear in the routing table.

Redistribution of routes into any routing protocol is conditioned by the appearance of the route in the routing table. Thus, if the interface goes down, the router removes the static route from its routing table, and the route is withdrawn from the routing protocol. When the static route reappears, the redistribution process inserts it into the routing protocol again.

Copyright © 2004, Cisco Systems, Inc. Customer-to-Provider Connectivity with BGP

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The backup edge router of the customer also uses a default static route toward the ISP, via the backup link. The backup edge router is also redistributing the default route into the IGP. However, the static route that is used is a floating static route, which is assigned a high administrative distance (AD), higher than the AD of the customer IGP. As long as the primary link works, the IGP provides the customer backup edge router with the primary default route. Because of the higher AD, the backup static default route is not installed into the backup router routing table. Because the static route is not in the routing table, it is not redistributed. If the primary link fails, the IGP no longer feeds the backup edge router with a default route. The backup static default route is the only remaining default route. Therefore, the router will install the floating default route into its routing table, and subsequently redistribute it into the IGP.

The backup edge router of the ISP can also use floating static routes, which are redistributed into the ISP BGP process.

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The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

BGP Backup with Static Routes (Cont.)

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Bgp Main And Backup Isp

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In the figure, the customer network and the ISP network are connected using leased lines with High-Level Data Link Control (HDLC) encapsulation. Both the primary and the backup edge routers in the customer network have a static default route toward the serial interface leading to the ISP. Both routers also do redistribution of the default route into the OSPF protocol, which is being used as an IGP within the customer network.

However, the static default route in the backup edge router is configured with an AD value set to 250. This AD value is higher than the AD values of any routing protocol. This configuration means that as long as the backup router receives the default route by OSPF, the static default route is not used.

When the primary link goes down, the static default route in the primary router is not valid. The OSPF protocol stops announcing the default route, because the default-information originate command makes OSPF contingent on the availability of that static default route in the routing table before announcement.

The backup router now installs its static default route in the routing table. The conditions for announcing the default route by OSPF are met and the rest of the customer routers see the backup router as the gateway of last resort.

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The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

BGP Backup with Static Routes (Cont.)

BGP Backup with Static Routes (Cont.)

© 2004 Cisco Systems, Ii

© 2004 Cisco Systems, Ii

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) v3.1 Copyright © 2004, Cisco Systems, Inc.

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

BGP Backup with Static Routes (Cont.)

• The BGP table on service provider backup router contains the floating static route.

BGP routing table entry for 11.2.3.0/24, version 7 Paths: (2 available, best #1, not advertised to EBGP peer) Advertised to non peer-group peers: 10.3.0.5 Local

Origin incomplete, metric 0, localpref 100, Height 32768, valid, SOUrced, best

Community: 387:31000 no-export Local

Origin incomplete, metric 0, localpref 100, valid, internal Originator: 1.0.0.2, Cluster list: 10.3.D.S Community: 387:31000 no-export

o1

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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.

Copyright © 2004, Cisco Systems, Inc. Customer-to-Provider Connectivity with BGP 5-37

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

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