Load Sharing with EBGP Multihop

Bgp Load Sharing Cisco

Because of recursive lookup, load sharing toward a BGP destination always occurs if there are several equal-cost IGP paths to the BGP next hop.

Equal-cost IGP paths are easily generated if the BGP next hop is not directly connected.

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When two adjacent routers have multiple links between them, you can configure the EBGP session from loopback interface to loopback interface. In this case, you must use the ebgp-multihop option to make the BGP session go into the active state. There must be static or dynamic routing in use to provide both routers with information on how to reach the loopback interfaces of each other. Otherwise, their EBGP session does not complete establishment.

Routing to the loopback interface of the neighboring router is required to establish the EBGP session and is also used in the recursive lookup when the routes are installed by the router in its routing table. The two routes to the loopback interface of the neighboring router should be equivalent for load sharing to occur.

After configuration, one single EBGP session is established between the two routers. This session is used to exchange the routing information. There is only one BGP route to each destination, and it has a next hop that refers to the loopback interface of the other router.

Before installing a route to a specific destination in its routing table, a router will perform a recursive lookup to resolve the next hop. In this case, the recursive lookup will result in finding two alternative routes. The router will install the BGP route to the final destination twice in the routing table (Forwarding Information Base [FIB]). The first time, the route is installed with one of the resolved next-hop addresses, and the second time with the other resolved next-hop address. Because multiple equal-cost paths exist, the router can load-share over the two paths, depending on the switching mode.

Copyright © 2004, Cisco Systems, Inc. Customer-to-Provider Connectivity with BGP 5-65

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.

Cisco.com router(config-router)#

neighbor ip-address ebgp-multihop [ TTL ]

• By default, EBGP neighbors must be directly connected.

• The ebgp-multihop command declares an EBGP neighbor to be distant (several hops away).

• Number of hops can be specified in the TTL parameter.

• Usually used to run EBGP between loopback interfaces for dial backup or load-sharing purposes.

• Use with extreme caution; routing loops can occur very easily.

© 2004 Cisco Systems, Inc. All rights reserved. BGP v3.1—5-18

By default, EBGP neighbors must be directly connected. Cisco IOS software verifies that an EBGP neighbor is reachable as directly connected over one of the router interfaces before the session goes into the active state. For an EBGP session, IP packets that carry the TCP segments with BGP information are also sent using a Time to Live (TTL) value set to the value 1. This value means that they cannot be routed.

The ebgp-multihop neighbor configuration command changes this behavior. Although the neighbor is several hops away, the session goes into the active state, and packets start to be exchanged. The TTL value of the IP packets is set to a value larger than 1. If no value is specified on the command line, 255 is used.

Use the ebgp-multihop command when you are establishing EBGP sessions between loopback interfaces for load-sharing purposes. You must take great care when using ebgp-multihop, because proper packet forwarding relies on all the intermediate routers along the path to the EBGP peer to make the correct forwarding decision. If the intermediate routers have a correct path to the EBGP peer, but a wrong path to the final destination, the packet may get into a routing loop.

5-66 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.

Load Sharing with EBGP Multihop (Cont.)

© 2004 Cisco Systems,

© 2004 Cisco Systems,

In the figure, the customer network and the ISP network are connected using two parallel links between a single router on the customer side and a single router on the ISP side.

In this case, only one EBGP session is configured between the customer and provider routers. The session should be established from the loopback interface in one router to the loopback interface in the other.

Each of the two edge routers has two static host routes that point to the loopback interface on the other router. The EBGP session is established from loopback to loopback using ebgp-multihop.

The customer receives an EBGP route from the ISP with the next hop set to 1.0.0.1. The customer edge router performs a recursive lookup and finds that it can reach 1.0.0.1 via 2.0.0.1 and via 2.0.0.5. These two routes are equivalent. Therefore, the route to the final destination is installed in the routing table of the customer router using both paths.

Depending on the switching mode in use, load sharing is done per packet, per destination, or per source and destination pair.

In this example, link-level procedures ensure that if one of the links goes down, the corresponding static link goes down. All BGP routes in the routing table that rely on the static route to the link that went down are invalidated. However, the BGP routes in the routing table that rely on the remaining link are still valid and used.

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

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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