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| 9.3 |  | Configuring BGP |
| 9.3.4 |  | EBGP multihop |

These example configurations have shown how to configure EBGP and IBGP. However, what is the difference between the configuration types? Figure presents some of the important characteristics between these BGP session types.

RTZ and RTY have established an EBGP session. EBGP peers are normally directly connected, but there are certain exceptions to this requirement. In contrast, IBGP peers merely require TCP/IP connectivity within the same AS. As long as RTY can communicate with RTW using TCP, both routers can establish an IBGP session. If needed, an IGP such as OSPF can provide IBGP peers with routes to each other.

In a typical configuration, an IBGP router maintains IBGP sessions with all other IBGP routers in the AS, forming a logical full mesh. This is necessary because IBGP routers do not advertise routes learned by way of IBGP to other IBGP peers, to prevent routing loops. In other words, if the IBGP routers are to exchange BGP routes with each other, configure a full mesh. An alternative to this approach is configuring a route reflector. Information on route reflectors can be obtained from the Cisco web site.

As noted, EBGP neighbors must be directly connected to establish an EBGP session. However, look again at RTW and RTU in Figure . These routers can maintain an EBGP session even though a non-BGP router, RTV, separates them. In this situation, EBGP is running across a non-BGP router using a configurable option called EBGP multihop. EBGP multihop is a Cisco IOS option that allows RTW and RTU to be logically connected in an EBGP session. This is despite the fact that RTV does not support BGP. The EBGP multihop option is configured on each peer with the following command:

> Router(config-router)#neighbor ip-address ebgp-multihop [hops]

This command enables the specification of how many hops, up to 255, separate the EBGP peers. The following commands could be applied to the routers in the example:

> RTW(config)#router bgp 200
> RTW(config-router)#neighbor 1.1.1.2 remote-as 300
> RTW(config-router)#neighbor 1.1.1.2 ebgp-multihop 2
> RTU(config)#router bgp 300
> RTU(config-router)#neighbor 2.2.2.1 remote-as 200
> RTU(config-router)#neighbor 2.2.2.1 ebgp-multihop 2

In general, EBGP multihop is designed to allow the development of economical AS edge router solutions. A single router with sufficient RAM and CPU power to support BGP is used to handle the BGP routing needs. This router does not have to be an expensive modular, chassis based system. EBGP multihop allows inexpensive edge routers to provide sufficient WAN interfaces for the autonomous systems connectivity needs.

| Lab Activity e-Lab Activity: EBGP Multihop This lab is to configure EBGP with multihop allows inexpensive edge routers to provide sufficient WAN interfaces for the autonomous systems connectivity needs.