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| 9.3 |  | Configuring BGP |
| 9.3.6 |  | Peering |

Any two routers that have formed a TCP connection in order to exchange BGP routing information are called peers or neighbors.

Figure demonstrates the different types of BGP peering sessions that will be encountered. An IBGP peering session is formed within AS 3, between the loopback address of RTA and a physical address of RTF. An EBGP session is also formed between AS 3 and AS 1 by using the two directly connected IP addresses of RTA and RTC. Another EBGP session is formed between RTF in AS 3 and RTD in AS 2, using IP addresses that are not on the same segment, multihop.

It is important to remember that the BGP peers will never become established unless there is IP connectivity between the two peers. In this example, OSPF is used to establish the required internal connectivity between RTD and RTE.

> Note: Click on the topology in Figure to view command outputs.

In the configuration from RTF, the

ebgp-multihop 2 command is seen being used as part of the neighbor configuration. This is an indication that the exterior BGP peer is not directly connected and can be reached at a maximum of two hops away. Remember that EBGP multihop is applicable only with EBGP, and not with IBGP.

The example also shows how the peer connection will look after the neighbors are in an established state. RTF sees neighbor 172.16.2.254 as an internal neighbor that belongs to AS3. The neighbor connection is running BGP Version 4 with a table version of two (2). The table version changes every time the BGP table is updated.

The other RTF neighbor, 192.68.12.1, is also in an Established state. This is an external neighbor that belongs to AS2. Notice that the display indicates that this neighbor is two hops away and is configured using the

ebgp-multihop command.