Content

| 9.6 |  | BGP Attributes |
| 9.6.3 |  | Next Hop behavior on multiaccess media |

Recall that a network link is considered multiaccess if more than two hosts can potentially connect to it. Routers on a multiaccess link share the same IP subnet and can physically access all other connected routers in one hop. Ethernet, Frame Relay, and ATM are examples of multiaccess media.

BGP speakers should always advertise the actual source of the route if the source is on the same multiaccess link as the speaker. In other words, RTC advertises a route learned from RTB, and RTC and RTB share a common multiaccess media. Then when RTC advertises the route, it should indicate RTB as being the source of the route. If not, routers on the same medium would have to make an unnecessary hop by way of RTC to get to a router that is sitting in the same segment.

In Figure , RTA, RTB, and RTC share a common multiaccess medium. RTA and RTC are running EBGP, while RTC and RTB are running OSPF. RTC has learned network 11.11.11.0/24 from RTB by way of OSPF and is advertising it to RTA by way of EBGP. Since RTA and RTB are running different routing protocols, it would seem logical that RTA would consider RTC (10.10.10.2) as its next hop to reach 11.11.11.0/24. However, this would not happen. The correct behavior is for RTA to consider RTB, 10.10.10.3, as the next hop because RTB shares the same medium with RTC.

When the media is broadcast, such as Ethernet and FDDI, physical connectivity is assumed and the next hop behavior is not an issue. However, when the media is nonbroadcast, such as Frame Relay and ATM, special care should be taken as described in the following section.

| Web Links Configuring BGP http://www.cisco.com/univercd/cc/td/doc/product/ software/ios122/122cgcr/fipr_c/ipcprt2/1cfbgp.htm