Allied Telesis BGP4 manual Verifying iBGP, Understanding Next-Hop Reachability Issues with iBGP

BGP and BGP4+ Introduction

4.Make sure each router has IP routes so that they can forward packets to the loopback interface IP address of the other router.

Verifying iBGP

iBGP neighbors use the same messages and neighbor states as eBGP peers. The same commands for BGP neighbor verification can be used to verify iBGP peers. The show ip bgp summary (BGP) command lists neighbors. The show ip bgp neighbors (BGP) and show bgp ipv6 neighbors (BGP4+) commands list many details specifically for the neighbor for BGP and BGP4+ respectively.

Understanding Next-Hop Reachability Issues with iBGP

With IGPs, the IP routes added to the IP routing table list a next-hop IP address. With few exceptions, the next-hop IP address routes exist in a connected subnet.

BGP advertises these particular IP addresses as the next-hop IP addresses because of a default behavior for BGP. By default, when a router advertises a route using eBGP, the advertising router lists its own update-source IP address as the next-hop address of the route. In other words, the next-hop IP address is the IP address of the eBGP neighbor, as listed on the neighbor remote-as (BGP and BGP4+) command. However, when advertising a route to an iBGP peer, the advertising router (by default) does not change the next-hop address.

The IP routing process can use routes whose next-hop addresses are not in connected subnets as long as each router has an IP route that matches the next-hop IP address. Two main options exist to ensure reachability to these next-hop addresses:

■Create IP routes so that each router can reach these next-hop addresses that exist in other ASNs.

■Change the default iBGP behavior with the neighbor next-hop-self (BGP and BGP4+) command.

Using neighbor next-hop-self to change Next-Hop Address

The second option for dealing with these unconnected next-hop IP addresses changes the iBGP configuration, so that a router changes the next-hop IP address on iBGP- advertised routes. This option requires the neighbor next-hop-self (BGP and BGP4+) command to be configured for the iBGP neighbor relationship.

With this command configured, the router advertises iBGP routes with its own update- source IP address as the next-hop IP address. Because the iBGP neighborship already relies on a working route for these update source IP addresses, if the neighborship is up, then IP routes already exist for these next-hop addresses.

IGP Redistribution and BGP Synchronization

Redistributing BGP routes into the IGP can solve routing loop problems. Redistributing BGP routes into the IGP prevents routing loops by giving routers the best exit point for each destination.

Redistributing eBGP-learned Internet routes into the IGP is the BGP feature started with the synchronization (BGP and BGP4+) command, where iBGP learned routes must be synchronized with IGP-learned routes for the same prefix before they can be used. If an iBGP learned route is to be considered to be a usable route, then that same prefix must be in the IP routing table and learned locally or via an IGP, such as OSPF.

	Software Reference Supplement for SwitchBlade® x8112, x908, x900 and x610 Series Switches
1.10	AlliedWare PlusTM Operating System - Software Version 5.4.3-2.6	C613-50032-01 REV D