BGP and BGP4+ Introduction
BGP and BGP4+ Synchronization
IGP synchronisation is a feature that is specifically required in the case where routing information is ‘transiting’ through an AS. That is, when:
■A BGP router on one side of the AS learns routes from external ASs by eBGP.
■This router passes the routes across the AS by iBGP to a BGP router on the other side of the network (passing through several IGP routers on the way, which simply pass on the iBGP updates).
■The BGP router on the other side of the network then sends the routes by eBGP to other external ASs.
The problem that can arise in this situation comes about because the iBGP session effectively tunnels route information across the AS, but packets destined to any of those routes have to cross the AS hop by hop.
If iBGP tunnels route information across the AS faster than the AS's IGP can pass the route information to all routers in between, then we end up with a situation where the BGP routers (and other ASs) know about routes that some of the IGP routers in the middle of the transit AS have not learnt yet. So, packets that are routed from an external AS into the transit AS may end up being dropped or misdirected somewhere in the middle of the AS when they arrive at an IGP router that has not yet learnt all the transiting routes.
To avoid this problem, we need to tell iBGP routers that even if they receive routes via the iBGP ‘express tunnel’, they must wait until the routes have also been received via the more pedestrian iBGP
This process of forcing a router to wait until a route has been learnt by IGP as well as iBGP is call IGP synchronisation. In other words, the routes that the router has learnt by iBGP must match up with the routes that it has learnt by IGP. Once a route has achieved this
IGP synchronization is disabled by default. It does not need to be enabled in an AS where there are no iBGP connections, or in an AS where all the internal route advertising is done by iBGP (i.e., when there is no other IGP in use in the AS).
To enable BGP synchronization for IPv4 routes, use the command below in the
Router Configuration mode:
awplus(config-router)#
synchronization This command enables synchronization between BGP and an IGP for IPv4 routes in Router Configuration mode.
To enable BGP4+ synchronization for IPv6 routes, use the command below in the
Address Family Configuration mode:
awplus(config-router-af)#
synchronization This command enables synchronization between BGP4+ and an IGP for IPv6 routes in Address Family Configuration mode.
| Software Reference Supplement for SwitchBlade® x8112, x908, x900 and x610 Series Switches |
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1.26 | AlliedWare PlusTM Operating System - Software Version |
