Tuesday, September 7, 2010

BSCI - BGP - Synchronisation

Synchronisation rule states that a BGP router can not advertise an external neighbor destination from iBGP peers unless that route is also known via an IGP (such as EIGRP, OSPF, RIPv2 etc.)

The thinking behind this is that in the event that a router along the path to the destination is not running BGP then you don't end up with a black hole with packets getting dropped. The IGP has a route the the destination so a path will still exist.

By default, Synchronisation is switched off in Cisco IOS an and there fore BGP can advertise a route without it first being advertised by an IGP.

There are 2 situations when you can safely switch off synchronisation:

  • When you have a fully meshed iBGP topology - resulting in the destination being reached with the need of an IGP
  • When the AS is NOT an transit AS - where all destination networks are within the AS and accessible due to you having a full mesh iBGP topology.
Configuration:
R1(config)#router bgp 123
R1(config-router)#synchronization
!
!This turns on synchronisation (which is disabled by default)

R1(config)#router bgp 123
R1(config-router)#no synchronization
!
!This turns off synchronisation

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