Friday, May 17, 2013


Advertising / Installing routes in BGP Table

Preconfiguration : 1. Whatever has been configured until the last post (i.e. 1.5 BGP NEXT HOP SELF)
2. Create three loopbacks : Loopback 1, Loopback 2, Loopback 3 on R6 which will be advertised to the BGP routers using the " network " command.

Synopsis :
You have already seen one way of advertising routes into BGP process. What is that ??? It's via redistribution of the IGP routes into BGP.
This method is great but it has some inherent caveats.
1. All the routes in the routing table learned by a particular IGP will be redistributed into BGP.
Now most of the enterprises may not want to advertise their entire network to the ISP,.
2. The above problem can be surmounted by the filtering, by means of route-maps. However, greater the enterprise network, more will be the complexity of the route-map filtering procedures. This can definitely add to the overhead to the router's resources

BGP "network" command enables you to advertise only those networks which you actually need to advertise to the ISP.

R6 configuration

R6(config)#router bgp 400

R6(config-router)#network mask

R6(config-router)#network mask

R6(config-router)#network mask



*May 15 13:00:08.856: %SYS-5-CONFIG_I: Configured from console by console


The R6's output will be as follows :

Fig 1.6.1
  See the Next Hop. The next hop value of means that the routes have been advertised to the BGP process on this router itself i.e. R6.

Similarly, the Weight of 32768 means these routes have been locally generated / advertised into BGP on this router itself.

The " i " value under " Path " column means the BGP route has been generated by the " network " command.
It would have been " ? " in case the BGP route is generated via " redistribution ".

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