Need Help with BGP

[LovejitSingh1313]

Hello Experts @Georg Pauwen  @balaji.bandi  @paul driver  @Richard Burts  @MHM Cisco World 

I have two sites where underlying ospf is configured and then BGP is configured.  Site A is advertising 189.75.62.x/24 to BGP and Site B is advertising 189.75.61.0/24 to BGP and its working fine. 

Site A is going to decommission so i want to use subnet 189.75.62.0/24 at site B . I tried following things

1. On site A, removed command network 189.75.62.x/24 and added same command at site B and refresh the BGP topology but it stopped working and subnet 189.75.62.x was totally unreachable.

Please let me know how should i approach this issue?

I also need to help with commands to get following results:

1. BGP command to show neighborship 

2. BGP command to show advertise routes

3. BGP command  to show learning routes 

Thanks 

Thanks

[Giuseppe Larosa · Accepted solution]

Hello @LovejitSingh1313 ,

modern BGP implementations default to

no auto-summary

this means that an exact match for the prefix needs to be found in the IP routing table in order for BGP to be able to advertise it.

>> If this is the case either use the aggregate-address command in BGP or create a static route to null 0 with high AD in order to have the prefix advertised as a /24.

You have found the following:

>> ip route vrf INTERNET 189.75.62.0 255.255.255.0 Null0 254 permanent

This means the prefix is actually subnetted in more specific subnets and the static route above is one of the two methods to build an aggregate route that can be advertised using network 189.75.62.0 255.255.255.0.

The other one if you are interested is to use the BGP aggregate-address command

router bgp <YourASN>

address-family ipv4 vrf INTERNET

aggregate-address 189.75.62.0 255.255.255.0

However, the aggregate-address would disappear if all the component subnets are removed from BGP table.

Here the static to Null0 with an high AD is used to create a permanent stable BGP advertisement.

The use of the keyword permanent in this case being a route to null0 that always exists can be considered not necessary.

Hope to help

Giuseppe