bgp issues

[carl_townshend]

Hi all

If I make changes to network statements, ie what networks my BGP router in HQ advertises out, would this cause an outage or make the BGP neighbours go down ? surely its ok add new networks without impacting the system?

also when adding a network statement into BGP does the IGP or static route have to match exactly the network statement ?

cheers

Carl                  

[milan.kulik]

Hi,

a)  changes to network statements will not make the BGP neighbours to go down.

You could cause an outage if you would remove some network statement incorrectly and your remote sites would lose connectivity to your HQ subnet.

b)  when adding a network statement into BGP there has to be an exact match (the same prefix/mask) in the routing table to get the prefix into BGP table (and advertised to the BGP neighbours).

So you need to configure a static route with an exact match or to receive a route with an exact match via IGP. The last possibility is a directly connected subnet (or loopback subnet) exactly matching.

HTH,

Milan

[jorkchristopher]

If using the static route, it would usually be configured as "ip route x.x.x.x x.x.x.x Null 0". This will allow BGP to advertise the subnet.

[smehrnia]

hi,

BGP can be configured to advertise whatever that is in the routing table, if a network is not in the routing table, bgp will not advertise it.

but u want to advertise a route to Null...?

Soroush.

[carl_townshend]

I thought bgp can advertise a route not in the routing table if you turn off synchronisation ?

[JohnTylerPearce]

There are two ways to say this.

BGP Synchroniztion Off: You can advertise a network in BGP as long as you have a route in your routing table that matches the network/prefix. You do not need to worry if your IGP has a route for this or not.

BGP Synchronization On: You can advertise a network in BGP, as long as yo uhave a route in your routing table that matched the network/prefix, and this route is included in your IGP.

[cadet alain]

Hi,

if the prefix is not installed in the BGP table it won't advertise it anyway and to install a prefix in the BGP table with a network command with auto-summary disabled then you have to have an exact match( prefix and prefix-length) between the network command and the prefix already installed in the RIB.

Regards.

Alain.

Don't forget to rate helpful posts.

[milan.kulik]

Hi,

a) BGP will nort advertise any route which is not in the routing table,

b) BGP synchronization is related to iBGP prefixes only. See

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800945ff.shtml#topic6

for details.

HTH,

Milan