How to advertise subnet between routers thru BGP?

[Robert Milanovich]

Lets say I have 3 routers named A, B, and C where routers A and C have one serial and one Ethernet. Router B has one Ethernet and 2 serials. A is connected to B via serial and then B is connected by the other serial interface to C. Each router also has its own AS defined in BGP with its local Ethernet network being advertised.

If each router has its own distinct Ethernet subnet advertised via BGP, what's the proper way to advertise the subnets in BETWEEN the routers? For example, if I am on the Ethernet segment of router C and I want to ping/telnet to the serial of router A... normally this would not be possible since router A would only advertise its ethernet segment. Would A advertise this segment or would B do it since they are both locally attached?

[mhussein]

I think it is possible to advertise host routes for each serial ip. I don't think it is possible to advertise the whole subnet that exists on serial links between the routers.

To advertise a host route for a serial interface ip, try:

router bgp nnnnn

network x.x.x.x 255.255.255.255 <----"x.x.x.x is the serial interface ip, /32 host mask"

!

ip route x.x.x.x 255.255.255.255 serialx/y/z

I am not sure what the expected behaviour is if you try to advertise a whole in-between-subnet via one router.

If you find this helpful, then please share your findings!

Regards,

Mustafa

[Harold Ritter · Accepted solution]

You should also be able to advertise the subnet if you'd like. If it is a /30 you would do the following:

router bgp

network x.x.x.x mask 255.255.255.252

Of course the opposite router sharing that link would receive the update but wouldn't install it in the RIB since it already has a connected route for this same subnet but the router not sharing that link would.

Hope this helps,

[Robert Milanovich]

So all that needs to be done is to pick one of the routers that's connected and to the /30 subnet and have it advertise the subnet. It seems like it wouldn't matter which one is used. Is this correct?

Thanks!

Rob

[Harold Ritter]

You are correct indeed.