Cisco BGP Advertised Routes

[Quoc Le]

Hello Community,

I have a question on BGP advertised routes. I have 2 routers peered together same AS 1 both advertising to the AS of 65000. When issuing the command "IP BGP neighbor x.x.x.x advertised-routes" :

Router 1 states:

*> 10.20.30.128/27 0.0.0.0 0 32768 i

Router 2 states:

*>i 10.20.30.128/27 192.168.203.50 0 100 0 i

The router with AS of 65000 is Juniper Router preferring Router 2. Router 1 is using the network statement to advertise 10.20.30.128/27 into BGP. Router 2 is learning vip iBGP peering. Shouldn't the Juniper Router prefer Router 1 since its being originated from there?

Thanks.

[milan.kulik]

Hi,

as you can read here http://www.cisco.com/c/en/us/td/docs/ios/iproute_bgp/command/reference/irg_book/irg_bgp3.html#wp1108213

"By default, for eBGP, the next hop to reach a connected network is the IP address of the neighbor that sent the update."

So from the Juniper router point of view both routers are sending the eBGP prefix with the same attributes (MED = 0, AS_PATH = 1) and each router is advertising its IP address as the next-hop.

So the best path choosing is based on the Juniper router configuration and Juniper best path selection algorithm described here:

http://www.juniper.net/documentation/en_US/junos13.3/topics/reference/general/routing-ptotocols-address-representation.html

You might look at the prefixes received on the Juniper router and check why the Router 2 was chosen.

Best regards,

Milan