BGP routes question

The_guroo_2 · ‎07-10-2011

guys a basic BGP question

we have two datacentres and we have a client (cleint and two data centres are conected via private MPLS link) now both data centres are conected with EBG with different private AS number

so the scenario is

Data centre A As 1

Datacentre B As 2

Clinet As 3

datacentre B has as path prepend configured

both data centre A and B are advertising same route to the client (to PE)

for example 192.168.1.0/24

but when i check on customer router i can only see 192.168.1.0/24 route from daatcentre A

data centre B is in different AS .....i am thinking that i shd see the route 192.168.1.0/24 on cutomer router from both AS 1 and 2 and As 1 shd be the prefered as AS 2 as prepend

Thanks guys for looking

milan.kulik · ‎07-11-2011

Hi,

if I understand your description correctly, there's an MPLS backbone between your BGP routers, i.e., the client is peering via BGP to a PE router?

In that case the PE router is advertising only the best BGP path for the 192.168.1.0/24 subnet to the client (BGP neighbor).

So you would see both paths on the PE router but not on the client router.

HTH,

Milan

The_guroo_2 · ‎07-11-2011

Hi Milan

You are right ....we have a private link......not internet link so all three links (2 data centres) and a client is having BGP with the PE router....in out case three PE......

but as far as i rem in old days i have seen (not in our case but from old job) but BGP we use to have two routes and BGP show the best route to the destination is it somehting change now

ashok_boin · ‎07-11-2011

Hi,

That's true when you are running eBGP peerings with service providers where you will generally get FULL BGP routing table entries.

In specific case of utilizing PE to CE routing protocol in MPLS, PE advertises the best BGP path as per response from Milan above.

Regards..

-Ashok.

With best regards...
Ashok

milan.kulik · ‎07-12-2011

Hi,

this is a basic BGP behaviour:

As long as you peer with one BGP neighbor you will get one route only for each subnet from him.

See "Note: Only the best paths received from BGP peers are propagated." in

http://www.cisco.com/image/gif/paws/19345/bgp_noad.pdf

So you would have to peer to multiple BGP neighbors to see multiple paths to a subnet.

HTH,

Milan

shehzadtesleem · ‎07-12-2011

Hi;

have you seen the output of sh ip bgp.

HTH.