iBGP route reflector affecting eBGP updates

[flumburt31]

Hello all, thanks for reading / replying.

I have taken over managing an ISP's Layer 3, and it needs some work to say the least. They are using BGP for both their eBGP (obviously) and IGP. My problem has to do with Route Reflection and our Dual Homeing eBGP peers.

Here is a diagram for reference: http://www.lucidchart.com/documents/view/4da4b2cc-edb4-43e8-a656-0d550ac1d2e5

We have two routers connected to different eBGP peers, both receiving full routes. Both routers have an iBGP session up with each other. The problem is that Router A is a route reflector client of Router B, but the routes received from Router A's eBGP neighbor have a heavier weight than those received from Router B.

Now Router B is bringing up a new eBGP peer, advertising only three subnets both ways. Router B has a heavier weight set on the routes received from the new eBGP peer, and so properly updates it's eBGP routing table to it's provider. However, since Router A prefers the routes of it's eBGP peer to those that were learned from Router B, it does not update it's eBGP routing table, and therefore does not advertise these routes to it's provider.

Last night I tried setting up an statement on Router A: every route learned from Router B that matches these three subnets, place a heavier weight on them than that of it's own eBGP provider.

This caused some issues, when the session was cleared with a soft inbound update, ALL routes from Router B took presidence over it's own eBGP peer; all routes went out Router B. yikes.

Can anyone recommend a method for Router A to prefer these three new subnets from Router B's new peer over it's own eBGP peer?

Here is what I used:

neighbor x.x.251.2 remote-as (LOCAL_AS)

neighbor x.x.251.2 soft-reconfiguration inbound

neighbor x.x.251.2 route-map set.ROUTERB.routes.weightin in

access-list 75 permit x.x.188.0 0.0.0.255 log

access-list 75 permit x.x.184.0 0.0.0.255 log

access-list 75 permit x.x.192.0 0.0.0.255 log

route-map set.ROUTERB.routes.weightin permit 10

match ip address 75

set metric 0

set local-preference 201

set weight 201

!

route-map set.ROUTERB.routes.weightin permit 20

set local-preference 50

set weight 50

[milan.kulik · Accepted solution]

Hi,

a) Route reflector configuration is irelavant here, as it has an influence only on prefixes received from an iBGP neighbor to be advertised to anothor iBGP neighbor which is not this case,

b) It seems your statement  "Router A has a weight of 35000 for all routes received from eBGP peer"

is not working.

If it were,

set weight 201

would not help, as 35000 would still beat it.

So it seems like the deafult weight=0 is applied on the routes coming from the eBGP neighbor.

That explains why

set weight 50

made all prefixes recived from B to be used.

HTH,

Milan

[flumburt31]

Thanks for the reply