eBGP multihome question

Answered Question
Sep 25th, 2009

Hi Everyone.

I have a question, I have 2 routers R1 in building #1, R2 in building #2 each having a different ISP with full routes. there is also a link between each router for traffic between the building and each building has their own /24.

what I would like to do is load balance the ebgp sessions and have backup incase one fails.

from what I have read I need to do the following:

1) a iBGP session between the router

2) include both network in both routers BGP config

3) prepend the network from the other building.

example

BGP in building A

router bgp 1111

network A.A.A.A

network B.B.B.B

prepend network B.B.B.B

BGP in building B

router bgp 1111

network A.A.A.A

network B.B.B.B

prepend network A.A.A.A

Thank you in advance

Correct Answer by mszeftawy about 7 years 5 months ago

Hi hcintron

One more point should be taken in consecration.

You have to create proper filtering for the routes sent from your routers, only advertise your range of addresses.

I mean don't rely on your service provider to filter the routes that came from your network, in case one of your service providers is not performing the proper filtering you will advertise provider A subnets to provider B and vise versa, which might make them use your network as a transit AS for the intra-provider traffic.

Correct Answer by Giuseppe Larosa about 7 years 5 months ago

Hello Hector,

your action plan looks like correct.

only point that if you want to perform selective prepending you need to use a route-map

access-list 11 permit B.B.B.B

route-map sel-prepend permit 10

match ip address 11

set as-path prepend yourASN yourASN yourASN

R1

router bgp yourASN

neigh isp.ipaddr route-map sel-prepend out

the same has to be done on R2

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Fri, 09/25/2009 - 11:04

Hello Hector,

your action plan looks like correct.

only point that if you want to perform selective prepending you need to use a route-map

access-list 11 permit B.B.B.B

route-map sel-prepend permit 10

match ip address 11

set as-path prepend yourASN yourASN yourASN

R1

router bgp yourASN

neigh isp.ipaddr route-map sel-prepend out

the same has to be done on R2

Hope to help

Giuseppe

Correct Answer
mszeftawy Fri, 09/25/2009 - 11:42

Hi hcintron

One more point should be taken in consecration.

You have to create proper filtering for the routes sent from your routers, only advertise your range of addresses.

I mean don't rely on your service provider to filter the routes that came from your network, in case one of your service providers is not performing the proper filtering you will advertise provider A subnets to provider B and vise versa, which might make them use your network as a transit AS for the intra-provider traffic.

hcintron Fri, 09/25/2009 - 14:58

Great.

Thank you Giuseppe and mszeftawy for letting me know that I was in the right track and the pointers.

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