BGP conditional advertisement + object tracking

[amar_5664]

Hi Experts,

Hope someone can help, we have multihomed environment advertising 4 prefixes to both ISPs and running an iBGP between two routers. Both ISPs send us default routes. Attached diagram to understand how we are connected.

Issue: traffic for prefix x.x.x.x/24 is taking a path from AS-2 which is what i dont want, reason being bandwidth is highly utilized. We have higher bandwidth for AS-1.

I have been trying to get a solution setup in much unconventional way to influence inbound path by not advertising 2 prefixes to AS-2. The issue we have is we are a private AS and cannot influence inbound traffic taken from dual-homed ISPs, part of the problem is AS-2 is now using AS-1 as a transit provider.

what i would like to do is for AS-2 link only advertise prefixes a.a.a.a/24 and b.b.b.b/24 whilst eBGP peer/interface is up for AS-1, if the link with AS-1 is down advertise a/b/x and y prefixes to AS2. This way i can control traffic destined to x/24 prefix will not take path through AS-1. I have been trying object tracking using ip sla's, prefix advertisements etc, is there a neater solution then what i have been trying.

[Abzal]

Hi amar,

If I understood you right as per image. You can inbound path from ISPs in the way you want. By making better path to network x.x.x.x/24, y.y.y.y/24 using AS-1 and a.a.a.a/24, b.b.b.b/24 using AS-2. In case of failure on of the router taking path through active router.

AS-1

access-list 2 permit a.a.a.a 0.0.0.255

access-list 2 permit b.b.b.b 0.0.0.255

route-map AS-1

match ip address 2

set as-path prepend 65101 65101

router bgp

neighbor remote as 1

network x.x.x.x 0.0.0.255

network y.y.y.y 0.0.0.255

network a.a.a.a 0.0.0.255

network b.b.b.b 0.0.0.255

neighbor route-map AS-1 out

AS-2

access-list 1 permit x.x.x.x 0.0.0.255

access-list 1 permit y.y.y.y 0.0.0.255

route-map AS-2

match ip address 1

set as-path prepend 65101 65101

router bgp

neighbor remote as 2

network x.x.x.x 0.0.0.255

network y.y.y.y 0.0.0.255

network a.a.a.a 0.0.0.255

network b.b.b.b 0.0.0.255

neighbor route-map AS-2 out

Hope it will help.

[amar_5664]

Hi Abzal,

Appreciate your response, but this will not influence larger part of the Internet Community. As we are Private AS when the prefixes will be advertised from AS1 to their upstreams it will not be propogated as any ISP would remove Private AS.

I have used their local communities but that is not helping much either.

[Abzal · Accepted solution]

There is one useful link

http://openweb.or.kr/wp-content/uploads/2012/03/BGP-MHing-HOWTO-whitepaper.pdf

But if local communities not helping you should contact to your ISP.

If you can aggreagate these subnets x.x.x.x/24 and y.y.y.y/24 into subnet with mask 23. It's better to advertise a summary from secondary router and more specific from primary.

Hope it will help.

[amar_5664]

Thanks abzal, appreciate your responses.