Multi-Homing and Public ASN

[sterdnotshaken]

I believe I know the reason's why a public ASN is required if I were doing multihoming, but would someone mind elaborating on why it would be needed verses having 2 separate routers uplinking to 2 separate carriers, each having separate iBGP adjacencies using separate private AS's? It is due to the fact that in a multihomed situation, without a public AS, the global BGP route table would have 2 separate destination public ASN's (the 2 separate carriers) as the destination for injected routes?

Thanks much!!!

[sean_evershed]

Hi,

There will be EBGP adjacencies between your routers and ISP A and ISP B. The Cisco Live Presentation Enterprise Multi-homed Internet Edge Architectures outlines the scenario you describe above.

Both ISP A and ISP B strip the private ASNs advertised for Company A's prefix and replace it with their own global ASNs.

ISP C's global routing table will only contain the single best path to Company A's prefix. See below the BGP path selection process:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

ISP C will use the shortest AS path in order to reach Company A's prefix.

You can check the BGP routing tables of many ISPs at the following site.

http://www.bgp4.as/looking-glasses

See below a snippet from the routing table for an Australian ISP

route-views.optus.net.au>sh ip route

Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP

       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

       E1 - OSPF external type 1, E2 - OSPF external type 2

       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

       ia - IS-IS inter area, * - candidate default, U - per-user static route

       o - ODR, P - periodic downloaded static route, H - NHRP

       + - replicated route, % - next hop override

Gateway of last resort is 203.202.125.1 to network 0.0.0.0

S*    0.0.0.0/0 [1/0] via 203.202.125.1

      1.0.0.0/8 is variably subnetted, 1191 subnets, 14 masks

B        1.0.0.0/24 [20/0] via 203.13.132.35, 7w0d

B        1.0.4.0/22 [20/0] via 202.160.242.71, 3w6d

B        1.0.16.0/23 [20/0] via 202.160.242.71, 1w2d

B        1.0.18.0/23 [20/0] via 202.160.242.71, 1w2d

B        1.0.20.0/23 [20/0] via 202.160.242.71, 1w2d

Since private ASNs are not global it is not possible for Company A to traffic engineer their ingress traffic. If you had a global ASN you could manipulate the BGP AS Path attribute before sending the prefix in order to influence return traffic. You might for example want your return traffic to use the link to ISP A rather than to ISP B.

Don't forget to rate all posts that are helpful.

Cheers

Sean

[sterdnotshaken]

Thanks Sean! So there are 1 of multiple options then...? One where I would have say 2 edge routers with private AS based adjacencies to 2 separate upstream Carriers and I share my routes with both carriers and out on the looking glass, I would see whichever of the to Carriers public AS's that is the shortest AS path to my network's route therein... Where another idea would be to procure my own AS # and have the same phyical layout as the previous option, but I would use my Public AS to peer with both Carriers Public AS's, thus making so that from the Looking Glass persective, I would see the sortest AS Path route there, which would have the destination AS of my Public AS?

Thanks for helping me with this!!

[sean_evershed · Accepted solution]

Hi,

In the scenario where you are peering with two different ISPs you need the following:

- PI address space

- Public ASN

You then need to design your outbound and inbound BGP routing policy.

For example you can use AS-path prepend to prefer one path over the other.

Cheers

Sean

[sterdnotshaken]

Excellent, that answers my question. Thanks Sean!

[mma]

Hi,

After "Both ISP A and ISP B strip the private ASNs advertised for Company A's prefix and replace it with their own global ASNs.", what would happen to the private IP addresses of Company A? What would ISP A and ISP B do with the private IP addresses of Company A?

As I am in a BGP, private AS, private IP, multihome situation with a single ISP and would like to know how the ISP will deal with the private IP space of Company A.