BGP Mulihoming with 2 co-operate sites

Unanswered Question
Dec 1st, 2008

Hi,

we have 2 co-operate sites. Each co-operate site has 2 different ISP's. Each co-operate site has an ASN number with /24 subnet. RIPE donot want to have 2 sites of the same company to have 2 ASN numbers. they have told us to to remove the ASN number on one site. As we have sites with 2 different ISP with dedicated link, we want to run the BGP so that we can take the beniefit of redundancy. How can we use one ASN number to run the BGP on these 2 sites?

your suggestion will be more helpfull.

Thanks

I have this problem too.
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Giuseppe Larosa Mon, 12/01/2008 - 09:53

Hello Tauseef,

public AS numbers are becoming rare so the RIRs are very careful in assigning them.

You have two sites and each site is connected to two different ISPs.

Each site advertises its own /24 and this is fine a network of this size can travel over the internet.

I guess your two sites are not directly connected by any means otherwise implementation would be easy.

There is a trick to make the two sites communicate even if they use the same AS number:

on each eBGP session to the ISPs you need to add

neighbor isp-x-y allow-as-in 1

in this way the advertisement coming from the other site can be accepted and installed in the routing table

note: if you use prepending you need to provide as parameter the maximum number of times your ASN can appear on routes coming from the other side: you need to change the 1 to the number you need to accept routes in any case.

However, in the long term I would consider to buy an EoMPLS service between the two sites so that you can merge them and use your routing protocols over it.

If you do so you can advertise both subnets on both sites using other tools to make one site preferred for subnet A and one preferred for subnet B.

Hope to help

Giuseppe

t4tauseef33 Wed, 12/03/2008 - 14:16

Thanks Giuseppe.

These 2 sites are not connected to each other through any way just the ISP connections.

lets say ISPA and ISPB are on site A and ISPC and ISPD on site B

ISPA ISPB ISPC ISPD

AS 50001 AS 50002 AS 60001 AS 60002

SITE A: AS 70001 SITE B: AS 70001

Can you give me a sample configuration for running the BGP in this case

ISPA Conneted router

neighbour ISPB-Local-Router remote-as 70001

neighbour ISPA remote-as 50001

ISPB Conneted router

neighbour ISPA-Local-Router remote-as 70001

neighbour ISPB remote-as 50002

ISPC Conneted router

neighbour ISPD-Local-Router remote-as 70001

neighbour ISPC remote-as 60001

ISPD Conneted router

neighbour ISPC-Local-Router remote-as 70001

neighbour ISPD remote-as 60002

how you will implement the allow-as-in? what is the change required on ISP?

Your help will be appricited.

Thanks

Jon Marshall Wed, 12/03/2008 - 14:40

Tauseef

neighbour ISPB-Local-Router remote-as 70001

neighbour ISPA remote-as 50001 allowas-in 1

neighbour ISPA-Local-Router remote-as 70001

neighbour ISPB remote-as 50002 allowas-in 1

neighbour ISPD-Local-Router remote-as 70001

neighbour ISPC remote-as 60001 allowas-in 1

neighbour ISPC-Local-Router remote-as 70001

neighbour ISPD remote-as 60002 allowas-in 1

The ISP does not need to to change anything because the allow-as-in command allows an EBGP speaking router to accept an EBGP advertised route with it's own AS number in it. The ISP does not need to do this. Only you need to do it at Site A & B.

Jon

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