Redundant internet using two locations.

Unanswered Question
Jul 30th, 2009

I am trying to accomplish ISP redundancy using two separate physical locations. Location 1 has 45Mbps connection to internet. Location 2 has a 45 Mbps connection to the internet. Between each location we have a 100Mbps layer 2 MPLS. I was going make the edge router in both locations part of the same AS. Location 1 I was going to create an eBGP relationship with its ISP and an iBGP with location 2 over the MPLS. Location 2 I was going to create an eBGP with its ISP and an iBGP with location 1. We have registered a /22 IP address block and AS number from ARIN. I was going to allocate a /23 to each location. Any comments or concerns about this would be greatly appriciated. Thanks.

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Rick Morris Thu, 07/30/2009 - 11:27

Sounds like you have it.

only routing detail would be routing internet out isp 2 via MPLS if isp 1 link fails. This will be done via metrics, but from the sound of the post it seems like you already understand that.

Giuseppe Larosa Thu, 07/30/2009 - 11:37

Hello Daniel,

your design looks like good.

There are a couple of notes about redundancy and fault tolerance:

by sending two /23 IP address blocks everything is fine when everything is working well and each site is associated to a single /23 address block.

What if link on siteA to ISP1 fails?

Can site1 still access the internet ?

Are you doing NAT on these devices and iBGP session is used for exchange of private ip subnets or NAT is made more internally and the iBGP session is used to exchange public ip addresses?

In latter case border router2 should advertise ip address block of site A out ISP2 with an AS path prepending or if this is not enough using BGP conditional advertising.

What if direct MPLS link fails ?

There is a need for communication between the two public IP address blocks?

if yes and if you are going to receive full BGP tables you may need to use

neighbor ispx allowas-in N

to be able to accept the advertisement from the internet that contains your own AS number as advertised by siteB

Hope to help


Giuseppe Larosa Thu, 07/30/2009 - 13:16

Hello Daniel,

there has been some threads about this question of BGP interconnections of two sites via internet using BGP that could be addressed with allowas-in.

It is starting to become more common, however having an MPLS link between the two sites is a very good point.

However, there are some setups that start without this inter-site link.

It depends on your needs : if it is useful to be able to make the two public address blocks to talk even when the direct mpls link fails you can think of it.

Hope to help



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