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BGP Disaster Recovery Scenario

Posted on 2004-09-27
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Last Modified: 2012-06-27
I'm not sure if any BGP expert out there has come accross this before but I will give my scenario:

I have one main site with multiple remotes (6 or so) in a hub and spoke topology. BGP running as the routing protocol as the network runs over MPLS VPN and thus the protocol needs to be TCP based.

There is HSRP and ISDN Backup at the centre. My  question is this:

Can I have a DR Site that runs BGP via another AS (or can be iBGP) that will advertise the same network as the main site but that network is weighted so that it becomes live if centre fails? i.e. If the link into the WAN from the centre site goes down (say 192.168.1.0/24), the backup network (192.168.1.0/24 Same but weighted?) becomes active?? So DR is automated? Or is this not possible?

Any help would be greatfully received.

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Question by:c1sc0m4n
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3 Comments
 
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by:PennGwyn
PennGwyn earned 400 total points
ID: 12164505
> Can I have a DR Site that runs BGP via another AS (or can be iBGP) that will advertise the same network as the main site but
> that network is weighted so that it becomes live if centre fails?

What organizations routinely do is advertise the same AS via BGP from different sites, with different wieghts.  In a common scenario, site A and B both advertise the AS's address blocks, but site A advertises a higher metric to subnets that are physically not at site A, and site B advertises a higher metric to subnets that are not physically at site B.  In normal operation, almost all traffic for site A comes to A and almost all for B comes to B, but if either site goes down then traffic for the whole block goes to the one that's up.

What you have in mind is a slightly similar subset:  site A advertises your whole block, and so does site B but with a higher metric.  (The normal way to achieve a higher metric in BGP is to prepend an extra instance of your AS number to the AS-list, so such routes appear to require an extra AS hop.)  Note that the backup site leads to the same network, and so advertises the same AS as the primary!

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LVL 79

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by:
lrmoore earned 400 total points
ID: 12164517
Sure, you can do exactly what you want using bgp conditional advertising..

http://www.cisco.com/en/US/tech/tk365/tk80/technologies_configuration_example09186a0080094309.shtml

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Author Comment

by:c1sc0m4n
ID: 12166665
Can one of you help me by providing example configs? I have read the Cisco Document but lack understanding of it! :-(

At the moment I have:

router bgp 100 < ------- Main Site
 no synchronization
 bgp log-neighbor-changes
 network 195.183.203.0
 redistribute static
 neighbor 172.16.1.40 remote-as 400 < ----------- Carrier
 neighbor 172.16.1.40 ebgp-multihop 255
 neighbor 172.16.1.40 update-source Loopback0
 neighbor 172.16.1.40 default-originate
 no auto-summary
!

router bgp 200 < ------------- DR Site (When Main Fails Network Below becomes active somehow??)
 no synchronization
 bgp log-neighbor-changes
 network 195.183.203.0
 redistribute static
 neighbor 172.16.1.40 remote-as 400
 neighbor 172.16.1.40 ebgp-multihop 255
 neighbor 172.16.1.40 update-source Loopback0
 neighbor 172.16.1.40 default-originate
 no auto-summary
!

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