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Is cross-site failover possible without L2 LAN extensions?


We have a site (site A) which has a /26 public IP range and another site (site B) which has a different /27 public IP range. The internet links are provided by the same ISP and both sites exist in the same AS.

We have asked our ISP to provide a facility whereby if Site A loses its internet connections, any traffic desitned for its /27 ip range is redirected to SIte B i.e. so that SIte B would then 'own' the /27 ip range and be able to responde to requests. We do have a layer 2 LAN extension between site A and site B but have specifically requested that this is not used as part of the failover mechanism. Also, for complete disclosure, there are 2 internet links at each site already configured for local failover so this cross-site failover scenario would only come into effect if both routers/internet links at a single site went down.

The ISP has stated that this redirection of Site A's public address space to Site B is only possible by running HSRP between their router at Site A and their router at Site B across our layer 2 links. Is this correct?

My experitse is mainly in Enterprise LAN switching but am surprised our ISP is saying this can not be acheived. Any opinions would be greatly appreciated.

2 Replies 2

Alessio Andreoli

Hi Paul,

many ISPs have many different settings but i do believe that what you are asking for is possible in the vast majority of scenarios.. However, you can't even set an iBGP session between your two routers?

Can't you advertise both the routes in both the sites with different priorities?

site a:

/26 preferred

/27 very bad metric/local preference or whatever you use

site b:

/27 preferred

/26 very bad metric/local preference or whatever you use

Your ISP is advertising both the prefixes and should not have any issue to agree this config. i still would insist for an iBGP session between your routers.



Thanks Alessio,

Can I ask a couple more questions?

As all routers (CE and PE) are all in the same AS, i assume iBGP would need to be running between them (am i correct in saying eBGP is only for between AS's?).

Is my interpretation of your response above correct....

  • Site A's routers have an iBGP relationship with its upstream PE router
  • Site A:advertises its own /26 as preferred and siteB's /27 with a low preference
  • Site B's routers have an iBGP relationship with its upstream PE router
  • Site B advertises its own /27 as preferred and site A's /26 with a low preference
  • All PEs run the same iBGP to share information
  • This alone is sufficient to provide site failover in the event of complete internet failure at one site
  • iBGP directly between the CE routers on our sites is optional but recommended? (not sure what benefit this provides?)

I have attached a diagram of the proposed scenario, including taking into account the locally resilient routers. I would be grateful if you could take a look at it and let me know if I have interpreted your response correctly.


Also, not shown in the diagram are the LAN segments which are on the inside of the firewalls (would be shown below each site's firewalls) which are completely different address spaces at each site.



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