iBGP loss of connectivity

[neil_titchener]

Hi,

I'd like some advice from the BGP experts amongst you on iBGP peering.  Our network has four iBGP peers, two at each of the DC sites with a single link connecting them.  Lets call them rt01 & rt02 at site one and rt03 & rt04 at site two. On the LAN side all four have connectivity between them.  On the WAN side all four are connected through a common eBGP relationship.  What Would happen to the BGP relationships if the link between the two sites went down which would leave two routers at site one and two at site two?  In this scenario all four routers would maintain their eBGP relationship over the WAN but we would now have iBGP relationship between rt01 and rt02 and another iBGP relationship between rt03 and rt04, both iBGP relationships using the same AS number. 

Another question is one router is offering a default route so would this be seen by all other routers in the same AS?

Thanks in advance.

[chrihussey · Accepted solution]

Yes the EBGP peering with the outside would be maintained. However the problem would be that if there are common networks at both DC sites then you would probably have issues as the EBGP would see the routes from all four. Optimally, you would want to set it up in a way that the if LAN connectivity was lost between the two DCs, then the networks specific to each DC would only be advertised in the eBGP.

As far as the default route is concerned, it would probably be best if at least one router at each DC was had a default route. In normal operation one could potentially be advertised over another and the other advertised only if the primary was lost.

Hope this helps

[neil_titchener]

Thanks for your reply, it's been very helpful.  The actual setup is more complicated than described but I wanted to put out a scenario to see what options were available to me.  Following your response I've run a few labs and can now see the consequences of various failure scenarios.

Thanks again.