Optimizing BGP

[Lewis Quin]

We are running BGP at two sites (different locations) that also has a L2 connection connecting the two sites.  There is a DMZ that is split between the two sites (public address space) using the L2 connection.  We announce the public DMZ address space (/23) out via BGP from both locations using and AS prepend to influence the inbound traffic into "Site A".  What I would like know is if we can do the following using BGP:

Break the /23 into two smaller networks (/24) bringing the first half into 'Site A', and the second half into 'Site B', however keep both sites capable of receiving the full /23  in the event of failure.

[Jon Marshall]

Yes you can as long as your ISP accepts /24s.

So at site A advertise site A's /24 as normal and site B's /24 with prepending

At site B advertise site B's /24 as normal and site A's /24 with prepending.

Alternatively you could just have site A advertise it's own /24 and the summary and site B advertise it's own /24 and the summary.

There is no need for prepending then as the summary is only used if the /24 is no longer available.

Up to you really which way you do it.

Jon

[Lewis Quin]

Perfect, so this is possible even though both /24's (a larger /23) have the same AS#?

[Jon Marshall · Accepted solution]

Yes it's possible.

See the last bit I added a about an alternative way to do it.

Really up to you which you prefer.

Jon

[Lewis Quin]

Awesome, thanks for the quick response!

[Jon Marshall]

No problem

Personally I would use the prepending solution rather than the summary one because then there is no ambiguity in what you are advertising from which site.

And because there is a direct link between sites under some circumstances you might still be advertising the summary even if the local /24 is not available.

Jon