BGP advertisements

Unanswered Question
Aug 15th, 2008
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My org is preparing to multi-home. We have a legacy /16 that we own as well as a /24 from our current provider. Our plans are to start advertising a /24 out of our /16 so we can readdress at our discretion out of provider space. My question is: Do we need to advertise both /24s from both ebgp routers, or would it suffice to only advertise the ISP's /24 from their connection, and our own /24 from the new connection?


Hope my question is clear. Please let me know if more info is needed.


Thanks in advance.


/rls

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Giuseppe Larosa Fri, 08/15/2008 - 07:53
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Hello Robert,

I hope I understood correctly your question.


To the second ISP that you add, you can only advertise the /24 taken from your own address range.

You should'nt advertise to provider ISP2 the address pool that ISP1 has given you to use that would be wrong.


You are free to advertise your own /24 to both if desired and on multiple eBGP routers if necessary.


Hope to help

Giuseppe


rsamuel708 Fri, 08/15/2008 - 11:09
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Giuseppe, our new ISP has given us their blessing to advertise the other ISP's /24 out the connection as well. What I'm looking for is reasons to do it or not. By the time I turn up our bgp sessions, we will not have re-addressed completely out of our existing range so it's not an option to only advertise our ISP-independent space only to both.


/rls

Giuseppe Larosa Fri, 08/15/2008 - 11:44
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Hello Robert,

in this case I would advertise the ISP1's address pool to ISP2 using AS path prepending just to have a backup in case the link and session with the first ISP1 fails.

As soon as readdressing is complete I would remove this ISP1's pool on both ISPs.

Verify with ISP1 that in case of failure they will be ready to route to you via ISP2.

In the internet ISP1's /24 will be part of ISP1's address blocks.

So it is probable that in any case traffic will go back to ISP1, ISP1 will not see your advertisement and has to be prepared to accept ISP2's backup advertisement.


It isn't a clean solution but acceptable during a transition phase.


Best Regards

Giuseppe

Edison Ortiz Fri, 08/15/2008 - 07:53
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You need to advertise whatever addresses you need for internal reachability.


I'm not sure what subnets are you using for email, www, and other type of services.


It wasn't clear in your post.


If you are able to re-ip all your internal subnet to the new /24, then just use that one.


HTH,


__


Edison.

rsamuel708 Mon, 08/18/2008 - 05:41
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Edison,


We currently use our ISP's netblock for about 95% of our services with only a very small portion being used out of our direct assignment ips....so, right now we'd really need to advertise both in order to have complete reachability. From earlier posts, I guess it would be acceptable to advertise only one from each, but then we would leave ourselves open to a failure in case one link went down...


So it seems as if advertising both would be the answer. You agree?


/rls

Edison Ortiz Mon, 08/18/2008 - 06:09
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So it seems as if advertising both would be the answer. You agree?


Yes.


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Edison.

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