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Re: Disaster recovery using as-prepend?
From: Warren Kumari <warren () kumari net>
Date: Thu, 16 Feb 2006 12:29:29 -0800


Part of the question is how bad it is for you if you DO get any traffic to your backup datacenter, the connectivity between the datacenters and the datacenters connectivity to the rest of the world.

Assuming that you do not have good connectivity between datacenters and that the datacenters have different connectivity to the outside world:

While pre-pending should get almost all of your traffic away from your backup DC, you cannot guarantee that it will not get any traffic while the primary is still up.

If your primary is connected to ISP_A and the backup is connected to ISP_B, customers connected to ISP_B MAY still flow to your backup DC (ISP_B will probably set local preference on all customer routes - you should be able to override this behavior with communities but not all providers support this (or honor it 100% of the time!))

Announcing a more specific from the primary is likely to work basically all the time (assuming a) your announcement is not too long to be listened to, b) ISP_A and ISP_B don't lose connectivity between themselves). This is not particularly polite however...

Another option is just not to announce the backup datacenter until the primary one goes away - see if you can do something like BGP Conditional Advertisement (or your vendor's version of the same).


Depending on just how bad having request arrive at the backup datacenter will drive just how paranoid you ned to be - if having your backup get traffic is going to make databases unhappy, etc then you MIGHT even want to consider a manual only failover - if your primary datacenter has a 20 second blip, the pain of dealing with requests that hit the backup during those 20 seconds MAY be greater than just being unavailable for 20 seconds... It all depends on your business, applications, etc, but prepending alone might not be the way to go.

Warren


On Feb 16, 2006, at 6:56 AM, Christopher J. Pilkington wrote:


My apologies if this question doesn't belong here.

We have a PI /24 we'd like to advertise out of our primary data center
for production use.  (Well, actually, we'll be advertising a more
specific from our /21 assignment, so already not too friendly... but I
digress.)

We have a disaster recovery site which will have a clone of the myriad
production servers.  We'd like to fail over to that site
automagically.

I'm thinking advertising the same prefix and just doing several
as-prepends.  However, now I'm not sure if this is a polite thing to
do or not.

Someone mentioned to me something with MEDs, but as soon as that term
was used, I started twitching, and couldn't follow the conversation.

Would a "good netizen" use the as-prepend method?  Or am I missing a
simpler/more polite solution?

-Christopher



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