Disaster recovery using as-prepend?

Warren Kumari warren at kumari.net
Thu Feb 16 20:29:29 UTC 2006

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  

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.


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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