diverse multisite advertising?

Joe Provo nanog-post at rsuc.gweep.net
Tue Nov 4 21:15:08 UTC 2008

On Tue, Nov 04, 2008 at 01:07:13PM -0800, Gregori Parker wrote:
> I've been reading up on BGP and asking around, but still havent found a
> hard yes/no on this...hopefully someone here can provide clarification.
> If I have a single ASN and an IP allocation from ARIN, can I advertise
> one half of my prefix from one site and the other half from another
> distinct non-connected site using the same ASN?  If so, are there
> implications with mutihoming to all unique ISPs (2 per location)?

You can do a lot of things.  If you want to do it is a matter related 
to what kind of network you run and what your goals are.  

> I seem to remember that an AS Path wouldnt be valid with the same AS
> appearing more than once in non-sequential order, i.e. at the beginning
> and the end in this case, but am unable to verify since everything I've
> been reading up on discusses BGP in single-site terms.

Your largest concerns will be
- how do intend your split sites see each other? GRE/IPSec tunnels are
  a popular if potentially fragile answer.
- deaggregating is antisocial, and depending on your allocation size &
  in what part of the bit-spectrum it lies, you may need to be concerned 
  about the deaggregates getting filtered.  Assuming a decent answer to
  the first point and you can 'safely' announce the aggregate in both 
  spots to attrach traffic beyind the propagation distance of your

You may wish to consider having the same set of providers in each 
location and examining approaches that make your interior appear more 
coherent than it is [eg deaggregates with sane MEDS and no-export to
allow you to TE across your paid links, and only let the aggregate 
out past their borders].


             RSUC / GweepNet / Spunk / FnB / Usenix / SAGE

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