AS prepend, MED or local_pref question.

[Steve Francis <[e-mail]>]

Hi.  
Apologies if this is not the correct forum for this question.

I wish to connect two separate routers in AS1 to two separate routers in
AS2.  I want half the networks in AS1 to use router1 going to AS2, and half
the networks to router 2 to go to AS2. And I want the return traffic to
follow the same paths, symmetrically.

Now, I can think of three ways to do this:
- at router 1, prepend a bunch of AS numbers to the routes advertisements
for the networks I want to come back via router2, and at router 2 prepend
for teh networks that should come back via router1.
- assign different communities to the two sets of routes, which are
announced both places, and apply a local_pref at the routers in AS2, so at
the router 1 peer in AS2, one set of routes would have a lower local_pref
applied, and a higher local_pref at router 2 peer in AS2.
- Use the MED attribute for the two sets of routes.

Is there a preferred way to do this?  Any advantage/disadvantage to any
specific approach?

Is it necessary for the protocol to provide so many ways to achieve the same
thing?  It seems to me that MED is redundant.

TIA
Steve Francis
Network Analyst
Communications Services
UCSB	
ph [phone]	fax [phone]