Re: Symmetrical Routing with a single router and two providers.....

[Enke Chen <[e-mail]>]

Bradon, 
This case is actually covered in <draft-ietf-idr-dpa-application-01.txt>. 
It is in Section 2.3, Policy 2 - Each link is used for traffic with the 
respective direct provider; In general one link is used as the primary, 
the other as backup. 

The simplest setup is the following: you (AS3) annouces route X with 
higher DPA value to AS1, and lower DPA value to AS2.  Your providers 
AS1 and AS2 configure AS-based "local_pref" to prefer the respective 
direct link to reach X.  You take partial routing from AS2 (i.e., 
AS2's customer routes only), and generate weighted default to favor 
the AS1 link for default.  

This setup allows you to use the link to AS2 to communicate with AS2's
customers,  and the link to AS1 to communicate every one else. (This 
achieves the objective of the shortest path -- you have to choose one 
to AS4 due to equal AS path length).  This setup also provides for 
the backup capability and preserves routing symmetry. 

It may take router vendors more than three weeks to roll out a new 
release with the DPA feature. (The implementation is on the way!)  
For now your best bet for implementint symmetry is to manipulate the 
AS path (i.e. add one additional AS while X is being announce to AS2) 
so that AS4 favors the AS1 link.  (That is a 10.3 feature of Cisco.) 

Hope this answers your question.  

-- Enke

> Date:    Mon, 26 Jun 1995 20:38:51 -0500 (CDT)
> From:    Photon - NOL Staff <[e-mail]>
> To:      [e-mail]

> 
> 
> 
> Please excuse my ignorance while you read on.....
> 
> 
> I've been keeping up as best as I can with BGP4, but I have obviously missed
> a few concepts here and there.  I see how the DPA helps in all of the listed
> configurations in draft-ietf-dpa-01.txt, but could someone explain to me how
> and if it could help this situation (which I face in about 3 weeks when our n
ew
> router comes in):
> 
>                              AS4
>                             /  \
>                            /    \
>                           / (NAP)\
>                         AS1------AS2
>                           \      /
>                            \    /
>                             \  /
>                              AS3
> 
> 
> In this diagram, AS3 is the end customer, and has a linkup to both AS1 and
> AS2.  AS1 and AS2 are both major "backbone" AS's with anywhere from 8-20k of
> customer routes.  Both of the (T1) links are coming from the same physical
> router in AS3, and AS3 has one flat, simple route internally, on one local
> ethernet.  Therefore AS3 has no need, desire or ability to split internally a
s
> far as routing is concerned.  AS3 would like to (optimally, but seeming 
> impossible) use the best/fastest path for each connection based on intelligen
t
> metrics, and not lose routing symmetry.  While both AS1 and AS2 will be used
> in a load balanced situation for AS3's normal traffic, AS3 also wants a BGP
> setup that will allow all traffic to go through one of its providers, should
> the other provider go down temporarily.  Since the two providers going down
> simultaneously is a rare occurance, this should provide for high availability
.
> 
> What kind of BGP setup could offer a decent method of good load balancing
> between AS1 and AS2 for customer AS3?
> 
> Would this be a situation in which AS3 would benenfit from taking on full
> routing, or would AS3 be better off taking the customer routes of AS1 and
> AS2, and using DPA and weighted default routes to acheive symmetry?
> 
> 
> Silently Going Insane,
> 
> 	Brandon
> 	[e-mail]
> 
> 
>