Re: [Idr] draft-jakma-mrai-dep-00 (fwd)

[Pedro Marques <[e-mail]>]

On Nov 25, 2008, at 11:59 PM, Olivier Bonaventure wrote:

Pedro,

In general my recommendation for MRAI values would be 0 for eBGP and 0

for iBGP. As the draft correctly explains calculating a value that best

fits the network is a very tricky business.

I would not recommend using the same MRAI timer for iBGP and eBGP. In a

large AS using route reflectors or confederations, we are often in a

situation where some routers of the AS learned an alternate path but did

not distribute it widely due to the alternate paths being hidden by

border routers

How does applying MRAI help in this case ? And how does having an higher value for eBGP rather than iBGP help ?

It seems that in your scenario the faster the route propagation the better.

(e.g. different local-pref or longer AS-Path), router

reflectors or at the borders of confederations. See e.g.

http://inl.info.ucl.ac.be/publications/quantifying-bgp-routes-diversity-insi

but discussions with operators showed that this is true for other networks.

In such large ASes, when a link fails iBGP should converge first in

order to allow all routers to learn the alternate path and advertise the

alternate path over eBGP sessions.

What if your alternate path happens to be being an eBGP peer ?

Dest prefix  is source by AS A

AS A connects to AS B preferentially and has fallback routing though AS C. Where C only advertises destination prefix if it is not otherwise know. AS D is transit for both B and C.

Convergence requires D to send withdrawal to C to receive the alternate path back.

It is not unheard off to have such arrangements.

Please do keep in mind that i didn't suggest that the values (0, 0) are the best for your network and for the scenario that you are trying to optimize. I'm merely suggesting that as general values for the generic case (0, 0) are hard to beat... and they work just fine.

This can be achieved, at least

partially, by using a longer MRAI timer for eBGP compared to iBGP.

As you note, the situation is different in BGP/MPLS VPNs environments or

when draft-walton-bgp-add-paths-06.txt is used.

I would not personally recommend to exempt withdraws from the MRAI on

eBGP sessions in large ASes.

Olivier

--

http://inl.info.ucl.ac.be , UCLouvain, Belgium

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