From: Alec Pun (clapun@graduate.hku.hk)
Date: Sun May 18 2003 - 00:48:21 GMT-3
Thanks very much. Can I put it in this way : admin distance is only used to
compare prefix learned from different routing protocols, e.g. EBGP vs OSPF.
However, when the same prefix is learned from both EBGP and IBGP, the best
path is determined by the qualification tests ?
Thanks.
alec
----- Original Message -----
From: "OhioHondo" <ohiohondo@columbus.rr.com>
To: "Alec Pun" <clapun@graduate.hku.hk>; <ccielab@groupstudy.com>
Sent: Sunday, May 18, 2003 3:12 AM
Subject: RE: BGP route decision
> Depending on the circumstances, the Best Route may be an EBGP learned
route
> or an IBGP learned route. BGP chooses a Best Route. Only the Best Route is
> sent to other BGP speakers AND the local routers IP routing table. Admin
> distance IS NOT a BGP factor in choosing the Best Route. If
synchronization
> is enabled, only synchronized routes are considered. After that, the Best
> Route is determined by passing all routes through the qualification tests
> below. Routes are disqualified until there is only one route left. For
> instance, if all routes have an equal WEIGHT and their NEXT-HOPS are all
> accessible -- and one of the routes has a higher Local_Preference than the
> others --- the route with the highest Local_Preference is chosen as the
Best
> Route. If the Local_Preference of all routes is the same, the next
criteria
> is evaluated. The list of qualifying tests are:
>
>
> 1) Next Hop address/network given for the reaching the network is
available
> in the IP routing
> 2) The route with the highest Weight attribute is chosen (the Weight
> attribute is Cisco proprietary)
> 3) The route with the highest Local_Preference attribute is chosen
> 4) Any routes that were learned Locally (next-hop = 0.0.0.0) have a
> preference
> 5) The route with the shortest AS_Path attribute is chosen
> 6) The route with the lowest Origin is chosen [internal (0), external (1),
> incomplete (2)]
> 7) The route with the lowest Multi-Exit Discriminator (MED) is chosen
> 8) The route learned via EBGP is chosen (iBGP routes are discarded at this
> point)
> 9) The route with the nearest IGP neighbor is chosen
> 10) The oldest route is preferred.
> 11) The route with the lowest BGP router-id is chosen
>
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
> Alec Pun
> Sent: Saturday, May 17, 2003 12:58 PM
> To: ccielab@groupstudy.com
> Subject: BGP route decision
>
>
> Hi group,
>
> If the same destination is learned from both a EBGP neighbor and IBGP
> neighbor, which will be the preferred ? 'Coz when I read the Internet
> Routing Architecture book by Halabi (page 330), I found the IBGP route is
> preferred over EBGP (for 192.68.11.0). Isn't that the admin distance for
> IBGP greater than EBGP ? (200 vs 20) Any hints are welcome. Thanks.
>
> alec
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