From: OhioHondo (ohiohondo@columbus.rr.com)
Date: Sat May 17 2003 - 16:12:45 GMT-3
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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