From: Fanglo MA (fangloma@pacific.net.hk)
Date: Mon May 19 2003 - 07:04:57 GMT-3
I would view in this way, correct me if I'm wrong, AD is used for router
to select routes from multiple route sources into routing table. BGP
decision process is a process to select learned routes and then put into
BGP database and to routing table selection process.
Regards,
Fanglo
_____________________________________________
Little minds are interested in extraordinary;
great minds in the commonplace.
- Elbert Hubbard.
On Sun, 18 May 2003, Alec Pun wrote:
> 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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