Let me take a crack at addressing this. I think people are confusing the
process a router follows to make routing decisions with the BGP best path
selection. Just because a BGP router chooses a best path does not mean that
the router will use that path when routing packets.
The router makes it decision as follows when routing packets:
1) The router will choose the path that has the longest prefix match.
2) If two routes are learned with the same prefix length, the router will
use the route with the lowest administrative distance.
eBGP routes have an AD of 20.
iBGP routes have an AD of 200.
In determining which BGP path is "best" for a particular prefix (NLRI -
Network Layer Reachability Information) which will get installed into its
local routing table and advertised to its BGP peers, the router considers
the criteria in the following order (each step only is used if all of the
preceding steps still leave a tie):
1) Is the next hop reachable? -- if not, don't consider that path as best.
2) Administrative Weight -- Cisco proprietary -- higher is better
3) Local Preference -- higher is better
4) Locally originated route --based on the Origin PA -- I is better than E,
which is better than ?
5) AS_Path -- the shortest AS_Path list is considered better.
6) Origin -- BGP NLRI that were learned from eBGP peers are considered
better than routes that were learned through iBGP peers
7) MED -- Multi-exit descrimintor - lower is better
8) Neighbor type: prefer routes that were learned from an eBGP peer over
routes learned from iBGP peers.
9) IGP Metric -- the smaller metric wins
Once a route is injected into the local routing table, the router doesn't
consult the BGP Best Path selection before choosing a route. It will use
the AD of the routes that are in the routing table.
Brian Valentine
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Suresh Mishra
Sent: Thursday, April 03, 2008 8:34 AM
To: Michael Berger
Cc: [email protected]
Subject: Re: [OSL | CCIE_RS] EBGP or IBGP
BGP is the only routing protocol where decision is made based on the
path attributes than Administrative distance which comes after Weight,
Local Pref, AS path, origin and Med in that order and AD is the
second last tie breaker in considering the best route.
Thanks
Suresh
On Thu, Apr 3, 2008 at 5:50 AM, Michael Berger <[EMAIL PROTECTED]> wrote:
> Hi Ajay,
>
> my understanding is that local pref comes much higher than eBGP vs iBGP
> in the decision process. So iBGP with higher local pref will be installed
in
> BGP table.
>
> Regards,
> Michael
>
>
> Ajay Chenampara wrote:
>
> >
> > Which is preferred: an eBGP learned route or an iBGP learnt route with
> higher value of local preference attribute?
> >
> > My understanding was that eBGP with the lower AD will always be chosen.
> But Jeff Doyle's Vol 2 routing policies section seems to indicate
> otherwise......
> >
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