From: Hunt Lee (ciscoforme3@xxxxxxxxxxxx)
Date: Tue Jul 30 2002 - 07:14:00 GMT-3
Brian is correct. One may also want to watch out for the shortest AS
Path, since it is higher in the decision process than EBGP over IBGP
(the route learnt via IBGP may have a shorter AS path than the one
via EBGP ;)
To skip the check of AS-PATH length, use:
bgp bestpath as-path ignore
Regards,
Hunt
--- Brian McGahan <brian@cyscoexpert.com> wrote: > Ted,
>
> As Chris stated, synchronization is the first requirement in the
> BGP decision process. After that, the process is as follows:
>
> Next-hop reachability
> Weight
> Local Preference
> AS-Path (shortest)
> Origin Code (EGP > IGP > Unknown)
> MED
> EBGP over iBGP routes
> Shortest internal path
> Router-ID (lowest)
>
> However, there is another criterion that is worth mentioning.
> Default local-preference for iBGP learned routes on a Cisco router
> is
> 100. Although local preference is higher in the decision process
> than
> EBGP over IBGP, this is not the default case. You must have a
> local-preference greater than the default (100) to choose the iBGP
> route
> over the EBGP route. Therefore if everything (except for the
> default
> local-pref of the iBGP route) is equal for two routes up to the
> EBGP
> over iBGP decision, the EBGP route will be chosen. Even though the
> iBGP
> route has a local-pref of 100, it chooses the EBGP route. Setting
> the
> iBGP route to have a local-pref of at least 101 will make it chose
> the
> iBGP route first.
>
> I have not seen this documented, and it may be version specific,
> so someone please correct me if I'm wrong.
>
> HTH
>
> Brian McGahan, CCIE #8593
> Director of Design and Implementation
> brian@cyscoexpert.com
>
> CyscoExpert Corporation
> Internetwork Consulting & Training
> http://www.cyscoexpert.com
> Voice: 847.674.3392
> Fax: 847.674.2625
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
> Behalf Of
> Chris Hugo
> Sent: Monday, July 29, 2002 3:53 PM
> To: Chris Hugo; Ted McDermott
> Cc: ccielab@groupstudy.com
> Subject: Re: BGP Route Preference Decision
>
> BTW,
> The word assumption was not to be meant as a put-down. I was trying
> to
> fit a reply post in my schedule and I was doing some other tasks
> and not
> thinking right.
> sincerely stated,
> chris hugo
>
> Chris Hugo <chrishugo@yahoo.com> wrote: Hi Ted,
> Before you make that assumption fix the sync issue :). Also take a
> look
> at Halabi's BGP (pg.152-159) bible which states their are three
> RIBs
> (RIB-IN, LOC-RIB, and RIB-OUT). BGP's input policy engine runs thru
> the
> Decision Process before the routes even touch the Loc-RIB (IP
> Routing
> Table).
> HTH and have fun with BGP,
> chris hugo
> Ted McDermott wrote: I'm listing a BGP and IP route table below for
> network
> 128.213.0.0. It appears that before the BGP
> decision-making process is entered, (i.e. next-hop
> reachable, then weight, then local-preference, etc.)
> the first criteria in the decision-making process is
> whether the route is learned via EBGP or IBGP, because
> of the administrative distance difference. Thus, if
> you have one route learned via EBGP and the other via
> IBGP, it will always take the EBGP, regardless of the
> other BGP parameters. Is that correct?
>
> Note the equal weight, local-preference, and the
> longer as-path of the preferred route.
>
> rtb#sho ip route 128.213.0.0
> Routing entry for 128.213.0.0/16
> Known via "bgp 100", distance 20, metric 0
> Tag 300, type external
> Last update from 192.208.10.5 13:55:00 ago
> Routing Descriptor Blocks:
> * 192.208.10.5, from 192.208.10.5, 13:55:00 ago
> Route metric is 0, traffic share count is 1
> AS Hops 4
>
> rtb>sho ip bgp 128.213.0.0
> BGP routing table entry for 128.213.0.0/16, version 8
> Paths: (2 available, best #1)
> Advertised to non peer-group peers:
> 203.250.13.41
> 300 500 400 200
> 192.208.10.5 from 192.208.10.5 (192.208.10.174)
> Origin IGP, localpref 100, valid, external,
> best, ref 2
> 200
> 128.213.63.2 from 203.250.13.41 (203.250.13.41)
> Origin IGP, metric 0, localpref 100, valid,
> internal, not synchronized, ref 2
>
>
This archive was generated by hypermail 2.1.4 : Sat Sep 07 2002 - 19:36:48 GMT-3