From: rob@warnerbeach.com
Date: Wed Apr 18 2007 - 13:09:53 ART
Hi Anees,
The no bgp always-compare med only disables the comparison for paths
from neighbours in DIFFERENT AS
During the best-path selection process, MED comparison is done only
among paths from the SAME AS
The bgp always-compare-med command is used to change this behavior by
enforcing MED comparison between all paths, REGARDLESS of the
autonomous system from which the paths are received.
The bgp deterministic-med command can be configured to enforce
deterministic comparison of the MED value between all paths received
from within the same autonomous system.
So your phase 3 scenario you have disabled comparisons for different
AS and yet both routes originate from within the same AS so med is
used before lowest router id in the decision making process.
Make sense?
HTH
Rob CCIE(R&S)
Quoting ccie anees <ccieanees@yahoo.com>:
> Hi,
>
> I don't understand even after reading the Cisco documents
>
> http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094934.shtml
>
> http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094925.shtml
>
> My topolgy
>
>
> ----------- R2 AS 65502
> |
> AS 65501 R1------ ---------- R3 AS 65503
> |
> ----------
> R4 AS 65504
> From R1:
>
> router bgp 65501
> no synchronization
> bgp router-id 1.1.1.1
> bgp log-neighbor-changes
> neighbor 100.100.102.2 remote-as 65502
> neighbor 100.100.103.3 remote-as 65503
> neighbor 100.100.104.4 remote-as 65503
> no auto-summary
> !
>
> Case 1:
> **********
> R1#show ip bgp 200.200.200.0
> BGP routing table entry for 200.200.200.0/24, version 2
> Paths: (3 available, best #1, table Default-IP-Routing-Table)
> Advertised to update-groups:
> 1
> 65502
> 100.100.102.2 from 100.100.102.2 (2.2.2.2)
> Origin IGP, metric 100, localpref 100, valid, external, best
> 65503
> 100.100.104.4 from 100.100.104.4 (4.4.4.4)
> Origin IGP, metric 25, localpref 100, valid, external
> 65503
> 100.100.103.3 from 100.100.103.3 (3.3.3.3)
> Origin IGP, metric 50, localpref 100, valid, external
> R1#
> Case 1 is fine. It works as it suppose to. The route with the
> lowest router ID becomes the best.
>
> Case 2
> #######
> router bgp 65501
> bgp always-compare-med
> !
> R1#show ip bgp 200.200.200.0
> BGP routing table entry for 200.200.200.0/24, version 2
> Paths: (3 available, best #1, table Default-IP-Routing-Table)
> Advertised to update-groups:
> 1
> 65502
> 100.100.102.2 from 100.100.102.2 (2.2.2.2)
> Origin IGP, metric 100, localpref 100, valid, external
> 65503
> 100.100.104.4 from 100.100.104.4 (4.4.4.4)
> Origin IGP, metric 25, localpref 100, valid, external, best
> 65503
> 100.100.103.3 from 100.100.103.3 (3.3.3.3)
> Origin IGP, metric 50, localpref 100, valid, external
>
> Great., It works as it supopose to. But I have issues with the third one.
>
> Case 3:
> *********
> router bgp 65501
> no bgp always-compare-med
>
> R1#show ip bgp 200.200.200.0
> BGP routing table entry for 200.200.200.0/24, version 2
> Paths: (2 available, best #1, table Default-IP-Routing-Table)
> Advertised to update-groups:
>
> 65503
> 100.100.104.4 from 100.100.104.4 (4.4.4.4)
> Origin IGP, metric 25, localpref 100, valid, external, best
> 65503
> 100.100.103.3 from 100.100.103.3 (3.3.3.3)
> Origin IGP, metric 50, localpref 100, valid, external
>
> Here it is comparing the MED value. Is it the default behavior? Do
> I suppose to add bgp deterministic-med to have the BGP to use MED?
> how come it is choosing the best path based on MED without the bgp
> deterministic-med?
>
> Also If I configure bgp always-copare-med, I don;t need to use bgp
> deterministic-med. Am i correct?
>
> Thanks,
>
> Anees.
>
>
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