From: ccie anees (ccieanees@yahoo.com)
Date: Mon Apr 16 2007 - 16:15:51 ART
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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