Subject: BGP anomalous Behaviour

From: Tulip Rasputin <[email-masked]>
Date: Tue, 20 May 2003 02:55:25 -0400 (EDT)

Hello,
I understand that this message is more pertinent to the BGP Working Group but since i am unable to
send mails to that WG i am posting it here - as its anyway related to routing.

Suppose a BGP Peer receives three UPDATEs for the same prefix (say a.b.c.d/mask) with the
following attributes:

Route 1:
AS_PATH 10 100
MED 10
NEXT_HOP NH1
calculated IGP cost to reach the next-hop NH1 is 6

Route 2:
AS_PATH 6 100
MED 1
NEXT_HOP NH2
IGP cost to reach NH2 is 5

Route 3:
AS_PATH 6 100
MED 0
NEXT_HOP NH3
IGP cost to reach NH3 is 12

Now this router starts running its decision process. I want to know if the final outcome of the
Decision Process depends upon the order in which the paths are processed.

Case 1: If the receiving router processes Paths 1,2 and 3 respectively in this order.

From 1 and 2 it will not compare the MEDs as they are from different ASes. Hence it looks at the
IGP cost which is less in case of 2. It thus selects Path 2 and discards 1.

Next it picks up 3. Here the AS is the same and hence compares the MED - and thus selects as Path
3 as the best one and uses this and this is the route which is propogated to everyone else.

Case II:
It processes as paths 2,3 and 1 respectively.

From 2 and 3 it compares the MED as they are from same AS. It selects 3 as the MED is small. Next
it compares this with 1 and selects the latter as the IGP cost to reach the NH is smaller there
(6<12).

Thus it now arrives at a different best path!!!

Please let me know if i am correct in my understanding.

Would appreciate the comments from the routing fraternity.

Thanks
Rasputin.

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