Subject: FW: BGP anomalous Behaviour

From: Mareline Sheldon <[email-masked]>
Date: Tue, 17 Jun 2003 21:14:26 -0700 (PDT)

Found this interesting mail from the archives.

Would love to hear from others on this.

Regards,
Mareline S.
----- Original Message ----- 
From: "Tulip Rasputin" <[email-masked]>
To: <[email-masked]>
Sent: Tuesday, May 20, 2003 12:25 PM
Subject: BGP anomalous Behaviour

> 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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