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RE: BGP Question Concerning MED attribute posted 09/10/2003
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At 5:36 PM -0400 9/10/03, Nordhoff, Michael G. (US - Hermitage) wrote:
Thanks for the clarification.  The intent is to minimize the potential for
asymmetrical routing that will most likely occur when the 2 ISPs I will be
peering with connect to the same peering points beyond their AS's.  For
example, I peer with Provider A and Provider B.  Provider A and Provider B
both peer with Provider C.  My intent was to ultimately influence the path
Provider C takes back to me.  Perhaps it was a good idea but not easily
implemented...

Thanks again for the feedback...

- MN

Even if you could send MEDs, the ISP isn't required to use them. The reality is that particular routing inside an ISP will only happen if you have a business agreement to do so. The way to signal traffic under such an agreement is with a mutually-agreed-to community.



-----Original Message----- From: Rob Laidlaw [mailto:laidlaw@xxxxxxxxxxxx] Sent: Wednesday, September 10, 2003 4:11 PM To: Nordhoff, Michael G. (US - Hermitage); ccielab@xxxxxxxxxxxxxx Subject: Re: BGP Question Concerning MED attribute

Routing TCP/IP vol.2, page 246.

     "When a BGP speaker learns a route from a peer, it can pass the route's
MED to any IBGP peers, but not to EBGP peers.  As a result, the MED has
relevance only between neighboring autonomous systems."

I don't believe there is anything you can do to pass this attribute.  Why do
you want to pass this information?  Its function is to allow a customer with
multiple connection to the same provider to pick their entry point.  If BGP
passed this info to the ISP's external neighbors, then the customer would be
able to choose paths in the ISP's network rathern then the ISP.

Rob

----- Original Message -----
From: "Nordhoff, Michael G. (US - Hermitage)" <mnordhoff@xxxxxxxxxxxx>
To: <ccielab@xxxxxxxxxxxxxx>
Sent: Wednesday, September 10, 2003 3:39 PM
Subject: BGP Question Concerning MED attribute


A quick question concerning the BGP MED attribute...



 Is it normal behavior for a BGP peer to NOT pass along MED information for
 prefixes learned from BGP peers in different AS's?  My scenario is this...
 R1 is attached to R2.  R3 is attached to R4.  R2 and R4 are attached to
R5.
 R1 advertises a prefix to R2 via EBGP as does R3 to R4.  R2 and R4 then
 advertise these prefixes to R5 via EBGP.  The MED attributes added by R1
and
 R3 are received by R2 and R4, respectively.  However, these MEDs are not
 being passed along to R5.  What is the reason for this behavior and what
can
be done to circumvent it?



Thanks in advance!



Mike Nordhoff - CCNP,CCDP

Deloitte & Touche, National WAN Services



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