BGP MED question

Unanswered Question
May 3rd, 2007
User Badges:

Multi-exit Discriminator (MED):

I know MEDs are propagated between autonomous systems and not propagated into a third autonomous system.


My question is: Are MEDs also propagated within the same autnomous system?

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Loading.
mohammedmahmoud Thu, 05/03/2007 - 14:08
User Badges:
  • Green, 3000 points or more

Hi,


MED = Multi Exit Discriminator

The Local AS advertise it to the remote AS in order to choose between multiple routes to enter the local AS

MED is known as external metric  passed between ASs but not forwarded to third AS (the neighbor AS uses it and never forward it to other ASs) accordingly it has no significance inside the local AS (but it is propagated as a PA).



HTH, please rate if it does help,

Mohammed Mahmoud.


sundar.palaniappan Thu, 05/03/2007 - 14:31
User Badges:
  • Green, 3000 points or more

Paras,


The answer to your question is Yes.


From section 1.1 of RFC2026;


If received over EBGP, the MULTI_EXIT_DISC attribute MAY be propagated over IBGP to other BGP

speakers within the same AS. An MED attribute received from a neighboring AS MUST NOT be propagated to other neighboring autonomous systems.


HTH


Sundar

pkapoor Thu, 05/03/2007 - 14:40
User Badges:

Thanks Sundar.


Let me clarify my question a bit further. Let's say Router_A in AS_100 advertises to Router_X in AS_200 that the MED for its AS is 10. Further, let's say that Router_B from AS_100 advertises to Router_Y in AS_200. However, it says that its MED is 5. My question is do Router_A and Router_B (which are both from AS_100) advertise to each other what MED they are going to use to advertise to AS_200?


Futher, how do they calculate the MED? Or is it something that the admin configures on Router_A and Router_B in order to influence which router will be used by AS_200 to ingress into AS_100?

Harold Ritter Thu, 05/03/2007 - 14:54
User Badges:
  • Cisco Employee,

You would normally set the MED using a outbound route-map to the router in the neighboring AS. Router A and B each send their MED separately. Routers in AS 200 select the best MED (i.e. the best exit point towards AS100).


The other way to set the MED is to automatically convert the IGP metric into a MED that will be advertised to the neighboring AS.


Hope this helps,

sundar.palaniappan Thu, 05/03/2007 - 15:08
User Badges:
  • Green, 3000 points or more

Paras,


As hritter pointed out MED is set using a outbound route-map on the router in the neighboring AS. In your case, both routerA & routerB would be sending the prefix with their own configured metric(s) to AS200. The routers in AS200 would choose the path with the lower metric to get to AS100 if the path selection gets to the MED level.


Here's a nice little example that should you a basic idea of how MED works.


http://cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094934.shtml


HTH


Sundar



Harold Ritter Thu, 05/03/2007 - 14:32
User Badges:
  • Cisco Employee,

I think your question is, once the neighbor in the remote AS receives the MED, does it propagate it to iBGP neighbors. The answer is yes.


Hope this helps,

Actions

This Discussion