BGP MED attribute

snarayanaraju · ‎11-10-2009

Hi Friends,

A basic conceptual query. Hope you experts donot bother to help me on this

I know that BGP MED is having OPTIONAL NON-TRANSITIVE attribute. But I also

know that MED is capable of moving to immediate next neighbour AS.

MED is a hint to external neighbors about the preferred path into an

autonomous system (AS) that has multiple entry points

But since it is passing to next AS, How it is catagorized as NON-TRANSITIVE

? Since it is seen in neighbour AS, I thought it should be TRANSITIVE . But

to my surprise it is OPTIONAL NON-TRANSITIVE

Please help me where i have understood wrongly

regards & Thanks in advance

sairam

Reza Sharifi · ‎11-10-2009

Hi Sairam,

Non-transitive means if you receive a MED form your neighbor AS, you will not propagate that MED to other ASs.

For example:

If AS-1 is connected to AS-2 and AS-2 is also connected to AS-3, When AS-2 receives a MED from AS-1 it will not advertise that MED to AS-3

HTH

Reza

simontibbitts · ‎11-10-2009

Hi Sairam.

I think you are getting confused with the term 'transitive' From the Jeff Doyle Routing TCP/IP Volume II book:

"If an optional attribute is TRANSITIVE, a BGP process should accept the path in which it is included, even if it doesn't support the attribute, and it should pass the path on to its peers.

If an optional attribute is NONTRANSITIVE, a BGP process that does not recognize the attribute can quietly ignore the Update in which it is included and not advertise the path to its other peer"

To summarize. TRANSITIVE and NONTRANSITIVE indicates what a router should do with the update if it does not understand it.

Simon

Tharak Abraham · ‎11-11-2009

Hi Sairam,

The MED attribute RECIEVED from a neighboring AS WILL NOT be propagated to other neighboring ASes and thats why its referred to as Optional-Non transitive and not because of the reason that an AS/router advertises this attribute.

HTH