Question about BGP MED attribute Optional , Non-transitive.

[mhiyoshi]

Dear all,

In my understanding , BGP MED attribute is Optional, Non-transitive so basically it can be received from eBGP peer at once but it is NOT transfer with the received MED value, however the received router can change the BGP attribute policy to transfer to other eBGP peer with received MED value is it possbile?

I think if the received router configures route-map with match ip address acl [routes] and set metric [same value] , it can be possible however it is NOT flexible bacause manually setting the BGP routes and metric.

So my conslution is just to modify AS-PATH attiribute like AS-PATH prepend can be possible for eBGP routing control because it's Transitive, Well-known, so basically send it end-to-end eBGP peer.

I appreciate to let me know if there is other good soltuion.

Best Regards,

Masanobu Hiyoshi

[Pavel Tarakanov · Accepted solution]

> BGP MED attribute is Optional, Non-transitive so basically it can be received from eBGP peer at once but it is NOT transfer with the received MED value

When a BGP speaker learns a route from a peer, the route MED is passed to other interior BGP (iBGP) peers, but not to exterior BGP (eBGP) peers.

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/217973-understand-bgp-med-attribute.html

> the received router can change the BGP attribute policy to transfer to other eBGP peer with received MED value is it possbile?

Yes

> So my conslution is just to modify AS-PATH attiribute like AS-PATH prepend can be possible for eBGP routing control because it's Transitive, Well-known, so basically send it end-to-end eBGP peer.

It will allow to propogate inforamtion further than adjecent AS. MED will affect only adjecent AS.

[paul driver · Accepted solution]

Hello

•  Well-known Mandatory: Recognised and Included in all BGP update messages.
• Well-known Discretionary: Recognised and may or not include in BGP update messages
• Optional Transitive: Even if not supported it still need to accept and send in update message.
• Optional Non-transitive: Can be ignored and not advertise to peers.

Well-known mandatory path attributes must be included with every prefix advertisement;
ORIGIN 
AS_PATH 
NEXT_HOP 

Well-known discretionary path attributes may or may not be included with a prefix advertisement.
LOCAL_PREF 
ATOMIC_AGGREGATE 

Optional path attributes do not have to be recognised by all BGP implementations. Optional
attributes can be set so that they are transitive and stay with the route advertisement from AS to AS.
AGGREGATOR 
COMMUNITY 

Optional non-transitive path attributes and cannot be shared from AS to AS.
MULTI_EXIT_DISC (MED) 
ORIGINATOR_ID 
CLUSTER LIST 

[MHM Cisco World · Accepted solution]

Simple words 

Transitive vs Non-transitive

Different is MED is pass only to one AS direct connect

As-path pass many AS

That big difference between two attributes 

MHM