bgp path attribute, optional non -transitive

[sarahr202]

Hi everybody

My book says  optional non-transitive path attribute  such  as MED may or may not be implemented by BGP software implementation.  Moreover if a bgp router does not recognize it, then bgp router  should not pass it on.

That means there are certain features such as MED which are optional and might not be implemented by all bgp software implementation.

Let say  we want to control how the inbound traffic from AS2 enters our AS1.   But before we do that don't we need to ensure if AS2's bgp implementation supports MED?

thanks and have a day.

[rais]

As you have mentioned, BGP may or may not implement optional features. We should ensure with our neighbors if they do or do not support a given attribute. We can control inbound traffic using other means such as AS-prepends.

If received over External BGP (EBGP), the MULTI_EXIT_DISC attribute MAY be propagated 
over Internal BGP (IBGP) to other BGP speakers within the same AS (see also 9.1.2.2).  
The MULTI_EXIT_DISC attribute received from a neighboring AS MUST NOT be propagated to 
other neighboring ASes....per RFC 4451

Thanks.

[Reza Sharifi · Accepted solution]

Hi Sarah,

All BGP implementation support med.  And yes, you are correct, we use MED to influence incoming traffic from another AS. So, if you have 2 ASN (1 and 2) and you have 2 links from AS1 to AS2 and AS 1 is advertising 172.16.1.0/24 over both links to you, you can manipulate MED (or know as metric) to prefer one link  over the other.  By default all MEDs are 0 which is the highest preference.   The range is from 0 to 4294967295.  It is also a none-transitive , which means AS 2 will not pass the same MED id it has learned form AS 1 to AS 3.

HTH

[sarahr202]

Hi Reza

"All BGP implementation support med"

Then it(MED) should fall under the category of either well-known mandatory or well-known discretionary.

thanks and have a day

[Reza Sharifi · Accepted solution]

Hi Sarah,

By noting all BGP implementation support MED, I meant to say that when you turn on BGP, (regardless of which vender's router you are using) they all support MED. They all actually support all the BGP attributes.  Now, as you already know some are well-knows mandatory and some are well-know discretionary.

HTH

[sarahr202]

Thanks Reza. I got it now.