11-12-2010 09:34 PM - edited 03-06-2019 02:01 PM
Optional attribute does not have to be supported.
If a BGP router does not understand this optional attribute, this attribute is removed if it is non-transitive.
I do not understand why MED belongs to this category?
thx,
Jingyi
Solved! Go to Solution.
11-13-2010 05:09 PM
shelley_wang wrote:
I understand MED is optional attribute because it is not required to be included in BGP advertisements like 5 well-known attributes.
For non-transitive attribute, if a BGP router does not understand this optional attribute, this attribute is removed.
For routes with attributes ORIGINATOR_ID and CLUSTER_ID, receiving BGP router only passes them on to routers inside the same Confederation.
But I don’t understand why MED is non-transitive.
Thx,
jingyi
Jingyi
MED is non-transitive because it is only relevent to the AS you advertise it to. The AS that receives this MED advertisement should not pass this on to any other AS's because it has no meaning there.
Jon
11-12-2010 11:01 PM
Hi Jingyi,
It may be because it is 6th in path selection criteria.
The MEd is considered as weak metric. In contrast with weight and LP,a router will prefer a path with smallest MED
value but only if weight,LP AS-path and origin code attribute are equal.
Using MED may not yield expected result if neighboring AS modifies any of the stronger attribute values.
You can see here that first few attribute (weight,LP) are used for outgoing traffic and then few are left for incoming traffic. If i leave evrything on
my peer he is having power to do anthing with my AS.
So understand why those mandatory attribute (designer keep only required one) like...origin: required to know how route is originated
As-path: required for loop avoidance..and next-hop: required to know who is originator.
Regards
Mahesh
11-13-2010 04:46 PM
I understand MED is optional attribute because it is not required to be included in BGP advertisements like 5 well-known attributes.
For non-transitive attribute, if a BGP router does not understand this optional attribute, this attribute is removed.
For routes with attributes ORIGINATOR_ID and CLUSTER_ID, receiving BGP router only passes them on to routers inside the same Confederation.
But I don’t understand why MED is non-transitive.
Thx,
jingyi
11-13-2010 05:09 PM
shelley_wang wrote:
I understand MED is optional attribute because it is not required to be included in BGP advertisements like 5 well-known attributes.
For non-transitive attribute, if a BGP router does not understand this optional attribute, this attribute is removed.
For routes with attributes ORIGINATOR_ID and CLUSTER_ID, receiving BGP router only passes them on to routers inside the same Confederation.
But I don’t understand why MED is non-transitive.
Thx,
jingyi
Jingyi
MED is non-transitive because it is only relevent to the AS you advertise it to. The AS that receives this MED advertisement should not pass this on to any other AS's because it has no meaning there.
Jon
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