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BGP MED

Answered Question
Nov 12th, 2010
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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

Correct Answer by Jon Marshall about 6 years 9 months ago

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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Mahesh Gohil Fri, 11/12/2010 - 23:01
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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

shelley_wang Sat, 11/13/2010 - 16:46
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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

Correct Answer
Jon Marshall Sat, 11/13/2010 - 17:09
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    2017 LAN, WAN

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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