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BGP attributes

Good day,

Somebody can help me to explain why community is an optional transitive attribute ? Why Aggregator and atomic aggregate are in different mode respectively optional transitive and well-know discretionary ?

Any suggest will be appreciated.



Re: BGP attributes

Not all attributes may be supported by all BGP implementations (I am talking about different vendors). Communities are optional attribute, because they need not be implemented or supported by all vendors.

Why is it transitive ? If transitive bit is set, the attribute is passed from one BGP neighbor to another. The whole purpose of communities are to group routes so that policies can be applied to routes based on matching communities.

This is the general behaviour.

a. If the neighbor does not support OT attributes and it receives an OT, it will pass on the attribute to its neighbor and mark it as partial

b. If the neighbor receives an optional non transitive attribute it will delete the attribute if it doesnt implement it in its BGP protocol stack.

Similarly, Aggregator is also OT.

Atomic aggergate is set to well known, so it means all bgp implementations should support it. You cannot afford to lose path attribute of a route, due to aggregation. Hence this attribute should be supported by all impl. of bgp and propagated to other neighbors as well..


PS: Please remember to rate helpful replies!


Re: BGP attributes


Here's my understanding of the attributes you are interested in:


This attribute was not part of the original BGP spec (RFC1771). It was subsequently introduced therefore it was not a good idea to make it a well-known mandatory otherwise existing implementations would not work with new ones implementing communities. The reason it is transitive is because the recognition of the community is not critical to the operation of BGP. If a router within the path does not recognise it, it simply indicates this by setting the Partial bit - however, there is no reason it should not pass it on to other routers which may recognise it.


This is optional since it is only applicable to aggregated routes. In addition, it is a purely informative attribute which does not affect BGP operation. Therefore, there is no need to make it non-transitive.

Atomic Aggregate:

This is a well-known attribute because all routers in the path have to understand it, in order to prevent them from de-aggregating the route it is associated with. If a router does not recognise this attribute, it could very well de-aggregate the route, which could potentially result in routing loops. It is discretionary because it is not necessary for non-aggregate routes.

Hope that helps - pls rate the post if it does.


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