1. Well Known
2. Optional
Well Known Attributes:
Well Known attributes are must be recognized by each compliant of BGP implementations. Well Known attributes are propagated to other neighbors also. This can be further divided into:
1. Mandatory:
It is BGP well known attributes. Mandatory attributes are must be present in all update message passed between BGP peers. It is present in route description. Must be supported and propagated.
There are four attributes under this category are as follows:
i) Weight:
Weight is a cisco defined attribute that is local to a router. The weight attribute is not advertised to neighboring routers. If the router learns about more than one route to the same destination, the route with the highest weight is preferred.
ii) AS Path:
When a route advertisement passes through an autonomous system, the AS number is added to an ordered list of AS numbers that the route advertisement has traversed.
iii) Origin:
The origin attribute indicates how BGP learned about a particular route. The origin attribute can have one of three possible values:
a) IGP:
The route is interior to the originating AS. This value is set when the network router configuration command is used to inject the route into BGP.
b) EGP:
The route is learned via Exterior Gateway Protocol (EGP).
c) Incomplete:
The origin of the route is unknown or is learned some other way. An origin of incomplete occurs when a route is redistributed into BGP.
iv) Next-hop:
The eBGP next-hop attribute is the IP address that is used to reach the advertising router. For eBGP peers, the next-hop address is the IP address of the connection between the peers.
2. Discretionary:
It is a BGP well Known attributes. Discretionary attributes may be present on update messages. Must be supported but propagation optional.
There are two attributes under this category are as follows:
i) Local Preference:
The local preference attribute is used to prefer an exit point from the local autonomous system. Unlike the weight attribute, the local preference attribute is propagated through-out the local AS. If there are multiple exit points from the AS, the local preference attribute is used to select the exit point for a specific route.
ii) Atomic Aggregate:
Notes that route summurization has been performed.
Optional Attributes:
Optional attributes are recognized by some implementation of BGP and expected that not recognized by everyone. Optional attributes are propagated to their neighbor based on the meanings. This can be further divided into:
1. Transitive:
Optional transitive attributes don't have to be supported, but must be passed onto peers. Marked as partial if unsupported by neighbor.
There are two attributes under this category are as follows:
The community attributes provides a way of grouping destinations, called communities, to which routing decisions (such as acceptance, preference and redistribution) can be applied. Route maps are used to set the community attribute. The predefined community attributes are as follows:
a) No-export:
Do not advertise this route to eBGP peers.
b) No-advertise:
Do not advertise this route to any peers.
c) Internet:
Advertise this route to the internet community; all routers in the network belong to it.
ii) Aggregator:
Identifies the router and AS where summarization was performed.
2. Non-Transitive:
Optional non-transitive attributes don't have to be supported, and can be ignored. Deleted if unsupported by neighbor.
There are three attributes under this category are as follows:
i) Multi-Exit Discriminator(MED):
The multi-exit discriminator(MED) or metric attribute is used as a suggestion to an external AS regarding the preferred route into the AS that is advertising the metric.
ii) Originator ID:
The originator ID attribute identifies a route reflector.
iii) Cluster List:
The cluster list attribute records the route reflector clusters the route has traversed.
