Router 1 is configured for BGP as dual-homed on the Cisco network. Which three BGP attributes are carried in every BGP update on this router (both IBGP and EBGP)? (Choose three.)
A.
origin
B.
router-ID
C.
AS-path
D.
local-preference
E.
next-hop
Explanation:
There are basically two major types of attribute:
i) Well Known.
ii) Optional.i)Well Known :
Well known attributes are must be recognized by each compliant of BGP implementations. Well known attributes are propagated to other neighbors also.
Further divided into :
1) Mandatory : It is BGP well known attributes. Mandatoryattributes are must be present in all update message passedbetween BGP peers. It is present in route description. Must be supported and propagated.
2) Discretionary: It is BGP well known attributes. Discretionary attributes may be present on update message. Must be supported; propagation optional.
ii)Optional :
Optional attributes are recognized by some implementation of BGP & expected that not recognized byeveryone. Optional attributes are propagated to theirneighbors based on the meanings.
Further divided into :
1) Transitive: Optional transitive attributes dont have to be supported, but must be passed onto peers. Marked as partial if unsupported by neighbor.
2) Non Transitive: Optional non-transitive attributes dont have to be supported, and can be ignored. Deleted if unsupported by neighbor.
Now the different BGP attributes are listed below:
1)Weight (Attribute Type Mandatory):
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.
2)Local preference (Attribute Type Discretionary ):
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 throughout 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.
3)AS path (Attribute Type Mandatory):
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.
4)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 the 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.
5)Multi-exit discriminator (Attribute Type – Non Transitive ):
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.
6)Next-hop (Attribute Type Mandatory):
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.
7)Community (Attribute Type – Transitive ):
The community attribute 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:
No-export: Do not advertise this route to EBGP peers.
No-advertise: Do not advertise this route to any peer.
Internet: Advertise this route to the Internet community; all routers in the network belong to it.
8)Atomic Aggregate (Attribute Type – Discretionary ):
Notes that route summarization has been performed.
9)Aggregator (Attribute Type – Transitive ):
Identifies the router and AS where summarization was performed.
10)Originator ID (Attribute Type – Non Transitive ):
Identifies a route reflector.
11)Cluster List (Attribute Type – Non Transitive ):
Records the route reflector clusters the route has traversed.