What are two mandatory BGP attributes? (Choose two.)

A.
route preference

B.
origin

C.
AS path

D.
originator

Explanation:
BGP Attributes

BGP attributes are a confusing array of information carried in a BGP update capable of indicating anything from path preference to various additional pieces of information about a route, either within an autonomous system or outside an autonomous system. There are four basic types of attributes:

* Well known mandatory attributes; these attributes must be recognized by all BGP speakers, and must be included in all update messages. Almost all of the attributes impacting the path decision process, described in the next section, are well known mandatory attributes.
* Well known discretionary attributes; these attributes must be recognized by all BGP speakers, and may be carried in updates, but are not required in every update.
* Optional transitive attributes; these attributes may be recognized by some BGP speakers, but not all. They should be preserved and advertised to all peers whether or not they are recognized.
* Optional non-transitive attributes; these attributes may be recognized by some BGP speakers, but not all. If an update containing an optional transitive attribute is received, the update should be advertised to peers without the unrecognized attributes.

Figure 1-8 illustrates the way in which attributes are included in a BGP update message.

AS Path

The AS_PATH is a well-known mandatory attribute and as described in the section BGP Path Vector Implementation earlier in this chapter, is the list of all autonomous systems the prefixes contained in this update have passed through. The local autonomous system number is added by a BGP speaker when advertising a prefix to an eBGP peer.

Origin Code

The ORIGIN is a well known mandatory attribute that indicates the origin of the prefix, or rather, the way in which the prefix was injected into BGP. There are three origin codes, listed in order or preference:

* IGP, meaning the prefix was originated from information learned from an interior gateway protocol
* EGP, meaning the prefix originated from the EGP protocol, which BGP replaced
* INCOMPLETE, meaning the prefix originated from some unknown source