KB.15.15

Optional transitive Transitive attribute between ASs. A BGP router not

supporting this attribute can still receive routes with this

attribute and advertise them to other peers.

Optional non-transitive If a BGP router does not support this attribute, it will not

advertise routes with this attribute.

The category of each BGP path attribute is described in the following table.

Table 36 BGP path attributes

CategoryName

Well-known mandatoryORIGIN

Well-known mandatoryAS_PATH

Well-known mandatoryNEXT_HOP

Well-known discretionaryLOCAL_PREF

Well-known discretionaryATOMIC_AGGREGATE

Optional transitiveCOMMUNITY

Optional non-transitiveMULTI_EXIT_DISC (MED)

Optional non-transitiveORIGINATOR_ID

Optional non-transitiveCLUSTER_LIST

Using BGP path attributes

ORIGIN

ORIGIN is a well-known mandatory attribute that defines the origin of routing information, that is,

how a route became a BGP route. There are three types:

IGP Has the highest priority. Routes added to the BGP routing table using the network

command have the IGP attribute.

EGP Has the second highest priority. Routes obtained via EGP have the EGP attribute.

Incomplete Has the lowest priority. The source of routes with this attribute is unknown, which

does not mean such routes are unreachable. The routes that are redistributed

from other routing protocols have this attribute.

AS_PATH

AS_PATH is a well-known mandatory attribute. This attribute identifies the autonomous systems

through which routing information carried in the Update message has passed. When a route is

advertised from the local AS to another AS, each passed AS number is added into the AS_PATH

attribute, allowing the receiver to determine the ASs for routing back the message. The number of

the AS closest to the receiver’s AS is leftmost, as shown in Figure 56 (page 304).

BGP path attributes 303