5-5In fact, all the BGP route attributes can be classified into the following four categories.z Well-known mandatory attributes, which can be identified by any BGP routers. Route attributes ofthis type are carried in Update messages. Without these attributes, routing information goes wrong.z Well-known discretionary attributes, which can be identified by any BGP routers. An Updatemessage can travel with or without this type of attributes.z Optional transitive attributes, which can be transmitted among ASs. Although attributes of this typemay not be supported by any BGP routers, routes with them can still be received and be forwardedto BGP speakers.z Optional non-transitive attributes, which is dropped on the BGP routers that do not support them. Inthis case, the attributes are not forwarded to other BGP routers.Table 5-1 lists basic BGP route attributes and the categories they belong to.Table 5-1 BGP route attributes and the corresponding categoriesBGP route attribute CategoryORIGIN Well-known mandatoryAS_PATH Well-known mandatoryNEXT_HOP Well-known mandatoryLOCAL_PREF Well-known discretionaryATOMIC_AGGREGATE Well-known discretionaryAGGREGATOR Optional transitiveCOMMUNITY Optional transitiveMULTI_EXIT_DISC(MED) Optional non-transitiveORIGINATOR_ID Optional non-transitiveCLUSTER_LIST Optional non-transitivePrimary route attributes1) ORIGINThe ORIGIN attribute holds the source of routing information. It indicates how a route becomes a BGProute. The following describes the possible values of the ORIGIN attribute.z IGP: BGP routes with their ORIGIN attributes being IGP have the highest priority. They are addedto the BGP routing table through the network command.z EGP: BGP routes with their ORIGIN attributes being EGP are obtained through EGP.z Incomplete: BGP routes with their ORIGIN attributes being Incomplete have the least priority. Thisvalue does not indicate that the BGP route is unreachable; it means the source of the BGP routecannot be determined. The ORIGIN attribute of a BGP route imported through the import-routecommand is Incomplete.2) AS_PATHThe AS_PATH attribute holds the numbers of all the ASs that a route passes from the source to thedestination. AS numbers in this attribute are in the order the route passes the ASs. Before a BGPspeaker advertises a route to the BGP speakers of other ASs, it adds the local AS number to the headof the AS number queue in the AS_PATH attribute. According to the AS_PATH attribute of a receivedBGP route, a router can retrieve the information about the ASs the route passes. In AS_PATH attribute,
