6-4-Selecting a BGP Path.ppt

Implementing BGP Selecting a BGP Path BGP Path Attributes BGP metrics are called path attributes. Characteristics of path attributes include: Well-known versus optional Mandatory versus discretionary Transitive versus nontransitive Partial Well-Known Attributes Well-known attributes Must be recognized by all compliant BGP implementations Are propagated to other neighbors Well-known mandatory attributes Must be present in all update messages Well-known discretionary attributes May be present in update messages Optional Attributes Optional attributes They are recognized by some implementations (could be private); but expected not to be recognized by all BGP routers. Recognized optional attributes are propagated to other neighbors based on their meaning. Optional transitive attributes If not recognized, marked as partial and propagated to other neighbors Optional nontransitive attributes Discarded if not recognized BGP Attributes BGP attributes include the following: AS path * Next-hop * Origin * Local preference MED Others * Well-known mandatory attribute AS Path Attribute A list of autonomous systems that a route has traversed: For example, on router B, the path to 192.168.1.0 is the AS sequence (65500, 64520). The AS path attribute is well-known, mandatory. Next-Hop Attribute The IP address of the next AS to reach a given network: Router A advertises network 172.16.0.0 torouter B in EBGP, with anext hop of 10.10.10.3. Router B advertises172.16.0.0 in IBGP torouter C, keeping 10.10.10.3as the next-hop address. The next-hop attribute is well-known, mandatory. Origin Attribute IGP (i) network command EGP (e) Redistributed from EGP Incomplete (?) Redistributed from IGP or static The origin attribute informs all autonomous systems in the internetwork how the prefixes were introduced into BGP. The origin attribute is well-known, mandatory. Example: Origin Attribute RouterA# show ip bgp BGP table version is 14, local router ID is 172.31.11.1 Status codes: s suppr