笔记丶TCPIPVolume2-BGP-20101214

 

The ORIGIN Attribute --
    ●IGP--BGP routes are given an origin of IGP if they are learned from an IGP routing table via the 'network' statement.
    ●EGP--The NLRI was learned from the Exterior Gateway Protocol.
    ●Incomplete--Routes that BGP learns through redistribution carry the incomplete origin attribute,because there is no way to determine the original source of the route.

The AS_PATH Attribute --
    ●An AS number is prepended to the AS_PATH only when the route is being advertised between EBGP peers.If the route is being advertised between IBGP peers-peers within the same autonomous system-no AS number is added.
    ○But,When influence the incoming traffic,can add multiple instances of its own AS number to the list sent to another AS(eg:(500,100,100,100)),The procedure of adding extra AS numbers to the AS_PATH is called AS path prepending.
    ●AS_PATH can avoid route loops. If a BGP router receives a route from an external peer whose AS_PATH includes its own AS number,the router knows that the route has looped.Such a route is dropped.

The NEXT_HOP Attribute --
    ●If the advertising router and receiving router are in different autonomous systems,the NEXT_HOP is the IP address of the advertising router's interface.
    ●If the advertising router and receiving router are in the same AS,and the NLRI of the update refers to a destination within the same AS,the NEXT_HOP is the IP address of the neighbor that advertised the route.
    ●If the advertising router ande the receiving router are internal peers and the NLRI of the update refers to a destination in a different AS,the NEXT_HOP is the IP address of the external peer from which the route was learned.

The LOCAL_PREF Attribute --
    ●This attribute is used only in updates between internal BGP peers.
    ●Default value is 100.The route with the highest LOCAL_PREF is selected.
    eg: There are two ISPs connected,LOCAL_PREF of routes which learned from ISP1 is 200,higher than ISP2(default 100).

The MULTI_EXIT_DISC Attribute --
    ●This optional nontransitive attribute is carried in EBGP updates and allows an AS to inform another AS of its preferred ingress points.
    ●MEDs are used only to influence traffic between two directly connected autonomous systems.
         ★If influence route preferences beyond the neighboring AS,can manipulate AS_PATH attribute.
    ●The lowest MED value is prefferred. (Default value is 0).
    ●MEDs from the two autonomous systems are not compared.MEDs are meant only for a single AS to demonstrate a degree of preference when it has multiple ingress points.

The ATOMIC_AGGREGATE and AGGREGATOR Attribute --
   
The COMMUNITY Attribute --
    ●INTERNET——All routes belong to this community by default.Does not have a value.
    ●NO_EXPORT(4294967041,or 0xFFFFFF01)——Routes received carrying this value cannot be advertised to EBGP peers or,if a confederation is configured,the routes cannot be advertised outside of the confederation..
    ●NO_ADVERTISE(4294967042,or 0xFFFFFF02)——Routes received carrying this value cannot be advertised at all,to either EBGP or IBGP peers.
    ●LOCAL_AS(4294967043,or 0xFFFFFF03)——Routes received carrying this value cannot be advertised to EBGP peersm,including peers in other autonomous systems within a confederation.

The ORIGINATOR_ID and CLUSTER_LIST Attribute --
    ●Used by route reflectors to prevent routing loops.