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Extended community attribute
To meet new demands, BGP defines the extended community attribute. The extended
community attribute has the following advantages over the COMMUNITY attribute:
Provides more attribute values by extending the attribute length to eight bytes.
Allows for using different types of extended community attributes in different scenarios to
enhance route filtering and control and simplify configuration and management.
The device supports the route target and Site of Origin (SoO) extended community attributes.
For information about route target, see
MPLS Configuration Guide
.
The SoO attribute specifies the site where the route originated. It prevents advertising a route
back to the originating site. If the AS-path attribute is lost, the router can use the SoO attribute to
avoid routing loops.
The SoO attribute has the following formats:
16-bit AS number:32-bit user-defined number. For example, 100:3.
32-bit IP address:16-bit user-defined number. For example, 192.168.122.15:1.
32-bit AS number:16-bit user-defined number
, where the minimum value of the AS number
is 65536. For example, 65536:1.
BGP route selection
BGP discards routes with unreachable NEXT_HOPs. If multiple routes to the same destination are
available, BGP selects the optimal route in the following sequence:
1.
The route with the highest Preferred_value.
2.
The route with the highest LOCAL_PREF.
3.
The route generated by the
network
command, the route redistributed by the
import-route
command, or the summary route in turn.
4.
The route with the shortest AS_PATH.
5.
The IGP, EGP, or INCOMPLETE route in turn.
6.
The route with the lowest MED value.
7.
The route learned from EBGP, confederation EBGP, confederation IBGP, or IBGP in turn.
8.
The route with the smallest IGP metric.
9.
The route with the smallest recursion depth.
10.
If all routes are received from EBGP peers and the peers have different router IDs, the route
that used to be an optimal route becomes the optimal route.
11.
The route advertised by the router with the smallest router ID.
If one of the routes is advertised by a route reflector, BGP compares the ORIGINATOR_ID of
the route with the router IDs of other routers. Then, BGP selects the route with the smallest ID
as the optimal route.
12.
The route with the shortest CLUSTER_LIST.
13.
The route advertised by the peer with the lowest IP address.
The CLUSTER_IDs of route reflectors form a CLUSTER_LIST. If a route reflector receives a route
that contains its own CLUSTER ID in the CLUSTER_LIST, the router discards the route to avoid
routing loops.
If load balancing is configured, the system selects available routes to implement load balancing.
BGP route advertisement rules
BGP follows these rules for route advertisement:
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