R0106-HP MSR Router Series Layer 3 - IP Routing Configuration Guide(V7)

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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 next hop metric.
9. The route with the shortest CLUSTER_LIST.
10. The route with the smallest ORIGINATOR_ID.
11. The route advertised by the router with the smallest router ID.
12. 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 follow these rules for route advertisement:
• When multiple feasible routes to a destination exist, BGP advertises only the optimal route to its
peers. If the advertise-rib-active command is configured, BGP advertises the optimal route in the IP
routing table. If not, BGP advertises the optimal route in the BGP routing table.
• BGP advertises only routes that it uses.
• BGP advertises routes learned from an EBGP peer to all BGP peers, including both EBGP and IBGP
peers.
• BGP advertises routes learned from an IBGP peer to EBGP peers, rather than other IBGP peers.