R21xx-HP FlexFabric 11900 Layer 3 IP Routing Configuration Guide

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{ NO_EXPORT—Routes with this attribute cannot be advertised out of the local AS or out of the
local confederation, but can be advertised to other sub-ASs in the confederation. For
confederation information, see "Settlements for problems in large-scale BGP networks."
{ No_ADVERTISE—Routes with this attribute cannot be advertised to other BGP peers.
{ No_EXPORT_SUBCONFED—Routes with this attribute cannot be advertised out of the local AS
or other sub-ASs in the local confederation.
You can configure BGP community lists to filter BGP routes based on the BGP COMMUNITY
attribute.
• 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 switch supports the route target (RT) extended community attribute. For more information about
the RT attribute, see MPLS Configuration Guide.
BGP route selection
BGP discards routes with unreachable NEXT_HOPs. If multiple routes to the same destination are
available, BGP selects the best 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
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.