Huawei S6700 Series Configuration Manual page 383

S6700 Series Ethernet Switches
Configuration Guide - IP Routing
7.
8.
9.
10. Prefers the route advertised by the switch with the smallest router ID.
11. Prefers the route learned from the peer with the smallest address if the IP addresses of peers
Policies for BGP Route Advertisement
On the S6700, BGP advertises routes based on the following policies:
l
l
l
l
l
l
Routing Selection Policies for Load Balancing
In BGP, the next-hop address of a generated route may not be the address of the peer that is
directly connected to the local switch. One common scenario is that the next hop is not changed
when a route is advertised between IBGP peers. Therefore, before forwarding a packet, the
switch must find a directly reachable address, through which the packet can reach the next hop
specified in the routing table. In this process, the route to the directly reachable address is called
a dependent route. BGP routes depend on these dependent routes for packet forwarding. The
process of finding a dependent route based on the next-hop address is called route iteration.
The S6700 supports iteration-based BGP load balancing. If load balancing is configured for a
dependent route (assume that there are three next-hop addresses), BGP generates the same
Issue 01 (2012-03-15)
l After the deterministic-med command is run, routes are not selected in the sequence
in which routes are received.
Prefers EBGP routes over IBGP routes.
EBGP is higher than IBGP, IBGP is higher than LocalCross, and LocalCross is higher than
RemoteCross.
If the ERT of a VPNv4 route in the routing table of a VPN instance on a PE matches the
IRT of another VPN instance on the PE, the VPNv4 route will be added to the routing table
of the second VPN instance. This is called LocalCross. If the ERT of a VPNv4 route from
a remote PE is learned by the local PE and matches the IRT of a VPN instance on the local
PE, the VPNv4 route will be added to the routing table of that VPN instance. This is called
RemoteCross.
Prefers the route with the lowest IGP metric to the BGP next hop.
NOTE
Assume that load balancing is configured. If the preceding rules are the same and there are multiple
external routes with the same AS_Path, load balancing will be performed based on the number of
configured routes.
Prefers the route with the shortest Cluster_List.
NOTE
If routes carry the Originator_ID, the originator ID is substituted for the router ID during route
selection. The route with the smallest Originator_ID is preferred.
are compared in the route selection process.
When there are multiple active routes, the BGP speaker advertises only the optimal route
to its peer.
The BGP speaker advertises only the preferred routes to its peer.
The BGP speaker advertises the routes learned from EBGP peers to all BGP peers
(including EBGP peers and IBGP peers) except the peers that advertise these routes.
The BGP speaker does not advertise the routes learned from IBGP peers to its IBGP peers.
The BGP speaker advertises the routes learned from IBGP peers to its EBGP peers.
The BGP speaker advertises all preferred BGP routes to the new peers when peer
relationships are established.
Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
7 BGP Configuration
364