H3C S7500E Series Ethernet Switches Operation Manual

Operation Manual – IPv4 Routing
H3C S7500E Series Ethernet Switches Chapter 5 BGP Configuration
5-30
 Note:
z Using a routing policy can set preferences for routes matching it. Routes not
matching it use the default preferences.
z If other conditions are identical, the route with the smallest MED value is selected as
the best external route.
z Using the peer next-hop-local command can specify the router as the next hop for
routes to a peer/peer group. If BGP load balancing is configured, the router specify
itself as the next hop for routes to a peer/peer group regardless of whether the peer
next-hop-local command is configured.
z In a “third party next hop" network, that is , the two EBGP peers reside in a common
broadcast subnet, the BGP router does not specify itself as the next hop for routes
to the EBGP peer, unless the peer next-hop-local command is configured.
z In general, BGP checks whether the AS_PATH attribute of a route from a peer
contains the local AS number. If so, it discards the route to avoid routing loops.
z You can specify a fake AS number to hide the real one as needed. The fake AS
number applies to routes to EBGP peers only, that is, EBGP peers in other ASs can
only find the fake AS number.
z The peer substitute-as command is used only in specific networking environments.
Inappropriate use of the command may cause routing loops.
5.6 Tuning and Optimizing BGP Networks
This task involves the following parts:
1) Configure BGP timers
After establishing a BGP connection, two routers send keepalive messages periodically
to each other to keep the connection. If a router receives no keepalive message from
the peer after the holdtime elapses, it tears down the connection.
When establishing a BGP connection, the two parties compare their holdtimes, taking
the shorter one as the common holdtime.
2) Reset BGP connections
After modifying a route selection policy, you have to reset BGP connections to make the
new one take effect, causing short time disconnections. The current BGP
implementation supports the route-refresh capability. With this capability enabled on all
BGP routers in a network, when a policy is modified on a router, the router advertises a
route-refresh message to its peers, which then resend their routing information to the
router. Therefore, the local router can perform dynamic route update and apply the new
policy without tearing down BGP connections.
If a router not supporting route-refresh exists in the network, you must configure the
peer keep-all-routes command to save all route updates, and then use the refresh