Multicast and Routing Guide K/KA/KB.15.15

CAUTION:
• Using a routing policy can set preferences for routes matching it. Routes not matching it use
the default preferences.
• If other conditions are identical, the route with the smallest MED value is selected as the best
external route.
• Using the neighbor next-hop-self command can specify the router as the next hop for
routes sent to a peer. If BGP load balancing is configured, the router specifies itself as the
next hop for routes sent to a peer regardless of whether the neighbor next-hop-self
command is configured.
• In a “third party next hop” network, that is, a BGP router has two eBGP peers in a common
broadcast subnet, the BGP router does not specify itself as the next hop for routes sent to such
an eBGP peer, unless the neighbor next-hop-self command is configured.
• BGP checks if 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.
• You can specify a fake AS number to hide the real one. The fake AS number applies to routes
sent to eBGP peers only, that is, eBGP peers in other ASs can only find the fake AS number.
• The neighbor as-override command is used only in specific networking environments.
Inappropriate use of the command may cause routing loops.
Tuning and optimizing BGP networks
Prerequisites
BGP connections have been created.
Configuring a BGP keepalive interval and holdtime
After establishing a BGP connection, two routers send keepalive messages periodically to each
other to keep the connection. If a router receives no keepalive or update message from the peer
within the holdtime, it breaks the connection.
If two parties have the same timer assigned with different values, the smaller one is used.
Follow these steps to configure BGP keepalive interval and holdtime.
RemarksUse the command...To do...
configurationEnter global configuration context
bgp as-numberEnter BGP context
timers {keepalive-time}
{hold-time}
Configure the global keepalive interval
and holdtime
Optional. By default, the keepalive
interval is 60 seconds, and holdtime is
180 seconds.
neighbor {ip-address} timers
{keepalive-time} {hold-time}
Configure the keepalive interval and
holdtime for a peer
314 Border Gateway Protocol (BGP)