Users Guide

Router A, Router B, and Router C belong to AS 100, 200, and 300, respectively. Router A acquired Router B — Router B
has Router C as its client. When Router B is migrating to Router A, it must maintain the connection with Router C without
immediately updating Router C’s configuration. Local-AS allows Router B to appear as if it still belongs to Router B’s old
network, AS 200, to communicate with Router C.
The Local-AS does not prepend the updates with the AS number received from the EBGP peer if you use the no prepend
command. If you do not select no prepend, the default, the Local-AS adds to the first AS segment in the AS-PATH. If you use
an inbound route-map to prepend the AS-PATH to the update from the peer, the Local-AS adds first.
If Router B has an inbound route-map applied on Router C to prepend 65001 65002 to the AS-PATH, these events take place
on Router B:
● Receive and validate the update.
●
Prepend local-as 200 to AS-PATH.
● Prepend 65001 65002 to AS-PATH.
Local-AS prepends before the route map to give the appearance that the update passed through a router in AS 200 before it
reaches Router B.
Graceful restart
OS10 offers graceful restart capability for BGP in helper mode only.
A BGP router whose neighbor is restarting is called a "helper."
If graceful restart is enabled on the restarting router, during restart, the helper maintains the routes that it has learned from its
neighbor.
After the switch over, the graceful restart operation begins. Both routers reestablish their neighbor relationship and exchange
their BGP routes again. The helper continues to forward prefixes pointing to the restarting peer, and the restarting router
continues to forward traffic to its peers even though those neighbor relationships are restarting. When the restarting router
receives all route updates from all BGP peers that are graceful restart capable, the graceful restart is complete. BGP sessions
become operational again.
Configure Border Gateway Protocol
BGP is disabled by default. To enable the BGP process and start to exchange information, assign an AS number and use
commands in ROUTER-BGP mode to configure a BGP neighbor.
BGP neighbor
adjacency
changes
All BGP neighbor changes are logged
Fast external
fallover
Enabled
Graceful restart Disabled
Local preference 100
4-byte AS Enabled
MED 0
Route flap
dampening
parameters
● half-life = 15 minutes
● max-suppress-time = 60 minutes
● reuse = 750
● suppress = 2000
Timers
● keepalive = 60 seconds
● holdtime = 180 seconds
Add-path Disabled
796 Layer 3