Motorola BSR 2000, Руководство по настройке и управлению

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BSR 2000 Configuration and Management Guide Release 1.0
12-26 MGBI 526360-001-00 Rev. B
Enabling and Disabling Synchronization
Synchronization is a feature of BGP that prevents a BGP speaker from advertising a
route before all routers within an AS have learned the route. Without synchronization,
traffic may be dropped as a result of intermediate non-BGP routers not having learned
routes when the AS provides transit service to other ASs.
Usually, a BGP speaker does not advertise a route to an external neighbor unless that
route is local or exists in the IGP. This feature allows routers within an AS to learn the
route before BGP makes it available to other ASs.The
no synchronization command
disables synchronization and allows the BGP router to advertise a network route
without waiting to learn it via IGP.
Figure 12-9 shows a situation that demonstrates the value of synchronization. Router
Los Angeles sends updates about network 192.56.0.0 to Router Boston. Router New
York receives updates about network 192.56.0.0 from Router Boston via IBGP.
Router New York wants to reach network 192.56.0.0 and sends traffic to Router
Albany. If Router Boston does not distribute network 192.56.0.0. into an IGP, Router
Albany cannot know that network 192.56.0.0 exists. Router Albany drops the packets
from Router New York destined for network 192.56.0.0. In addition, Router New
York advertises to AS 200 that it can reach 192.56.0.0 before Router Albany learns
about the network via IGP. This means that traffic coming from Router Chicago to
Router Albany through Router New York with a destination of 192.56.0.0 is dropped.
Synchronization solves this problem by not allowing BGP to advertise a route before
all routers within the same AS have learned about the route. In
Figure 12-9 , Router
New York waits to hear about network 192.56.0.0 before it sends an update to Router
Chicago.