NOTE: When you create an address family for a VRF, synchronization is automatically
disabled for that address family.
If synchronization is enabled and if redistribution is configured for the networks in Figure
36 on page 131, router NY checks its IGP routing table for a route to 192.56.0.0/16 when
it learns about the prefix from the IBGP session with router Boston. If the route is not
present, the prefix is not reachable through router Albany, so router NY does not advertise
it as available. Router NY keeps checking its IGP routing table; if the route appears, router
NY knows that it can pass traffic to the prefix and advertises the route by means of EBGP
to router Chicago.
In practice, service providers rarely redistribute BGP into an IGP because existing IGPs
cannot handle the full Internet routing table (about 100,00 routes). Instead, all routers
in an AS typically run BGP; in these cases it is advisable to turn synchronization off
everywhere.
Disabling Synchronization
Because the routes learned by means of EBGP are extensive, redistributing those routes
into your IGP consumes processor and memory resources. You can disable synchronization
if your AS does not pass traffic from one AS to another or if all the transit routers in your
AS run BGP. Figure 37 on page 132 shows the same configuration as in the previous
example, except that all the routers in AS 100 now run IBGP. As a result, all the routers
receive updates learned by the area border routers from external BGP speakers.
Figure 37: Disabling Synchronization
If synchronization is disabled, a BGP speaker propagates a BGP route learned from a
peer only if it is the best route to the prefix in the IP routing table. However, the speaker
does advertise the routes that it originates.
The following commands show how to configure routers Boston, NY, and Chicago (shown
in Figure 37 on page 132) with synchronization disabled for routers NY and Boston. The
no synchronization command enables router NY to put the route to 192.56.0.0/16 in its
IP routing table and advertise it to router Chicago without learning about 192.56.00/16
from router Albany. The command also enables router Boston to put the route to
Copyright © 2010, Juniper Networks, Inc.132
JunosE 11.2.x BGP and MPLS Configuration Guide
To Next Page
Loading...
Need help?
General
