TCP/IP Tutorial and Technical Overview - IBM Redbooks

if R3 needs to communicate with a device in AS 3. R3 passes the traffic to R2.
However, because R2 does not have any routes to AS 3 networks, the traffic is
dropped.
If R3 advertises the 10.0.0.0/8 network to AS 4, the problem continues. If AS 4
needs to communicate with a device in AS 3, the packets are forwarded from R5
to R3. R3 forwards the packets to R2 where they are discarded.
EBGP
BGP
R4
10.0.0.0/8
AS 3
BGP
R1
Figure 5-26 BGP synchronization
AS 1
R2
This situation is addressed by the synchronization rule of BGP. The rule states
that a transit AS will not advertise a route before all routers within the AS have
learned about the route. In this example, R3 will not advertise the existence of
the networks in AS 3 until R2 has built a proper routing table.
IBGP
There are three methods to implement the synchronization rule:
► Enable BGP on all devices within the transit AS. In this solution, R2 has an
IBGP session with both R1 and R3. R2 learns of the 10.0.0.0/8 network at the
same time it is advertised to R3. At that time, R3 announces the routes to its
peer in AS 4.
BGP
R3
BGP
R5
AS 4
EBGP
Chapter 5. Routing protocols 227