Refer to the exhibit.
Autonomous systems 200 and 300 have EBGP sessions established with
their directly connected routers in autonomous system 100. IGP has been configured on all routers
in autonomous system 100 and they successfully exchange routing updates. Traffic originated in
autonomous system 200 cannot reach the destination autonomous system 300. What
configuration should be done on the routers in autonomous system 100 in order for the traffic
coming from autonomous system 200 to be forwarded to autonomous system 300?
A.
IBGP session must be established between routers R1 and R3, and the synchronization must
be turned on.
B.
IBGP session must be established between routers R1 and R3, and the synchronization must
be turned off.
C.
IBGP session must be established between routers R1 R2 and R2 R3, and the synchronization
must be turned on.
D.
IBGP session must be established between routers R1 R2 and R2 R3, and the synchronization
must be turned off.
E.
IBGP speakers within autonomous 100 must be fully meshed, and the synchronization must be
turned on.
F.
IBGP speakers within autonomous 100 must be fully meshed, and the synchronization must be
turned off.
Explanation:
The synchronization rule states that if an AS provides transit service to another AS, BGP should
not advertise a route until all of the routers within the AS have learned about the route via an IGP.
To understand why this rule exists, let’s take an example if this rule is not there.Suppose Rt-A wants RT-B to access its local LAN 1.1.1.0, so it advertises this network through
R1. R1 and R3 are running IBGP so R1 sends this update to R3 through R2 (using the next-hopself to use its own interface’s IP address). In turn, R3 announces to RT-B that it can reach 1.1.1.0
via R3.
Now Rt-B really wants to send traffic to 1.1.1.0 so it will send to R3. R3 does a look up and sees
that the network can be reachable via R1. It then does a lookup for R1s IP address and sees that
it is reachable via R2 -> so it forwards packets to R2. But R2, running IGP (like OSPF), does not
find an entry for 1.1.1.0 so R2 drops all the packets for that network – a black-hole is created!
That is why the BGP synchronization rule is born. With this rule, when R3 receives an
advertisement for 1.1.1.0 from R1, it adds that route to its BGP table and before sending
advertisement to RT-B, it first checks its IGP routing table to see whether an entry exists for that
route. In this example, R3s IGP routing table does not know how to reach 1.1.1.0 so R3 will not
advertise this network to RT-B. This route is only advertised to RT-B when IGP makes an entry in
the routing table for 1.1.1.0.
Well, now you understand the importance of BGP Synchronization rule but now I wish to explain
why this rule causes trouble in fully-meshed IBGP!Synchronization prevents fully-meshed IBGP from working properly. Because no IGP is running so
R3 cannot advertise any route to RT-B even if no black-hole exists in this topology.
Note: A “fully-meshed” can be a physical fully-meshed topology or a topology where all routers in
the same AS established IBGP connections with each other (although they do not need to be
directly connected). So in the topology above, the connection between R1 & R3 is represented by
a dashed line, which means it can be physically connected or not (but an IBGP connection must
be established on both routers).
Therefore if all routers in AS 100 is fully-meshed, the synchronization rule must be turned off.
