Refer to the exhibit. Autonomous systems 200 and 300 have EBGP sessions established
with their directly connected routers in autonomoussystem 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? Select the best response.
A.
IBGP session must be established between routers R1and R3, and the synchronization
must be turned on.
B.
IBGP session must be established between routers R1and R3, and the synchronization
must be turned off.
C.
IBGP session must be established between routers R1R2 and R2 R3, and the
synchronization must be turned on.
D.
IBGP session must be established between routers R1R2 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-hop-self 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 soit 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 R1′s 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, R3′s IGP routing table does not know how to reach 1.1.1.0 so R3
will not advertise this network to RT-B. This routeis 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 connectionswith 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 canbe 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
