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Examine the exhibit.

Which set of configuration commands will enable the BGP router RTR2 to announce the network prefix 1

You have determined that RTR2 is not advertising the CIDR summary address 192.168.0.0 to the other
routers in AS 65100.
Which set of configuration commands will enable the BGP router RTR2 to announce the network prefix
192.168.0.0/16 to the other routers in the AS 65100?

A. router bgp 65100
neighbor 172.16.1.2 remote-as 65100
neighbor 192.168.3.2 remote-as 65100
network 192.168.3.0

B. router bgp 65100
neighbor 172.16.1.2 remote-as 65101
neighbor 192.168.3.2 remote-as 65100
network 192.168.0.0

C. router bgp 65100
neighbor 172.16.1.2 remote-as 65100
neighbor 192.168.3.2 remote-as 65100
network 192.168.0.0 mask 255.255.0.0
ip route 192.0.0.0 255.0.0.0 null 0

D. router bgp 65100
neighbor 172.16.1.2 remote-as 65101
neighbor 192.168.3.2 remote-as 65100
network 192.168.0.0 mask 255.255.0.0
ip route 192.168.0.0 255.255.0.0 null 0

Explanation:
Issuing the following commands will cause RTR2 to advertise the CIDR block 192.168.0.0/16 to the other
routers by using BGP:
RTR2(config)# router bgp 65100
RTR2(config-router)# neighbor 172.16.1.2 remote-as 65101
RTR2(config-router)# neighbor 192.168.3.2 remote-as 65100
RTR2(config-router)# network 192.168.0.0 mask 255.255.0.0
RTR2(config-router)# ip route 192.168.0.0 255.255.0.0 null 0
The network command specifies the address that will be inserted into the BGP table. Without the mask
keyword, the classful network will be assumed. Because 255.255.0.0, or /16, is not the natural mask for any
Class C address, the mask keyword must also be specified. Thus, 192.168.0.0 and 255.255.0.0 identify the
desired address and mask of the 192.168.0.0/16 network prefix.

The router checks the IP forwarding table for an exact match before it advertises the route. Without a
matching entry in the IP forwarding table, that route will not be advertised. RTR2 must be able to advertise a
CIDR block and not the individual subnets. A static route is required because BGP requires that a match of
the network prefix be present in the forwarding table when using the network command with the mask
keyword. Therefore, to ensure an exact match for the identified prefix exists in the IP forwarding table, and
to ensure that the prefix will always be advertised, a static route for 192.168.0.0/16 to null 0 is also required.
The syntax for the network command is shown below:
network network-number [ mask network-mask ] [ route-map map-tag ]
The parameters are:
mask – This parameter is optional and identifies the network or subnetwork to advertise.
route-map – This parameter is optional and identifies a preconfigured route-map that will be used to filter
specific addresses from being advertised.
The following command set is missing the mask keyword in the network command and the command to
create a static route to null 0. The address used in the network command is also incorrect. It should
192.168.0.0:
router bgp 65100
neighbor 172.16.1.2 remote-as 65100
neighbor 192.168.3.2 remote-as 65100
network 192.168.3.0

The following command set is missing the mask keyword in the network command and the command to
create a static route to null 0:
router bgp 65100
neighbor 172.16.1.2 remote-as 65101
neighbor 192.168.3.2 remote-as 65100
network 192.168.0.0

The following command set uses an incorrect mask (255.0.0.0) in the command that creates the static
route to null 0. It should be 255.255.0.0:
router bgp 65100
neighbor 172.16.1.2 remote-as 65100
neighbor 192.168.3.2 remote-as 65100
network 192.168.0.0 mask 255.255.0.0
ip route 192.0.0.0 255.0.0.0 null 0

Objective:
Layer 3 Technologies
Sub-Objective:
Describe, configure, and verify BGP peer relationships and authentication
References:
Internetworking Case Studies > Using the Border Gateway Protocol for Interdomain Routing > Controlling
the Flow of BGP Updates > CIDR and Aggregate Addresses > Aggregation and Static Routes

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