You have two routers in your OSPF area 0. Router 1 is connected to Router 2 via its Serial 1 interface, and to
your ISP via the Serial 0 interface. Router 1 is an ASBR.
After your assistant configures a default route on Router 1, you discover that whenever either router receives
packets destined for networks that are not in the routing tables, it causes traffic loops between the two routers.
To troubleshoot, you execute the show run command on Router 1. Part of the output is shown below:
<output omitted>
IP route 0.0.0.0 0.0.0.0 serial 1
Router ospf 1
Network 192.168.5.0 0.0.0.255 area 0
Default-information originate
Which command or set of commands should you execute on Router 1 to stop the looping traffic while
maintaining Router 2’s ability to send traffic to the Internet?
A.
Execute the no default-information originate command.
B.
Execute the no ip route 0.0.0.0 0.0.0.0 serial 1 command and then execute the ip route 0.0.0.0 0.0.0.0 serial
0 command.
C.
Execute the default-information originate always command.
D.
Execute the no network 192.168.5.0 area 0 command and then execute the network 192.168.5.0
255.255.255.0 area 0 command.
Explanation:
You should execute the no ip route 0.0.0.0 0.0.0.0 serial 1 command followed by the ip route 0.0.0.0 0.0.0.0
serial 0 command. The original configuration command was executed on the wrong interface on Router 1 by
your assistant. It should be executed on Serial 0, which is the connection to the ISP. The show run command
indicates that with the current configuration, if Router 2 receives a packet not in its table, it sends it to Router 1,
and then Router 1 sends it back out on Serial 1. This redirects the packet back to Router 2, and the loop
begins. By changing the configuration to Serial 0, Router 1 will start forwarding all traffic not in the routing table
to the ISP.
You should not execute the no default-information originate command. This command instructs Router 1 to
NOT inject the default route into area 0, which is the desired behavior. Running this command would stop the
loop, but would leave Router2 with no default route to send packets to the Internet.
You should not execute the default-information originate always command. It will not change the existing
looping behavior. The addition of the always parameter instructs Router 1 to inject a default route into area 0,
even if one does not exist on Router 1. This is unnecessary, since Router 1 does have a default route
configured, and will not change the existing looping behavior. To advertise a default route to other OSPF
routers, you should run this command:
Router1(config-router)#default information originate
You should not execute the no network 192.168.5.0 area 0 command followed by the network 192.168.5.0
255.255.255.0 area 0 command. There is nothing wrong with the original network command. Also, the network
192.168.5.0 255.255.255.0 area 0 command uses an incorrect mask type. The mask must be in the wildcard
format. Moreover, since it is incorrect, this will have the effect of disabling OSPF on the network connecting the
two routers.
Objective:
Routing Fundamentals
Sub-Objective:
Configure, verify, and troubleshoot single area and multi-area OSPFv2 for IPv4 (excluding authentication,
filtering, manual summarization, redistribution, stub, virtual-link, and LSAs)Cisco > Home > Support > Technology Support > IP > IP Routing > Configure > Configurations Examples and
Technotes > How OSPF Injects a Default Route into a Normal Area