A network administrator creates a static route that points directly to a multi-access interface, instead of the next-hop IP address. The administrator notices that Cisco Express Forwarding ARP requests are being sent to all destinations. Which issue might this configuration create?
A. Low bandwidth usage
B. Low bandwidth usage
C. Cisco Express Forwarding routing loop
D. High bandwidth usage
E. IP route interference
Correct Answer: C
Explanation/Reference:
If the serial connection fails between R2 and R4, all traffic is routed with the use of a floating static route out Ethernet 0/0 because R2 has a floating static route that points to the multi-access interface
Ethernet 0/0, and not to the Ethernet IP address 192.168.12.1 of R1. Therefore, for all unknown destinations, Router R2 sends out an ARP request through the
Ethernet0/0 interface.
In this case, R2 has lost the more specific route to the 10.10.34.0 network. Therefore, when the data packet arrives for the hosts on this network, it generates an ARP request via the Ethernet interface. Since Proxy ARP is enabled by default on R1’s Ethernet interface and it has a default route that points to R3, it responds back with an Proxy ARP reply with its own MAC address. Hence, R2 sends all traffic to R1, and R1 forwards all traffic with the use of its default route (0.0.0.0/0) out to AS 12, and consequently to 10.10.34.4 via the Internet.
When R2 receives the proxy ARP reply from R1, it creates a /32 valid Cisco Express Forwarding adjacency that points out interface Ethernet 0/0.
This Cisco Express Forwarding entry does not age out until the proxy ARP router R1 is present on the Ethernet segment.
Thus, the /32 Cisco Express Forwarding entry continues to be used to Cisco Express Forwarding-switch the packets, even after the serial connection between R2 and R4 is back up and the routing table default route points out Serial 8/0 towards AS 10. The result is a routing loop.
http://www.cisco.com/c/en/us/support/docs/ip/express-forwarding-cef/26083-trouble-cef.html
