Last Updated on August 7, 2021 by Admin 3
You administer the network in the topology diagram.
All routers are running EIGRP. All interface delay values are set to their defaults. On each active interface, the ip route cache command and the no ip routecache cef command have been configured.
The variance 2 command has been issued on RouterA.
An IP address of 192.168.51.50 has been assigned to the FastEthernet 1/0 interface on RouterC.
You issue the show ip route 192.168.51.50 command on RouterA and receive the following output:
Which of the following is indicated by the asterisk in the output? (Select the best answer.)
- The path through RouterB is the active path for a new flow.
- The path through RouterB is the active path for a new packet.
- The path through RouterB is the active path for the next flow and the next packet.
- The path through RouterB is down, so the traffic can only flow through RouterD.
- The path through RouterD is the active path for a new flow.
- The path through RouterD is the active path for a new packet.
- The path through RouterD is the active path for the next flow and the next packet.
- The path through RouterD is down, so the traffic can flow through only RouterB.
The asterisk in the output of the show ip route 192.168.51.50 command indicates that the path through RouterD, which is connected to RouterA’s Serial 0/0 interface, is the active path for a new destinationbased flow to 192.168.51.50. RouterA is performing per-destination load balancing, which is a type of load balancing that decides where to send packets based on the destination address of the packet. Therefore, the path of the network flow, which is a sequence of packets, is decided based on the destination address.
Per-destination load balancing typically occurs when fast switching is enabled on a router. In this scenario, the ip routecache command has been issued on each active interface on RouterA. In addition, Cisco Express Forwarding (CEF), which can be configured to use either per-destination load balancing or perpacket load balancing, has been disabled on each interface. Therefore, RouterA is fastswitching packets.
When an asterisk appears in the output of the show ip route ip address command on a router that is performing perdestination load balancing, it is an indicator that the next flow to the destination specified by ipaddress will flow over the path indicated by the asterisk. Therefore, the following line of output indicates that traffic to the destination of 192.168.51.50 will flow through RouterD because RouterD is connected to RouterA’s Serial 0/0 interface:
* 192.168.1.2, from 192.168.1.2, 00:00:16 ago, via Serial0/0
In this scenario, Enhanced Interior Gateway Routing Protocol (EIGRP) on RouterA is performing unequalcost load balancing of traffic to the destination network of 192.168.51.0 by sending some traffic over RouterB and some traffic over RouterD. Packets for a different destination address on the same network as 192.168.51.50 in this scenario might flow over RouterB instead.
You can determine the number of destinations that are likely to flow over RouterB as compared to RouterD by examining the Traffic share count line of the output from the show ip route 192.168.51.50 command. In this scenario, 120 per-destination flows will flow over RouterD for every 97 per-destination flows that flow over RouterB. This number can also be represented as a traffic flow ratio of 97:120. A Cisco router calculates the traffic flow for a load-balanced path by dividing each path’s metric into the largest metric and rounding down to the nearest integer. That integer becomes the traffic share count value for that path. You can influence the traffic flow ratio of load-balanced paths by adjusting the delay values of interfaces along the paths.
Neither the path through RouterB nor the path through RouterD is load balancing packets, because RouterA is not process switching packets. However, if process switching were enabled, RouterA would load balance the path to the 192.168.51.0/24 network by varying the packets sent over each path instead of varying the destination based flows. Although perpacket load balancing guarantees that an equal amount of traffic will traverse each link, packets could arrive out of order at their destinations.
Neither the path through RouterB nor the path through RouterD is down. When a path learned by a dynamic routing protocol goes down, the path is removed from the routing table. For example, if the path through RouterD were down in this scenario, the output of the show ip route 192.168.51.50 command on RouterA would display only the path through RouterB, as shown in the following output: