EIGRP Questions 2
When a router has no EIGRP feasible successor and the successor route to that destination network is in active status (the successor route is down, for example) a route recomputation occurs. A route recomputation commences with a router sending a query packet to all neighbors. Neighboring routers can either reply if they have feasible successors for the destination or optionally return a query indicating that they are performing a route recomputation. While in Active state, a router cannot change the next-hop neighbor it is using to forward packets. Once all replies are received for a given query, the destination can transition to Passive state and a new successor can be selected.
From the output we see a line “Gateway of last resort is 192.168.4.1 to network 0.0.0.0”. Gateway of last resort refers to the next-hop router of a router’s current default route. Therefore all the traffic through this router to destination networks not matching any other networks or subnets in the routing table will be sent to 192.168.4.1 (which is on Serial2) -> packet destined for 192.0.2.156 (or an unknown destination) will be forwarded via Serial2.
An weird thing in the output above is the missing of the asterisk mask (*) which represents for the candidate default route. To set the “Gateway of last resort is 192.168.4.1 to network 0.0.0.0” as the output above we can use these commands:
ip route 0.0.0.0 0.0.0.0 192.168.4.1
ip default-network 192.168.4.0
But these commands will create an static routing in the routing table with an asterisk mask. Maybe the output shown above is missing that route.
For more information about the command ip default-network please visit: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094374.shtml.
From the output of “show ip route” command on RouterB, we learn that RouterB does not learn any networks in RouterA. Also the “172.16.0.0/26 is a summary, 00:00:03, Null0” line tells us this netwok is summarized.
Note: EIGRP performs auto-summarization each time it crosses a border between two major networks. For example, RouterA has networks of 172.16.x.x. It will perform auto-summarization when sending over network 10.1.1.0/30, which is in different major network (172.16.0.0/16 and 10.0.0.0/8 are called major networks in this case).
All the above networks can be summarized to 10.0.0.0 network but the question requires to “represent the LANs in Phoenix but no additional subnets” so we must summarized to 10.4.0.0 network. The Phoenix router has 4 subnets so we need to “move left” 2 bits of “/24”-> /22 is the best choice -> D is correct.
From the line “D 192.168.20.64/26 [90/2195456] via 192.168.20.9, 00:03:31, Serial0/0″ we can see the IP address 192.168.20.9 belongs to network 192.168.20.8/30 and this network is between R1 and R3 -> Packet from 192.168.20.192/26 network destined to a host on the LAN attached to router R1 will go directly from R3 to R1.
From the routing table we learn that network 192.168.10.0/30 is learned via 2 equal-cost paths (192.168.10.9 &192.168.10.5) -> traffic to this network will be load-balancing.
Below is an example of the “show ip eigrp neighbors” command (from 9tut.com)