Two routers are connected by a serial link, and are configured to run EIGRP on all interfaces. You
examine the EIGRP neighbor table on both routers (using the show ip eigrp neighbor command)
and see that the router connected over the serial link is listed as a neighbor for a certain amount of
time, but is periodically removed from the neighbor table.
None of the routes from the neighbor ever seem to be learned, and the neighbor transmission
statistics (SRTT, RTO, and Q Count) seem to indicate that no packets are being transmitted
between the neighbors. Which would most likely cause this problem?
A.
While multicast packets are being successfully sent over the link, unicast packets are not
B.
There is a bug in the EIGRP code that needs to be fixed.
C.
This is correct behavior for the first few minutes of EIGRP neighbor formation. After four or five
cycles, it should straighten itself out and the neighbor
D.
The hello or hold intervals are set differently on the two routers.
Explanation:
EIGRP uses five packet types:
•Hello/Acks•Updates
•Queries
•Replies
•Requests
Hellos are multicast for neighbor discovery/recovery. They do not require acknowledgment. A hello
with no data is also used as an acknowledgment (ack). Acks are always sent using a unicast
address and contain a nonzero acknowledgment number.
Updates are used to convey reachability of destinations. When a new neighbor is discovered,
update packets are sent so the neighbor can build up its topology table. In this case, update
packets are unicast. In other cases, such as a link cost change, updates are multicast. Updates
are always transmitted reliably. Queries and replies are sent when destinations go into Active
state. Queries are always multicast unless they are sent in response to a received query. In this
case, it is unicast back to the successor that originated the query. Replies are always sent in
response to queries to indicate to the originator that it does not need to go into Active state
because it has feasible successors. Replies are unicast to the originator of the query. Both queries
and replies are transmitted reliably.
Request packets are used to get specific information from one or more neighbors. Request
packets are used in route server applications. They can be multicast or unicast. Requests are
transmitted unreliably.