EIGRP Troubleshooting

 Common EIGRP troubleshooting scenario causes and solution.

Problem-1#: EIGRP neighbor not coming up.
Cause  :
1.AS number mismatch between neighbor.
2.K value mismatch between routers.
3.IP is on different subnets
4.Primary and secondary address on interface mismatch between routers.
5.layer 2 switch/hub connecting different subnets devices in same VLAN. For example VLAN leaking from vlan 1 to vlan2 by using back to back cable. 
6. Access-list blocking EIGRP multicast packets on interface.

Commands:
Show ip eigrp neighbors.


Solution:
a) Make sure same AS number is in use.
b) Make sure same K value is in use.
c) Make sure same IP subnets on both side.
d) Make sure Primary to Primary address match on both side and secondary to secondary.
e) Make sure there is no leaking.     


Problem-2# EIGRP neighbor is established but not exchanging routes and getting reset.
Cause :
1. Mismatch mask between the neighbor.
2. Hold time getting expired
3. Retry limit exceeded.
4. Vlan  leak is happening or cross VLAN domain due to some redundant link or wrong cabling.

Commands:
Show ip eigrp neighbors.

Solution :
a) Make sure there is no mismatch mask between neighbors.
b) Make sure there is no packet loss on ports (End-CPU to End-End CPU)
c) Make sure Unicast rechability is available and can ping end to end.
d) Make sure there is no access-list blocking the communication unicast communication between neighbors.
e) Make sure COPP is not blocking the unicast packet between neighbors.
f) Make sure there is no MTU mismatch between the neighbors.
g) Make sure EIGRP neigobors on the same subnet and no VLAN are leaking to other subnets.

Extra information :
Mismatch mask causes the routing loop in the network causing acknowledge packet not to processed by router.



Problem#-3: Router stuck in active.
Cause   :
    1.An active route gets stuck for one of the following reasons:
    2.Bad or congested links
    3.Low router resources, such as low memory or high CPU on the router
    4.Long query range
    5.Excessive redundancy
    6. Access-list /Copp blocking the unicast packet between neighbors.

Commands:
 show ip eigrp topology active
 Show ip eigrp neigbors

Solution:
  a) Use summarization.
  b) Use eigrb stub
  b) Use Hierarchy design.

Extra information:
    The query stops if one of the following occurs:
    All queries are answered from all the neighbors.
    The end of network is reached.
    The lost route is unknown to the neighbors.


Problem#-4 : Routes not advertising to neigbor:
Cause :
1.Access-list with distribute list is misconfigured.
2.Routers has discontigous network.
3.EIGRP split horizon is enabled

Command: Debug ip eigrp

Solution:

a) Make sure access-list allow the networks in distribute list.
b) Make sure "no auto summary" is enabled.
c) Make sure EIGRP split is disabled on Hub router in Hub/spoke topology.
d) OR Use sub-interface with different subnet between Hub/Spoke.


Problem#-5 : Routes are not installing into routing table.
Cause :
1.Auto or manual summarization configured.
2.Higher administrative distance
3.Duplicate router IDs

Solution:
a) More of design issue. Summarization should not happen at two places. Enable the "no auto-summary" or remove the manual summarization.
b) Reduce the administrative distance of eigrp if it is due to higher AD. Routes will shows as inacceible feasible distnace(metric-45300020202)
c)Make sure router-ID are unique for eigrp on each router. If there is duplicate router ID, EIGRP thinks its a loop and does not install the route in the table.

Extra information:

[Every time autosummarization or manual summarization takes place, EIGRP installs the summary route with the next hop to Null 0. This is a loop-prevention mechanism for EIGRP's summary routes. In this case study, this is exactly what happens—EIGRP does not install a route from its neighbor that falls within its summary range. ]