Core Issue
Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietary enhanced distance vector routing protocol, relying on the Diffused Update Algorithm (DUAL) to calculate the shortest path to a destination. It functions by discovering neighbors, exchanging routing information with them and installing the best routes in the routing table. It speeds up convergence by storing backup routes in the topology table and using them when the primary route fails. These are the most common EIGRP issues:
- Failure to form neighbor relationship with other EIGRP neighbors on the same segment.
- Routes missing from the topology table or routing table.
- Routes not being redistributed into EIGRP.
- EIGRP routes stuck in Active state.
- Load balancing not working properly.
- User receiving EIGRP Not On Common Subnet error messages.
- User can ping but cannot telnet across a Frame Realy network.
The issue is mostly related to configuration errors such as these:
- Packet filters and route filters denying EIGRP packets and routes.
- Split-horizon functioning over Non-Broadcast Multi-access (NBMA) networks such as Frame Relay and ATM.
- Missing broadcast keyword in Frame Relay and ATM map statements.
- Misconfigured IP addresses.
- Improper configuration of variance.
- Frame relay header compression is on only on one side of the network.
Resolution
To resolve the issue of a user being able to ping but not telnet across a Frame Relay network, issue the debug telnet and debug ip tcp transactions commands on both sides. If the debug shows that it is receiving bad segments from the the other side, you can take a look at the configuration. There is a possibility that Frame Relay Header Compression is turned on only on one side of the link. If this is the case, turn off header compression to solve the problem.For information on other issues, refer to these documents:
Describe the next hop
Directly connected
How are Neighbors Connected
Connected via Frame Relay
Type of Filtering
Access lists / Packet filtering
Route / Prefix filtering
Source
Single device
Source and Destination Physical Connectivity
Source and destination exist on same subnetwork
EIGRP Errors, Warnings, and Log Messages
Receiving "Not On Common Subnet" error message
Seeing routes stuck in active (SIA) error message
show ip eigrp neighbors
Neighbor does not exist in neighbor table
Neighbor exists in neighbor table
