Eigrp neighborship troubleshooting

1. EIGRP Neighborship Troubleshooting
As was shown in the OSPF neighborship troubleshooting article earlier, there are a
number of things that need to be configured correctly for everything to end up
working as planned. This article takes a look at these requirements from the
perspective of Enhanced Interior Gateway Routing Protocol (EIGRP) and shows the
different commands that can be used to ensure proper EIGRP neighborship
configuration and communications between devices.
EIGRP Neighborship Requirements
From the perspective of EIGRP, there are a couple of things that must match for a
EIGRP neighborship to establish; keep in mind that these are different from other
routing protocols (like OSPF), these include:
1. The devices must be in the same autonomous system (AS)
2. The devices must have the same authentication configuration
3. The devices must have the same k-values
EIGRP Neighborship Configuration Verification and Troubleshooting
Starting from the top of the list, the interfaces connecting devices must be on the
same autonomous system. To display the various commands and what to look for,
Figure 1 shows a simple lab has been setup with two devices that are connected
together via an Ethernet connection.
Figure 1 - Simple Lab
The first thing that is going to be checked by the EIGRP device is whether the remote
device is in the same autonomous system. No other processing will occur on the
device until both devices have been configured with the same area. The
troubleshooting for this type of problem requires someone with access to the
remote device; this is because there is nodebug command that can be used that will
show an obvious AS mismatch. The only way to confirm that this is the problem is to
verify the configuration of the devices and verify the AS is the same on each. Figure 2
and 3 below show the EIGRP configuration on both R1 and R2; R2 has been
configured with the incorrect AS (AS 1).
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2. Figure 2 - Mismatched Autonomous Systems (R1)
Figure 3 - Mismatched Autonomous Systems (R2)
The second entry on the list was that each device must have matching authentication
configuration; before any other information is exchanged between the devices they
must agree on an authentication type (if any is configured). EIGRP has a number of
different errors messages that are given depending on the specific authentication
based failure. The message that is shown in Figure 4 is given when a local device that
is not configured with EIGRP authentication is attempting to communicate with a
remote device that has an existing authentication configuration.
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3. Figure 4 - Local Missing Authentication Configuration (Complete)
The message that is shown in Figure 5 is given when the local device has been
partially configured with EIGRP authentication. In this example, EIGRP
authentication has been configured on the connecting interface but the key chain
has not yet been configured (no live authentication keys).
Figure 5 - Local Missing Authentication Configuration (No key chain configured)
For the next couple of authentication examples, the local device has already been
configured correctly for EIGRP authentication but the remote device has not. The
message that is shown in Figure 6 is given when a remote device has not been
configured with any authentication configuration.
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4. Figure 6 - Remote Missing Authentication Configuration (Complete)
The message shown in Figure 7 is given when the remote device has EIGRP
authentication configured but does not have a configuration that matched with the
local configuration (commonly this is a key string mismatch).
Figure 7 - Remote Authentication Configuration Mismatch
The final entry on the list was that the k-values must match between the
communicating devices. Typically, the default values are not changed but if they are,
each of the neighboring devices must share the same values. The message that is
shown in Figure 8 is given if a mismatch is detected between EIGRP devices;
no debug command is required for this message to be displayed.
http://blog.router-switch.com/

5. Figure 8 - k-value Mismatch
Summary
A neighborship is the first thing that must be established before any communication
will happen between devices. Each of the different routing protocols has their own
requirements that must be met before this neighborship will establish. This article
takes a look at the elements that must match for EIGRP neighborships to establish
and what commands to use to troubleshoot which misconfiguration potentially exists.
Hopefully, the information in this article, when committed to memory, will help in
future EIGRP configuration endeavors.
More Related Cisco Networking Tips:
How to Configure EIGRP on a Cisco Router?
http://blog.router-switch.com/