Troubleshooting EIGRP Neighbor Adjacency

EIGRP is an advanced distance vector routing protocol that has to establish a neighbor relationship before updates are sent. Because of this the first thing we’ll have to do is check if the neighbor adjacency is working properly. If this is the case we can continue by checking if networks are being advertised or not. In this lesson I’ll show you everything that can go wrong with EIGRP, how to fix it and in what order. Let’s get started with the neighbor adjacency!

There are a number of items that cause problems with EIGRP neighbor adjacencies:

• Uncommon subnet: EIGRP neighbors with IP addresses that are not in the same subnet.
• K value mismatch: By default bandwidth and delay are enabled for the metric calculation. We can enable load and reliability as well but we have to do it on all EIGRP routers.
• AS mismatch: The autonomous system number has to match on both EIGRP routers in order to form a neighbor adjacency.
• Layer 2 issues: EIGRP works on layer 3 of the OSI-model. If layer 1 and 2 are not working properly we’ll have issues with forming a neighbor adjacency.
• Access-list issues: It’s possible that someone created an access-list that filters out multicast traffic. EIGRP by default uses 224.0.0.10 to communicate with other EIGRP neighbors.
• NBMA: Non Broadcast Multi Access networks like frame-relay will not allow broadcast or multicast traffic by default. This can prevent EIGRP from forming EIGRP neighbor adjacencies.

Let’s take a look at these different issues…

EIGRP Uncommon Subnet

The first EIGRP issue is easy to spot, I’ll use the following two routers for this:

Here’s the configuration of these two routers:

R1(config)#interface FastEthernet 0/0
R1(config-if)#ip address 192.168.12.1 255.255.255.0
R1(config-if)#router eigrp 12
R1(config-router)#network 192.168.12.0

R2(config)#interface FastEthernet0/0
R2(config-if)#ip address 192.168.21.2 255.255.255.0
R2(config)#router eigrp 12
R2(config-router)#network 192.168.21.0

The uncommon subnet error is easy to spot. In the example above we have 2 routers and you can see I configured a different subnet on each interface.

After enabling EIGRP the following errors pops up:

R1# IP-EIGRP(Default-IP-Routing-Table:12): Neighbor 192.168.21.2 not on common subnet for FastEthernet0/0

R2# IP-EIGRP(Default-IP-Routing-Table:12): Neighbor 192.168.12.1 not on common subnet for FastEthernet0/0

Both routers complain that they are not on the same subnet.

R2(config-router)#interface FastEthernet0/0
R2(config-if)#ip address 192.168.12.2 255.255.255.0
R2(config)#router eigrp 12
R2(config-router)#no network 192.168.21.0
R2(config-router)#network 192.168.12.0

I’ll change the IP address on R2 and make sure the correct network command is configured for EIGRP.

R1# %DUAL-5-NBRCHANGE: IP-EIGRP(0) 12: Neighbor 192.168.12.2 (FastEthernet0/0) is up: new adjacency

R2#  %DUAL-5-NBRCHANGE: IP-EIGRP(0) 12: Neighbor 192.168.12.1 (FastEthernet0/0) is up: new adjacency

Voila! We now have an EIGRP neighbor adjacency.

R1#show ip eigrp neighbors 
IP-EIGRP neighbors for process 12
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
0   192.168.12.2            Fa0/0             13 00:05:15    3   200  0  3

R2#show ip eigrp neighbors 
IP-EIGRP neighbors for process 12
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
0   192.168.12.1            Fa0/0             11 00:05:32 1263  5000  0  3

We can verify this by using the show ip eigrp neighbors command.

Lesson learned: Make sure both routers are on the same subnet.

EIGRP K-Values Mismatch

Another common EIGRP issue is K-values mismatch. Take a look below:

This time the IP addresses are correct but we are using different K values on both sides. R1 has enabled the K-values that use bandwidth, delay, load and reliability. R2 is only using the ones for bandwidth and delay. Here’s what you will see on the console:

R1# %DUAL-5-NBRCHANGE: IP-EIGRP(0) 12: Neighbor 192.168.12.2 (FastEthernet0/0) is down: K-value mismatch

R1# %DUAL-5-NBRCHANGE: IP-EIGRP(0) 12: Neighbor 192.168.12.2 (FastEthernet0/0) is down: Interface Goodbye received

This error is easy to spot because your console will give you the “K-value mismatch” error on both routers. This is what the configuration of R1 looks like:

R1#show run | section eigrp
router eigrp 12
 network 192.168.12.0
 metric weights 0 1 1 1 1 0
 auto-summary

We can verify our configuration by looking at both routers. You can see the K values were changed on R1. Let’s change R2 so that we use the same K values:

R2(config)#router eigrp 12
R2(config-router)#metric weights 0 1 1 1 1 0

This should help, this is what the console shows us:

R1# %DUAL-5-NBRCHANGE: IP-EIGRP(0) 12: Neighbor 192.168.12.2 (FastEthernet0/0) is up: new adjacency

R2# %DUAL-5-NBRCHANGE: IP-EIGRP(0) 12: Neighbor 192.168.12.1 (FastEthernet0/0) is up: new adjacency

After changing the K values we have an EIGRP neighbor adjacency. In case you missed the console messages you can also see it here:

R1#show ip eigrp neighbors 
IP-EIGRP neighbors for process 12
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
0   192.168.12.2            Fa0/0             13 00:02:11   13   200  0  6

R2#show ip eigrp neighbors 
IP-EIGRP neighbors for process 12
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
0   192.168.12.1            Fa0/0             13 00:02:42   19   200  0  6

Another problem solved! Lesson learned: Make sure the K-values are the same on all EIGRP routers within the same autonomous system.

EIGRP AS Number Mismatch

This is a typical monday morning problem. EIGRP uses an AS number and it has to match on both EIGRP routers. Here’s the topology:

There’s a mismatch in the AS number. When we configure EIGRP we have to type in an AS number. Unlike OSPF (which uses a process ID) this number has to be the same on both routers. Let’s take a look:

R1#show ip eigrp neighbors 
IP-EIGRP neighbors for process 12

R2#show ip eigrp neighbors 
IP-EIGRP neighbors for process 21

Unlike the other EIGRP configuration mistakes this one doesn’t produce an error message. We can use show ip eigrp neighbors and see that there are no neighbors. Use your eagle eyes to spot for differences and you’ll quickly see that we are not using the same AS number. Here’s the configuration:

R1#show run | section eigrp
router eigrp 12
 network 192.168.12.0
 auto-summary

R2#show run | section eigrp
router eigrp 21
 network 192.168.12.0
 auto-summary

Let’s fix this, I’ll change the configuration of R2:

R2(config)#no router eigrp 21

router eigrp 12
 network 192.168.12.0
 metric weights 0 1 1 1 1 0
 auto-summary

After making the change, the neighbor adjacency will appear:

R1# %DUAL-5-NBRCHANGE: IP-EIGRP(0) 12: Neighbor 192.168.12.2 (FastEthernet0/0) is up: new adjacency

R2# %DUAL-5-NBRCHANGE: IP-EIGRP(0) 12: Neighbor 192.168.12.1 (FastEthernet0/0) is up: new adjacency

R1#show ip eigrp neighbors 
IP-EIGRP neighbors for process 12
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
0   192.168.12.2            Fa0/0             11 00:01:44   13   200  0  3

R2#show ip eigrp neighbors 
IP-EIGRP neighbors for process 12
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
0   192.168.12.1            Fa0/0             12 00:01:53    7   200  0  9

After changing the AS number life is good. Lesson learned: Make sure the AS number is the same if you want an EIGRP neighbor adjacency.

EIGRP Passive Interface

if you checked the AS number, K values, IP addresses and you still don’t have a working EIGRP neighbor adjacency then you should think about security. Maybe an access-list is blocking EIGRP and/or multicast traffic. Here’s the topology:

Once again two EIGRP routers and no neighbor adjacency. What is going on?

R1#show ip eigrp neighbors 
IP-EIGRP neighbors for process 12

R2#show ip eigrp neighbors 
IP-EIGRP neighbors for process 12

We see that there are no neighbors but the AS number matches. Let’s take a closer look:

R1#show ip protocols                 
Routing Protocol is "eigrp 12"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Default networks flagged in outgoing updates
  Default networks accepted from incoming updates
  EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
  EIGRP maximum hopcount 100
  EIGRP maximum metric variance 1
  Redistributing: eigrp 12
  EIGRP NSF-aware route hold timer is 240s
  Automatic network summarization is in effect
  Maximum path: 4
  Routing for Networks:
    192.168.12.0
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: internal 90 external 170

R2#show ip protocols 
Routing Protocol is "eigrp 12"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Default networks flagged in outgoing updates
  Default networks accepted from incoming updates
  EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
  EIGRP maximum hopcount 100
  EIGRP maximum metric variance 1
  Redistributing: eigrp 12
  EIGRP NSF-aware route hold timer is 240s
  Automatic network summarization is in effect
  Maximum path: 4
  Routing for Networks:
    192.168.12.0
  Passive Interface(s):
    FastEthernet0/0
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: internal 90 external 170

If you look at the output of show ip protocols you can see that the network has been advertised correctly. If you look closely on R2 you can see that we have a passive interface. Let’s get rid of it!