EIGRP Neighbor and Topology Table Explained

The EIGRP topology table is something most networking students find difficult to read. In this lesson I’ll give you an example of what it looks like and the information you can find it it. We’ll do this by looking at some real routers. Here’s the topology I will use:

EIGRP Neighbor Adjacency Lab

I’m using two routers with a loopback interface each and EIGRP has been configured. The routers have become EIGRP neighbors as we can see here:

R1#show ip eigrp neighbors 
IP-EIGRP neighbors for process 1
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
0   192.168.12.2            Fa0/0             10 00:06:06   19   200  0  27

In the output above I’m looking at the EIGRP neighbor table of R1. As you can see we have one neighbor (192.168.12.2) which happens to be R2 on interface FastEthernet 0/0. What else do we find here?

  • H (Handle): Here you will find the order when the neighbor adjacency was established. Your first neighbor will have a value of 0, the second neighbor a value of 1 and so on.
  • Hold: (sec): this is the holddown timer per EIGRP neighbor. Once this timer expires we will drop the neighbor adjacency. The default holddown timer is 15 seconds. On older IOS versions only a hello packet would reset the holddown timer but on newer IOS versions any EIGRP packet after the first hello will reset the holddown timer.
  • Uptime: How long the neighbor has been up.
  • SRTT (Smooth round-trip time): The number of milliseconds it takes to send an EIGRP packet to your neighbor and receive an acknowledgment packet back.
  • RTO (Retransmission timeout): The amount of time in milliseconds that EIGRP will wait before retransmitting a packet from the retransmission queue to this neighbor.
  • Q Cnt (Q count): The number of EIGRP packets (Update, Query or Reply) in the queue that are awaiting transmission. Ideally you want this number to be 0 otherwise it might be an indication of congestion on the network.
  • Seq Num (Sequence number): This will show you the sequence number of the last update,query or reply packet that you received from your EIGRP neighbor.

Excellent so that’s how EIGRP stores neighbor information! Our next stop is of course to take a look at the EIGRP Topology table:

R1#show ip eigrp topology 
IP-EIGRP Topology Table for AS(1)/ID(1.1.1.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status 

P 1.1.1.0/24, 1 successors, FD is 128256
        via Connected, Loopback0
P 2.2.2.0/24, 1 successors, FD is 156160
        via 192.168.12.2 (156160/128256), FastEthernet0/0
P 192.168.12.0/24, 1 successors, FD is 28160
        via Connected, FastEthernet0/0

Now that’s a lot of information to look at! Let me break it down for you in chunks:

R1#show ip eigrp topology 
IP-EIGRP Topology Table for AS(1)/ID(1.1.1.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status 

P 1.1.1.0/24, 1 successors, FD is 128256
        via Connected, Loopback0
P 2.2.2.0/24, 1 successors, FD is 156160
        via 192.168.12.2 (156160/128256), FastEthernet0/0
P 192.168.12.0/24, 1 successors, FD is 28160
        via Connected, FastEthernet0/0

If you look at the red fonts you can see that we are looking at the EIGRP topology table for AS (Autonomous System) number 1. Keep in mind that the AS number has to match on EIGRP routers in order to become neighbors!

R1#show ip eigrp topology 
IP-EIGRP Topology Table for AS(1)/ID(1.1.1.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status 

P 1.1.1.0/24, 1 successors, FD is 128256
        via Connected, Loopback0
P 2.2.2.0/24, 1 successors, FD is 156160
        via 192.168.12.2 (156160/128256), FastEthernet0/0
P 192.168.12.0/24, 1 successors, FD is 28160
        via Connected, FastEthernet0/0

Look at those codes…Update, Query and Reply should ring a bell since I discussed them a few pages ago. Let’s focus on those codes that I didn’t explain before…

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Forum Replies

  1. Thank you so much for this clear explanation

  2. Hi Rene,

    How does EIGRP come up with the local metric for the interface it’s using to reach the destination network? And where can we see it. Thank you

  3. Hi Victor,

    EIGRP uses this formula to calculate the metric:

    https://networklessons.com/eigrp/eigrp-k-values-formula/

    Here is an example of a directly connected gigabit interface with a network that is advertised in EIGRP:

    R1#show ip eigrp topology 192.168.12.0/24
    EIGRP-IPv4 Topology Entry for AS(1)/ID(11.11.11.11) for 192.168.12.0/24
      State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2816
      Descriptor Blocks:
      0.0.0.0 (GigabitEthernet0/1), from Connected, Send flag is 0x0
          Composite metric is (2816/0), route is Internal
          Vector metric
    ... Continue reading in our forum

  4. Hi Rene,
    Thank you for the explaintions, is clear and easy to understand.

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