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EIGRP Tables



As we talked about a little before, EIGRP (Enhanced Interior Gateway Routing Protocol) has three different tables.

These EIGRP tables are:

• Neighbour Table
• Topology Table
• Routing Table

EIGRP Table Types

EIGRP Table Types

EIGRP Neighbour Table keeps all the router that EIGRP neighbourship is established with. EIGRP Neighbour Table is established by Hello messages.

EIGRP (Enhanced Interior Gateway Routing Protocol) has a specific Neighbour Table for each Layer 3 protocols. For example, if you are using both IPv4 and IPv6 protocols, you will have two different EIGRP Neighbour Tables.

You can see the EIGRP Neighbour Table with “show ip eigrp neighbors ” command.

In EIGRP Neighbour Table you can see the ip address of the neighbour, the port you access to the neighbour, the port’s uptime etc.

You can see an output of “show ip eigrp neighbors ” command is given below:

Router#show ip eigrp neighbors 
IP-EIGRP neighbors for process 100
H   Address         Interface      Hold Uptime    SRTT   RTO   Q   Seq
                                   (sec)          (ms)        Cnt  Num
0   10.0.0.2        Fa4/0          11   00:00:08  40     1000  0   12
1   20.0.0.2        Fa5/0          10   00:00:08  40     1000  0   11

EIGRP Topology Table keeps all the routes to all the destinations. It keeps all the Successor and Feasible Successor routes. We can say also, “it keeps the best routes and the alternatives”.

You can check EIGRP Topology Table with “show ip eigrp topology ” command.

An example output of “show ip eigrp topology” command is given below:

Router#show ip eigrp topology 
IP-EIGRP Topology Table for AS 100/ID(192.168.1.2)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - Reply status

P 10.0.0.0/24, 1 successors, FD is 28160
         via Connected, FastEthernet4/0
P 20.0.0.0/24, 1 successors, FD is 28160
         via Connected, FastEthernet5/0
P 30.0.0.0/24, 1 successors, FD is 30720
         via 10.0.0.2 (30720/28160), FastEthernet4/0
P 40.0.0.0/8, 1 successors, FD is 30720
         via 20.0.0.2 (30720/28160), FastEthernet5/0
P 192.168.1.0/24, 1 successors, FD is 28160
         via Connected, FastEthernet0/0
P 192.168.2.0/24, 1 successors, FD is 30720
         via 10.0.0.2 (30720/28160), FastEthernet4/0
P 192.168.3.0/24, 1 successors, FD is 30720
         via 20.0.0.2 (30720/28160), FastEthernet5/0
P 192.168.4.0/24, 1 successors, FD is 33280
         via 10.0.0.2 (33280/30720), FastEthernet4/0

In EIGRP Topology Table, there are some route states. These states are:

Active State : The state that router has no route and trying to find a route. Queries are sent through the all interfaces.
Passive State : The state that the router has choosen a route.

SIA (Stuck In Active) : The mode when no reply is get in 3 minutes. The cause of Stuck In Active can be any high CPU and memory usage, packet drops, unidirectional links etc.

Routing Table is the table which keeps the Best Routes to any destinations. This Best Routes are Successors

You can see the Routing Table with “show ip route eigrp” command.

An example output of “show ip route eigrp” command is given below:

Router#show ip route Eigrp 
     30.0.0.0/24 is subnetted, 1 subnets
D       30.0.0.0 [90/30720] via 10.0.0.2, 00:04:28, FastEthernet4/0
D    40.0.0.0/8 [90/30720] via 20.0.0.2, 00:04:28, FastEthernet5/0
D    192.168.2.0/24 [90/30720] via 10.0.0.2, 00:04:28, FastEthernet4/0
D    192.168.3.0/24 [90/30720] via 20.0.0.2, 00:04:28, FastEthernet5/0
D    192.168.4.0/24 [90/33280] via 10.0.0.2, 00:04:28, FastEthernet4/0

These tables are filled with updates. In EIGRP, updates are not periodic. Whenever a change occurs in the EIGRP network, triggered updates are sent about that change.

EIGRP (Enhanced Interior Gateway Routing Protocol) is a reliable protocol. It has window size one. If there is no acknowledgement that the packet is received, then the next one is not sent. Even if any packet is not acknowledged this packet is retransmited at EIGRP. After 16 unacknowledged retransmission.

Feasible Distance, Advertised Distance, Successor and Feasible Successor



In EIGRP (Enhanced Interior Gateway Routing Protocol), there are some new terms that explains different routes to a destination according to their metrics. These terms are :

• Feasible Distance
• Advertised Distance
• Successor
• Feasible Successor

Let’s check these EIGRP specific terms one by one:

Feasible Distance (FD) : The cost of the route to the next hop, plus, next hop to the destination.

Advertised Distance (AD) : The cost of the route from the next hop to the destination.

EIGRP Feasible Distance,Advertised Distance

EIGRP Feasible Distance,Advertised Distance

Think about the basic topology above. From Router A and Router B to the destination router, let’s check the Feasible Distance and Advertised Distance values.

As you can see above, Router A has 5 metric to its next hop Router B. And from the Router B to the destination, the cost is 5+5 = 10. So, for Router A, Advertised Distance is 10 (5+5) and Feasible Distance is 15 (5+10).

This logic is same for Router B. Let’s think that our starting point is Router B. At this time, Router B has 5 metric to its neighbour Router C. And from its neighbour Router C to Destination, the cost is 5. So, for Router B, the Advertised Distance is 5 and Feasible Distance is 5+5 = 10.

Now, let’s check the other two important terms, successor and feasible successor.

Successor : The Best Route to any destination. The route that has the best Feasible Distance.

Feasible Successor : The Second Best Routes to any destination.

EIGRP Successor, Feasible Successor

EIGRP Successor, Feasible Successor

As you can see above, we have three ways from Router A to the destination router. These three ways are writen differently in different tables of EIGRP.

In the Neighbour Table of Router A, all the neighbours connected to Router A exist. As you can see three of them is in the Neighbour Table.

After route calculations, the topology table is build and Feasible Distances and Advertised Distances are determined. According to these values, The Best Route to the destination, Successor, is selected. And the second best route is also selected as Feasible Successor. These two routes are in Topology Table of Router A.

Lastly, in the Routing table, the Successor exists. Ithe Best Route to the destination. In other words, the lowest metric. If Successor goes down, then Feasible Successor become active in Routing Table. This backup best route provides fast convergence characteristic to EIGRP.

EIGRP Article Series…
EIGRP – Part 1
EIGRP – Part 2 (EIGRP Tables)
EIGRP – Part 3 (EIGRP Packet Types and Neighborship)
EIGRP – Part 4 (Other EIGRP Properties)

EIGRP Configuration with Packet Tracer

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About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

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