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topology table in eigrp

Topology Table – Exclusive Introduction

Topology Table – Exclusive Introduction

The EIGRP topology table contains all of the information about all known routes received from EIGRP neighbors. As an EIGRP router learns routes from its neighbors, those routes are stored in its EIGRP topology table. If a neighbor is advertising a possible route, it has to be using that route to forward packets to the destination network.

The topology stores all possible routes in the topology table database. It also provides the information used to select the best possible route, which is copied into the routing table. Topology tables are stored in Random Access Memory (RAM) and are re-created whenever a router starts up. Routing tables update all routing information from the topology table.

The “show ip eigrp topology” command illustrates the topology table as shown in figure 1. The topology table displays lists of all successors and Feasible Successors (FSs) calculated by DUAL. Only the successor with the lowest metric value is installed into the IP routing table. We should explain the entries marked in the red rectangle.

  • P– This parameter shows that the route is in the passive state. All routes in the topology table must be in the passive state for a stable routing domain. If the DUAL recalculates for a new path, the routes become is an active state and the parameter has changed to display A instead of P.
  • 168.2.0/24 – This is the destination network which is displayed just after the P or A. The address will also be displayed in the routing table if the route becomes the successors.
  • 1 successors– This parameter displays the number of successors for this network. If multiple equal-cost paths found to this network, there will be multiple successors.
  • FD is 258816– FD (Feasible Distance), It is the EIGRP metric to reach the destination network. This is the metric value we have calculated earlier. The metric is also displayed in the routing table.

Figure 2 illustrates the entries of the next hop of the successor router.

  • via 10.10.10.2– This field identifies the Next-hop address of the successor, 10.10.10.2 is the IP address of successor router. The IP address is also shown in the routing table.
  • 258816– FD to 192.168.2.0/24, the metric value is shown in the IP routing table.
  • 5120– This is the Reported Distance (RD) of the successor router and is R3’s cost to reach this network.
  • GigabitEthernet 0/1– This is the outbound interface of the local router to reach this network, also shown in the routing table.

The second subentry as shown in figure 3 is the entry for Feasible Successor, if there is not a second entry, then there is no Feasible Successor

  • via 10.10.10.6– This is the Next-hop address of the Feasible Successor, 10.10.10.6 is the IP address of R1.
  • 1660928– R1’s new Feasible Distance to 192.168.2.0/24, if R2 became the new successor. The entry would also be displayed in the IP routing table.
  • 5376– Reported Distance of the Feasible Successor.
  • GigabitEthernet 0/0 – This is the outbound interface used to reach a Feasible Successor if this router becomes the successor.
EIGRP topology table

What is EIGRP Topology Table

EIGRP uses a neighbor table, topology table, and IP routing table. The neighbor table maintains a state of neighbors. The topology table is used to store information about all known routes received from all neighbors.

EIGRP Update messages send the routers’ EIGRP topology tables. The EIGRP topology table is a database of possible routes. Each router chooses its best routes and installs these routes in its respective IP routing table uses the information in the topology table.

Each EIGRP router maintains an EIGRP topology table for each IPv4 and IPv6. It also includes route entries for every destination that the router learns from its directly connected EIGRP neighbors. The figure below illustrates the continuance of the earlier route discovery process from the previous article “EIGRP Neighbor Adjacency”.

When a router R1 receives an update from neighbor router R2, it adds the routing information into its EIGRP topology table and replies with an EIGRP acknowledgment. The figure now illustrates the update of the topology table.

EIGRP topology table

  1. Router R1 replies with an EIGRP acknowledgment packet informing Router R2 about the confirmation of receiving the update of the routing information.
  2. Router R1 sends an EIGRP update to Router R2 advertising the routes in its topology table, except route learned from Router R2.
  3. Router R2 receives the EIGRP update from Router R1 and adds this information to its topology table.
  4. Router R2 replies to Router R1’s EIGRP update packet with an EIGRP acknowledgment packet.

Convergence

When Router R1 receives the EIGRP update packets from Router R2, R1 updates its IP routing table using the information in the topology table with the best path to each destination, including the metric and the next-hop router. Similarly to Router R1, Router R2 updates its IP routing table with the best path routes to each network. This is the EIGRP converged state for both routers.

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