Defending against lameness since 2008

Here is some hopefully relevant information about EIGRP that you might expect to see on the exam. EIGRP uses IP protocol 88 and sorts the best and backup routes with the Diffusing Update Algorithm (DUAL). It uses the following five types of packets to communicate.

1. Hello - Used to identify neighbors.
2. Update - Used to advertise routes.
3. Query - Used to ask about routes where the best route has been lost.
4. Reply - Used to answer the queries.
5. Ack - Used to acknowledge the update, query and reply messages.

The hello packet uses the multicast address of 224.0.0.10 to communicate with its neighbors. EIGRP supports unequal-cost load sharing. This is done by taking the feasible distance and multiplying it by the variance.

• Feasible Distance (FD) - Full metric for the route to the destination network.
• Advertised Distance (AD) - Metric of the next-hop router to the destination network.
• Successor - Primary path to the destination network. Kept in the topology and routing table.
• Feasible Successor (FS) - Backup path to the destination network. Kept in the topology table only. To be considered a feasible successor, the advertised distance must be less than the feasible distance of the successor.

EIGRP maintains three tables.

1. Neighbor Table - Verifies all acknowledgements are received.
2. Topology Table - Used to understand paths through the network.
3. Routing Table - Contains the best paths from the topology table.

You can configure stub routers in EIGRP. Stub routers are most commonly used in hub-and-spoke networks and use less resources on the spoke routers. Limited EIGRP information is sent between the stub and core router since it has only one neighbor.

The real metric can be calculated by using the formula 256*(bandwidth + delay)

• bandwidth - Defined as 107 kbps divided by the slowest link along the path.
• delay - Constant value on every interface type and is stored in microseconds.

Tags: advertised distance, DUAL, eigrp, feasible distance, feasible successor, successor

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