dual,eigrp

Answered Question
Aug 16th, 2009

Hi every body.

i have few questions about eigrp topology database.

On one hand my book says topology database stores the every possible next hop to reach a subnet. On the other hand the same book says topology database stores successor and feasible routes only . So what is correct?

Dual:

What are the functions dual performs?

Here is my understanding:

1) Dual calculates successor route and feasible successor route whereever possible.

2) Dual installs the successor route in routing table

3) In case of successor route failure and there is no feasible successor route present in database, dual finds the alternate route by using eigrp query/reply protocol.

Is this all to dual?

Thanks a lot .

Correct Answer by Richard Burts about 7 years 6 months ago

Dual is an algorithm designed to evaluate potential paths to a destination and to identify paths that are determined to be loop free.

1) Dual does calculate and determine successor and possibly feasible successor routes based on information advertised from neighbor routers.

2) Dual does not necessarily install successor routes in the routing table. Dual nominates routes (successor routes) for installation in the routing table. But it is a different process that actually installs the route in the routing table. And if Dual nominates a route for installation but there is another route learned from a source with a better administrative distance, then the Dual route is not installed in the routing table.

3) in the case of failure of a successor route where there is no feasible successor then Dual does use the query process to determine whether there is a replacement route that is loop free.

You may find that this link provides helpful information about Dual:

www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f07.shtml

Among the things that you will find are this explanation:"The Topology Table is populated by the protocol dependent modules and acted upon by the DUAL finite state machine. It contains all destinations advertised by neighboring routers." So this answers your question about whether the topology table stores only successor and feasible successor routes: it stores more than just successor and feasible successor routes. The issue may be examined by comparing the output of show ip eigrp topology (which shows successor and feasible successor routes) and the output of show ip eigrp topology all (which will show additional routes that are neither successor nor feasible successor routes).

The link also includes this brief description of Dual which you might find helpful:"The DUAL finite state machine embodies the decision process for all route computations. It tracks all routes advertised by all neighbors. The distance information, known as a metric, is used by DUAL to select efficient loop free paths. DUAL selects routes to be inserted into a routing table based on feasible successors. A successor is a neighboring router used for packet forwarding that has a least cost path to a destination that is guaranteed not to be part of a routing loop."

HTH

Rick

Correct Answer by Lucien Avramov about 7 years 6 months ago

There is a topology table, neighbor table stored on the IOS device. A destination entry is moved from the topology table to the routing table when there is a feasible successor.

A very good document explaining dual:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f07.shtml#dual_example

If you are interested to understand the DUAL algorithm more in details, you can read the IEEE paper:

http://ccrg.soe.ucsc.edu/publications/jj.dual.ton93.pdf

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Correct Answer
Lucien Avramov Sun, 08/16/2009 - 19:50

There is a topology table, neighbor table stored on the IOS device. A destination entry is moved from the topology table to the routing table when there is a feasible successor.

A very good document explaining dual:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f07.shtml#dual_example

If you are interested to understand the DUAL algorithm more in details, you can read the IEEE paper:

http://ccrg.soe.ucsc.edu/publications/jj.dual.ton93.pdf

Correct Answer
Richard Burts Sun, 08/16/2009 - 20:04

Dual is an algorithm designed to evaluate potential paths to a destination and to identify paths that are determined to be loop free.

1) Dual does calculate and determine successor and possibly feasible successor routes based on information advertised from neighbor routers.

2) Dual does not necessarily install successor routes in the routing table. Dual nominates routes (successor routes) for installation in the routing table. But it is a different process that actually installs the route in the routing table. And if Dual nominates a route for installation but there is another route learned from a source with a better administrative distance, then the Dual route is not installed in the routing table.

3) in the case of failure of a successor route where there is no feasible successor then Dual does use the query process to determine whether there is a replacement route that is loop free.

You may find that this link provides helpful information about Dual:

www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f07.shtml

Among the things that you will find are this explanation:"The Topology Table is populated by the protocol dependent modules and acted upon by the DUAL finite state machine. It contains all destinations advertised by neighboring routers." So this answers your question about whether the topology table stores only successor and feasible successor routes: it stores more than just successor and feasible successor routes. The issue may be examined by comparing the output of show ip eigrp topology (which shows successor and feasible successor routes) and the output of show ip eigrp topology all (which will show additional routes that are neither successor nor feasible successor routes).

The link also includes this brief description of Dual which you might find helpful:"The DUAL finite state machine embodies the decision process for all route computations. It tracks all routes advertised by all neighbors. The distance information, known as a metric, is used by DUAL to select efficient loop free paths. DUAL selects routes to be inserted into a routing table based on feasible successors. A successor is a neighboring router used for packet forwarding that has a least cost path to a destination that is guaranteed not to be part of a routing loop."

HTH

Rick

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