# EIGRP Sucessor route storgae

Member Posts: 800
Question:
Where are EIGRP sucessor routes stored

routing table
topology table
neighbor table
neiighbor table and topology table
routing table and topology table

My answer was just the routing table, which was marked wrong, upon so research i have read that it is stored in the topology table and the neighbor table.... i thought it was only the feasibly sucessor that was in the topology table....

So i guess that both the feasible sucessor and the sucessor are stored in the topology table but only the sucessor is placed in thr routing table.. is this right?
CCIE# 38186
showroute.net

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My book says:
A successor route is the best route to a remote network. A successor route is used by EIGRP to fwd traffic to a destination and is stored in the rounting table. It is backed up by a feasible successor route that is stored in the topology table (if available).
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This is a confusing topic, but the successor route is stored in the routing table and a copy is stored in the topology table. The feasible successor is stored solely in the topology table.

Here is a quote from my text,
"The EIGRP distance vector algorithm, DUAL, uses the information gathered in the neighbor and topology tables and calculates the lowest cost route to the destination. The primary route is called the successor route. When calculated, DUAL places the successor route in the routing table and a copy in the topology table.

DUAL also attempts to calculate a backup route in case the successor route fails. This is called the feasible successor route. When calculated, DUAL places the feasible route in the topology table. This route can be called upon if the successor route to a destination becomes unreachable or unreliable."
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When eigrp discovers a neighbor and forms an adjacency the routing table of the neighbor is learned and placed in the topology database.So the topology database is created from the routing tables of all neighbors.The shortest path to each destination is calculated which is known as the feasible distance per destination.The route with the shortest path is placed in the routing table.All other routes are still in the topology table feasible successors and non feasible successors.Depending on variance and max paths, you might have several routes placed in the routing table to the same destination.The key thing to remember is every route to every destination is in the topology table.
Networking, sometimes i love it, mostly i hate it.Its all about the \$\$\$\$
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w0rd...that makes sense.
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ok thanks for that, so im doing some testing, i have done a show ip eigrp topology command and got the following output

router4#sh ip eigrp topo
IP-EIGRP Topology Table for process 1

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

P 192.168.4.0/8, 1 successors, FD is 281600
via Summary(281600/0), Null0
P 192.168.4.1/0, 1 successors, FD is 281600
via Connected, Ethernet0
P 172.16.0.0/8, 1 successors, FD is 28160
via Summary(28160/0), Null0
P 172.16.0.2/0, 1 successors, FD is 28160
via Connected, Serial0

how can i tell which one is the sucessor from this output?
CCIE# 38186
showroute.net
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.The key thing to remember is every route to every destination is in the topology table.

Not true. This is a common misconception. Every mathmatically loop free destination is in the topology table, not every destination. You can lab this up and mess with BW statements to make a route that is actually loop free appear questionable so it does not appear in the topology table of its neighbor.

Yankee
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Yankee wrote:
.The key thing to remember is every route to every destination is in the topology table.

Not true. This is a common misconception. Every mathmatically loop free destination is in the topology table, not every destination. You can lab this up and mess with BW statements to make a route that is actually loop free appear questionable so it does not appear in the topology table of its neighbor.

Yankee

I'll have to disagree a little with this, every destination will be in the topology table.The paths to the every destination will also be in the topology table once they adhere to the split horizon rule.So a neighboring router cannot be a successor for a destination network if its current successor for that destination is the local router im referring to.So rephrasing my original statement.
The key thing to remember is every route to every destination is in the topology table as long as it adheres to the split horizon rule!
Networking, sometimes i love it, mostly i hate it.Its all about the \$\$\$\$
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came accross this question...

i have no idea what address to use... im normally pretty good a subnetting questions but this one has stumped me... any help??

CCIE# 38186
showroute.net
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^^ sorry. didnt mean to post that here
CCIE# 38186
showroute.net
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Ed says "I'll have to disagree a little with this...."

I'm sorry you disagree, but I deal with a large EIGRP network daily (have for years) and have done the lab testing I asked you to try. I did not make that post lightly and I assure you I am correct, but lab it up as I suggested and see for yourself.

DUAL proves mathmatically a route is loop free and therefore safe. It does not prove a route is a loop, so it may exclude unattractive routes to be safe. This not a flaw in the design, it is intentional and safe.

Yankee
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Yankee wrote:
Ed says "I'll have to disagree a little with this...."

I'm sorry you disagree, but I deal with a large EIGRP network daily (have for years) and have done the lab testing I asked you to try. I did not make that post lightly and I assure you I am correct, but lab it up as I suggested and see for yourself.

DUAL proves mathmatically a route is loop free and therefore safe. It does not prove a route is a loop, so it may exclude unattractive routes to be safe. This not a flaw in the design, it is intentional and safe.

Yankee

I understand eigrp very well, the topology database will hold all routes except routes that are invalid due to split horizon.Routes that are invalid due to AD being greater or equal to FD will not be shown when you do " show ip eigrp topology" but they will be shown when you do "show ip eigrp topology all-links" , these routes which matimatically could be loops are still present in the topology table but are not used by the DUAL process.
Networking, sometimes i love it, mostly i hate it.Its all about the \$\$\$\$
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okay, I tried.

Yankee