EIGRP successors and feasible successors

[surfbum99]

Hi,

In EIGRP is it the case that the "Feasible successors" are stored in the Topology table and the "successors" are stored in the routing table?

I had an exam question the other day, saying where are the successors stored?

1. Routing table
2. Topology table
3. Both Routing table and topology table
4. none of the above.

 

[leedsit]

Hi,

the successors and feasible successors are stored in the topology table, however only the successor is entered into the routing table... So i would say the answer is both.

LEEroy
MCNE6,CCNA2,3/4 CCNP,CWNA,CCSA,Project+

 

[surfbum99]

Mmm in a web page I read :

EIGRP uses the Neighbor Table to list adjacent routers. The Topology Table lists all the learned routes to a destination whilst the Routing Table contains the best route to a destination, which is known as the Successor. The Feasible Successor is a backup route to a destination which is kept in the Topology Table.

http://www.rhyshaden.com/eigrp.htm

 

[leedsit]

For the Route to be listed in the Routing table, it must be in the Topology table, on which the DUAL algorithm is ran in order to produce successors, feasible successors etc. If the successor is not in the topology table, then technically the Feasible successor would then be the successors. So still stand by the successors should be in the routing table and the topology table.

LEEroy
MCNE6,CCNA2,3/4 CCNP,CWNA,CCSA,Project+

 

[surfbum99]

Right many thanks, and only Feasible Successors are in the topology table.

 

[leedsit]

Hmmm,

I keep finding the word "moved" when talking about the tables.... maybe I`ll lab this one up, looks like I need reminding...

Feasible successor
A destination entry is *****moved****** from the topology table to the routing table when there is a feasible successor. A feasible successor is a path whose reported distance is less
than the feasible distance, and it is considered a backup route. EIGRP will keep up to six feasible successors in the topology table. Only the one with the best metric (the successor).is placed in
the routing table. The show ip eigrp topology command will display all the EIGRP feasible successor routes known to a router.

Successor
A successor route (think successful!) is the best route to a remote network. A successor route is used by EIGRP to forward traffic to a destination and is stored in the routing
table. It is backed up by a feasible successor route that is stored in the topology table—if one is available.
By using the feasible distance, and having feasible successors in the topology table as backup links, the network can converge instantly, and updates to any neighbor are the only traffic sent
from EIGRP."


LEEroy
MCNE6,CCNA2,3/4 CCNP,CWNA,CCSA,Project+

 

[surfbum99]

surely the successor is the feasible successor after its been moved from the topology table to the routing table. Therefore successors can't be in the topology table just feasible successors.

 

[leedsit]

Just looked at my lab book.

Show ip route

D 10.1.4.0 [90/5376] via 10.1.1.1, 00:01:19 Eth 0
[90/5376] via 10.1.6.1, 00:01:19 S0/0

Show ip eigrp topology

P 10.1.4.0/24, 2 Successors, FD is 5376
via 10.1.6.1 [5376/5120), Serial 0
via 10.1.1.1 [5376.5120), Ethernot 0

So it appears it is in both tables.

I`ll lab up later and post the results.

LEEroy
MCNE6,CCNA2,3/4 CCNP,CWNA,CCSA,Project+

 

[surfbum99]

mmmm, if a feasible successor, turns into a successor when it gets moved from the topology table to the routing table, why would that successor be still in the topology table.

 

[leedsit]

Hi,

I have set up a lap with 3 routers, 1 router is in the middle and the two outer routers are connected with serial cables, both interfaces have the same bandwidth. on each of the external routers im injecting 5.5.5.5 /32, so the middle router will so two routes. Because the bandwidth command is the same on the serial interfaces, it should product Equal cost paths.

Show ip route gives..

Router-1#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is not set

1.0.0.0 255.255.255.0 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Serial0
2.0.0.0 255.255.255.0 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, Serial1
5.0.0.0 255.255.255.255 is subnetted, 1 subnets
D 5.5.5.5 [90/896000] via 1.1.1.1, 00:01:40, Serial0
[90/896000] via 2.2.2.1, 00:01:40, Serial1

and show Ip eigrp topology shows

Router-1#show ip eigrp topology
IP-EIGRP Topology Table for process 1

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

P 5.5.5.5 255.255.255.255, 2 successors, FD is 895744
via 2.2.2.1 (896000/128256), Serial1
via 1.1.1.1 (896000/128256), Serial0
P 1.1.1.0 255.255.255.0, 1 successors, FD is 768000
via Connected, Serial0
P 2.2.2.0 255.255.255.0, 1 successors, FD is 767744
via Connected, Serial1

As you can see, there are two successors which are in the Routing table, so it would seem the successor is in the Topology and routing table at the same time.

the next output is the same, execpt one interface has higher bandwidth

show ip route

Router-1#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is not set

1.0.0.0 255.255.255.0 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Serial0
2.0.0.0 255.255.255.0 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, Serial1
5.0.0.0 255.255.255.255 is subnetted, 1 subnets
D 5.5.5.5 [90/896000] via 1.1.1.1, 00:03:22, Serial0
Router-1#

show ip eigrp topology

Router-1#show ip eigrp topology
IP-EIGRP Topology Table for process 1

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

P 5.5.5.5 255.255.255.255, 1 successors, FD is 895744
via 1.1.1.1 (896000/128256), Serial0
via 2.2.2.1 (2560640000/128256), Serial1
P 1.1.1.0 255.255.255.0, 1 successors, FD is 768000
via Connected, Serial0
P 2.2.2.0 255.255.255.0, 1 successors, FD is 767744
via Connected, Serial1









LEEroy
MCNE6,CCNA2,3/4 CCNP,CWNA,CCSA,Project+

 

[surfbum99]

So a "successor" is definately in the routing table and the topology table. and a "feasible successor" is only in the topology table?

 

[leedsit]

I would say you are spot on with that comment.

LEEroy
MCNE6,CCNA2,3/4 CCNP,CWNA,CCSA,Project+

 

[surfbum99]

excellent thanks for yer help