eigrp path selection question

[dannan lin]

hi:

i want to know how to see all the successor routes in routing table.

below is the scenario.

from above topology, it is obvious there are two routes to loopback interface at R2 .  one is via 23.1.1.2/24 another one is via 13.1.1.1/24

however from the routing table and the toplogy table i can only see one successor route which is via 23.1.1.2 ? why is this ?

below are outputs of routing table and toplogy table.

routing table:

toplogy table :

However if i disconnect the 23.1.1.2 /24 s3/0 interface , the new route to loopback appears (via f0/0)

why EIGRP does not show me all the routes to loopback 2.2.2.0 network  while it shows 2 successor routes to 12.1.1.0 network ????

Thanks in advance.

[cadet alain]

Hi,

It can't show you 2 successors if there is only one, it's not a magician.There is also only one successor for the other subnet. if you do a sh ip eigrp topology all-links then you'll see the other path is not a FS because its RD is not < than the FD of the successor and it can't be a successor as its FD is greater than the successor FD. Don't forget the metric takes the minimum bandwidth + the cumulative delay into account.The bandwith is the same for both path but going via f0/0 has a higher delay.

Regards.

Alain.

[dannan lin]

thanks cadet :

but from what i understand about EIGRP the condition of being a FS is when RD is < FD .

so in my case,

i typed in "show ip eigrp top all" command and output looks like below

R3>sh ip eigrp topology all-links
IP-EIGRP Topology Table for AS 100

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

P 13.1.1.0/24, 1 successors, FD is 28160
         via Connected, FastEthernet0/0
P 12.1.1.0/24, 1 successors, FD is 20514560
         via 13.1.1.1 (20514560/20512000), FastEthernet0/0
         via 23.1.1.2 (21024000/20512000), Serial3/0
P 2.2.2.0/24, 1 successors, FD is 20640000
         via 23.1.1.2 (20640000/128256), Serial3/0
P 23.1.1.0/24, 1 successors, FD is 20512000
         via Connected, Serial3/0

as you can see there is  still one successor (via s3/0) only .

if i shut serial link (s3/0)

and typed the same command again , the output is like below

R3>sh ip eigrp topology all-links
IP-EIGRP Topology Table for AS 100

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

P 13.1.1.0/24, 1 successors, FD is 28160
         via Connected, FastEthernet0/0
P 12.1.1.0/24, 1 successors, FD is 20514560
         via 13.1.1.1 (20514560/20512000), FastEthernet0/0
P 2.2.2.0/24, 1 successors, FD is 20642560
         via 13.1.1.1 (20642560/20640000), FastEthernet0/0
P 23.1.1.0/24, 1 successors, FD is 20512000
         via Connected, Serial3/0

alright, the new successor is there with higher cost, but its FD is indeed higher than RD ( 20642560 > 20640000)

for this reason it should be quilfied as a successor route. but there is no way i can see it from routing table or toplogy table .

any thoughts?

thanks 

[cadet alain · Accepted solution]

Hi,

when the serial interface is not shut then the  FD= 20640000 but when you shut it the new successor has a FD= 20642560

so higher than the successor when the serial is up so it couldn't not be a successor because the successor is the route with the lower FD.What puzzles me though is that it doesn't appear in the topology table when you do a sh ip eigrp topology all-links.Could you clear ip eigrp neighbour  or clear ip route * on R3 when the serial and fastethernet are up because we should see the non FS via fastethernet in the topology table.

but from what i understand about EIGRP the condition of being a FS is when RD is < FD

yes but the RD of the FS must be < FD of successor

Alain.

[dannan lin]

hi cadet:

thanks for your help,

i clear the routing table but nothing changes.  and packet tracer does not support  " clear ip eigrp neighbour " command

i think maybe it is just another bug inside the Packet Tracer.

anyway much appreciated.