eigrp not send update

[Arie]

I have three routers, all interfaces and connections configured well, I enabled eigrp by "router eigrp 1" on all of them. On one of them i added one classfull network. The problem is when i enter debug eigrp on that router it not show any message that would proofe that it is sending updates to neighbors. Also rest routers don't get anything (routers tables checked) . What could be the reason?

[Julio E. Moisa]

Yeap exactly, that is reason to have redundancy on the network infrastructures in order to avoid downtimes

:-)

[Arie]

Fine, great i didn't know that i have to provide subnet which is placed between routers to exchange information.

One more question, let's say one of the inner subnets of R1 has 172.16.4.0 subnet and subnet between R2 and R1 is 172.16.7.0.

If i would add clasfull network to the command network: network 172.16.0.0 then automatically 172.16.4.0 will be included in broadcast as well as 172.16.7.0, right? No matter if i would specify auto summary or no auto suummary in this case both subnets will match the mask therefore both will be included?

If that sentence is correct and to this particular example i consider two situations:

Target: i would like to exclude 172.16.4.0 (R1)

1) case when we have auto summary enabled :

i am not able to do that as both subnets are clasfull subnets so cant do it as their wildcards would be the same so R1 will broadcast: 172.16.0.0 (both .4 and .7 included)

2) case when we have no auto summary

i could do either:

network 172.16.7.0 0.255.255 / network 172.16.7.0 (same effect)

(this way only this concrete subnet .7 will be added and not .4 as auto summary will distinguish both networks)

Am i right with all of this? If yes i am happy :)

[Julio E. Moisa]

Yeap you are correct :-)

A supernet 172.16.0.0 0.0.255.255 will include the subnet 172.16.4.0 and 172.16.7.0 because they are part of supernet.

Just a comment on the second case:

netwokr 172.16.7.0 0.0.255.255 will not have the same effect if you type172.16.7.0 (without the wildcard) why?

Because you are not specifying the wildcard so Eigrp will take the subnet 172.16.7.0 as a classful so it will include under the Eigrp 1 the following

R2(config)#router eigrp 1
R2(config-router)#network 172.16.7.0
R2(config-router)#no aut
R2(config-router)#no auto-summary
R2(config-router)#end
R2#
R2#sh
*Mar 1 02:08:58.179: %SYS-5-CONFIG_I: Configured from console by cons
R2#
R2#sh run | section eigrp
router eigrp 1
network 172.16.0.0
no auto-summary
R2#

I recommend to use no auto-summary under the Eigrp process to receive the subnet with the correct subnet mask on the routing table

:-) 

[Arie]

so when specifying concrete networks its better to put wildcard, otherwise it will put classfull network mask the closest to the left right?

Can you look at this topic as well?:

https://supportforums.cisco.com/discussion/13223861/eigrp-one-subnet-not-updated

[Julio E. Moisa]

Yeap I prefer to configure the wildcard to be more specific and use no auto-summary, It is much better to have control of the network.

I have seen the topic, the serial interfaces are not on the same subnet, they must be on the same segment. 

:-)