eigrp... the five basics

1.  configure eigrp neighbor routers with the same autonomous system number...

ie,

dls1#config t
Enter configuration commands, one per line.  End with CNTL/Z.
dls1(config)#router eigrp 1

2.  eigrp will be configured only for interfaces with a matching network command

ie,

dls1(config)#router eigrp 1
dls1(config-router)#network 172.16.1.0 0.0.0.255
dls1(config-router)#

eigrp will discover neighbors on that interface by sending multicast hello's.
eigrp will advertise the subnet to the neighbors for that interface

3.  if no wildcard mask is provided, the classful network will be assumed
ie,

dls1(config)#router eigrp 1
dls1(config-router)#network 172.16.1.0 0.0.0.255
dls1(config-router)#netw 172.16.100.0

4.  if no wildcard is provided, eigrp is configured for all interfaces associated with the classful network

5.  if a wildcard is provided, the corresponding matching logic is in effect


wildcard mask easy review/easy method

a wildcard mask uses the same matching logic as an acl...

if the network is 10.1.5.0 /18 equals 255.255.192.0

why?

because 16 (or the 1st two octets of all one's) plus 2 (the third octet has the first instance of a zero, or less than 255)

we know the action is in the third octet because of the first instance of  a zero bit

128 192 224 240 248 252 254 255
127   63   31  15    7      3     1   host

256 - 192 = 64 minus 1

the value of the third octet is 63 for the wildcard or 0.0.63.255

you could subtract from 255 but subtracting from 256 and remembering to then minus one is easier

for review on this go here:

http://insearchofthecert.blogspot.com/2011/12/wildassmasks.html

but chances are if you need to review subnetting and wildcard masks, this is what you really need...

http://insearchofthecert.blogspot.com/#uds-search-results

perhaps namor could help with that...