eigrp... it never goes away...

it is often nice to go far away until you realize it's time to go back home...

neighbor eigrp routers need to be in the same AS

ios matches interfaces to eigrp that are configured with the network command
   once configured those interfaces attempt to discover neighbors using multicast hellos
   and will advertise their connected subnets (networks)

without  a wildcard mask, only classful networks will be advertised

without a wildcard mask, eigrp is enabled for all interfaces directly connected to that classful network

a configured wildcard mask uses acl logic for network comparison

shows the enabled eigrp interfaces:

r4#sh ip eigrp int
EIGRP-IPv4 Interfaces for AS(1)
                              Xmit Queue   PeerQ        Mean   Pacing Time   Multicast    Pending
Interface              Peers  Un/Reliable  Un/Reliable  SRTT   Un/Reliable   Flow Timer   Routes
Lo0                      0        0/0       0/0           0       0/0            0           0
Se1/1                    1        0/0       0/0          41       0/24         220           0
Se1/2                    1        0/0       0/0          24       1/48         132           0
Se1/3                    1        0/0       0/0          15       5/195        259           0

shows the network configuration for each enabled routing process:

r4#sh ip protocols
*** IP Routing is NSF aware ***

Routing Protocol is "eigrp 1"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Default networks flagged in outgoing updates
  Default networks accepted from incoming updates
  EIGRP-IPv4 Protocol for AS(1)
    Metric weight K1=1, K2=0, K3=1, K4=0, K5=0
    NSF-aware route hold timer is 240
    Router-ID: 172.18.1.1
    Topology : 0 (base)
      Active Timer: 3 min
      Distance: internal 90 external 170
      Maximum path: 4
      Maximum hopcount 100
      Maximum metric variance 1

  Automatic Summarization: disabled
  Maximum path: 4
  Routing for Networks:
    172.18.1.0/24
    172.20.0.0
  Routing Information Sources:
    Gateway         Distance      Last Update
    172.20.15.21          90      10:25:59
    172.20.15.17          90      10:25:59
    172.20.15.13          90      10:25:59
  Distance: internal 90 external 170

shows eigrp neighbors:

r4#sh ip eigrp neigh
EIGRP-IPv4 Neighbors for AS(1)
H   Address                 Interface              Hold Uptime   SRTT   RTO  Q  Seq
                                                   (sec)         (ms)       Cnt Num
1   172.20.15.17            Se1/2                    10 10:29:42   24   288  0  399
2   172.20.15.21            Se1/3                    14 12:00:12   15  1170  0  404
0   172.20.15.13            Se1/1                    12 12:00:12   41   246  0  402

 shows all known successor and feasible successor routes for the router:

r4#sh ip eigrp topo
EIGRP-IPv4 Topology Table for AS(1)/ID(172.18.1.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 192.168.17.0/24, 1 successors, FD is 3651840
        via 172.20.15.13 (3651840/2297856), Serial1/1
        via 172.20.15.17 (6151936/2297856), Serial1/2
        via 172.20.15.21 (21152000/2297856), Serial1/3
P 172.20.10.0/24, 1 successors, FD is 3139840
        via 172.20.15.13 (3139840/128256), Serial1/1
        via 172.20.15.17 (5639936/128256), Serial1/2
        via 172.20.15.21 (20640000/128256), Serial1/3
P 172.20.15.16/30, 1 successors, FD is 5511936
        via Connected, Serial1/2
P 172.20.15.20/30, 1 successors, FD is 20512000
        via Connected, Serial1/3
P 172.20.15.12/30, 1 successors, FD is 3011840
        via Connected, Serial1/1
P 172.20.15.8/30, 2 successors, FD is 11023872
        via 172.20.15.13 (11023872/10511872), Serial1/1
        via 172.20.15.17 (11023872/10511872), Serial1/2
        via 172.20.15.21 (21024000/10511872), Serial1/3
P 172.18.1.0/24, 1 successors, FD is 128256
        via Connected, Loopback0
P 172.20.15.0/30, 3 successors, FD is 21024000
        via 172.20.15.13 (21024000/20512000), Serial1/1
        via 172.20.15.17 (21024000/20512000), Serial1/2
        via 172.20.15.21 (21024000/20512000), Serial1/3
P 172.20.15.4/30, 1 successors, FD is 3523840
        via 172.20.15.13 (3523840/2169856), Serial1/1
        via 172.20.15.17 (6023936/2169856), Serial1/2
        via 172.20.15.21 (21024000/2169856), Serial1/3
P 192.168.16.0/24, 3 successors, FD is 21152000
        via 172.20.15.13 (21152000/20640000), Serial1/1
        via 172.20.15.17 (21152000/20640000), Serial1/2
        via 172.20.15.21 (21152000/20640000), Serial1/3

shows the routing table:

r4#sh ip route | excl L | inc loop
      

Gateway of last resort is not set

      172.18.0.0/16 is variably subnetted, 2 subnets, 2 masks
C        172.18.1.0/24 is directly connected, Loopback0
      172.20.0.0/16 is variably subnetted, 10 subnets, 3 masks
D        172.20.10.0/24 [90/3139840] via 172.20.15.13, 10:32:27, Serial1/1
D        172.20.15.0/30 [90/21024000] via 172.20.15.21, 10:32:27, Serial1/3
                        [90/21024000] via 172.20.15.17, 10:32:27, Serial1/2
                        [90/21024000] via 172.20.15.13, 10:32:27, Serial1/1
D        172.20.15.4/30 [90/3523840] via 172.20.15.13, 10:32:27, Serial1/1
D        172.20.15.8/30 [90/11023872] via 172.20.15.17, 10:32:27, Serial1/2
                        [90/11023872] via 172.20.15.13, 10:32:27, Serial1/1
C        172.20.15.12/30 is directly connected, Serial1/1
C        172.20.15.16/30 is directly connected, Serial1/2
C        172.20.15.20/30 is directly connected, Serial1/3
D     192.168.16.0/24 [90/21152000] via 172.20.15.21, 10:32:27, Serial1/3
                      [90/21152000] via 172.20.15.17, 10:32:27, Serial1/2
                      [90/21152000] via 172.20.15.13, 10:32:27, Serial1/1
D     192.168.17.0/24 [90/3651840] via 172.20.15.13, 10:32:27, Serial1/1

eigrp establishes routes by:

sending hello messages

exchanging complete topologies intitially, then partially thereafter, including changes only  

adding routes to the routing table with the lowest metric