Diagram 1 shows that We have two Area 0 in a network, between them we have Area 1 and Area 2.

Diagram 1: Two Area 0 in one network

A simple solution will be to have two virtual-links between R3 – R1/R2, and R4-R3. Another will be to have two GRE tunnels between R3-R1/R2 and R3-R4. Or mix of both, virtual-link between R3-R1/R2 and GRE tunnel between R4-R3.

One interesting and very easy method is to have two instances of OSPF in R3, and perform mutual redistribution. How it works?

R4 is part of Area 0, while R1/R2 are part of Area 0 as well. What remains in R3, which is not connected directly to Area 0 by any side. But, if we break OSPF instance in two. we end up having two full proper OSPF networks. The right of R3 we have Area 2 connected to Area 0. while the Left of R3 we have Area 1 connected to Area 0. Hence, doing mutual redistribution at R3 between those two OSPF networks will yield a full network database.

First, to check the neighbor:

#show ip eigrp neighbors
IP-EIGRP neighbors for process 10
H   Address                 Interface   Hold Uptime   SRTT   RTO  Q  Seq Type
(sec)         (ms)       Cnt Num
8   10.254.26.1             Vl1           12 5d22h      25   200  0  76435
7   10.254.58.1             Vl1           12 5d22h      25   200  0  4134
3   10.254.0.13             Vl1           10 5d22h      23   200  0  571
1   10.254.0.12             Vl1           11 5d22h      22   200  0  54511
0   10.254.58.2             Vl1           14 5d22h      18   200  0  3354
10  10.15.255.254           Gi2/9         13 4w0d        1   200  0  764
19  10.60.255.254           Gi3/8         12 13w4d       2   200  0  4008
16  10.10.255.254           Gi3/1         11 15w4d       1   200  0  1007
13  10.24.255.254           Gi3/6         10 15w4d      21   200  0  1010
5   10.5.255.254            Gi2/3         11 16w4d       1   200  0  37489
31  10.17.255.254           Gi2/11        10 16w4d      15   200  0  54827
2   10.3.255.254            Gi2/1         14 16w6d       2   200  0  4024
47  10.4.255.254            Gi2/2         14 17w0d      16   200  0  2925

H is the sequence of neighbor discovery. Interface, is where the neighbor is located. Hold is the timer responsible to consider the neighbor dead in case Hellos ceased to receive. up-time is obvious. SRTT is the time between transmission of hello to receiving acknowledgment. RTO – in the case of multicast failure, the router will send a unicast to the neighbor. RTO is the time wait for acknowledgment for the unicast packet. Q number of queued packets. Seq Num is the sequence number of the last EIGRP packet received.

Second is to check the topology, it will indicate the cost, and how many routes are available for a unique destination network.

#show ip eigrp topology
IP-EIGRP Topology Table for AS(10)/ID(10.250.0.10)

Codes: P – Passive, A – Active, U – Update, Q – Query, R – Reply,
r – reply Status, s – sia Status

P 10.18.24.0/22, 2 successors, FD is 28928
via 10.254.0.11 (28928/28672), Vlan1
via 10.5.255.254 (28928/28672), GigabitEthernet2/3
P 10.9.255.252/31, 1 successors, FD is 3072
via 10.254.0.11 (3072/2816), Vlan1
P 0.0.0.0/0, 1 successors, FD is 281600
via Rstatic (281600/0)
P 10.10.0.0/20, 1 successors, FD is 3072
via 10.10.255.254 (3072/2816), GigabitEthernet3/1
P 10.11.255.254/31, 1 successors, FD is 2816
via Connected, GigabitEthernet3/3
P 10.10.255.254/31, 1 successors, FD is 2816
via Connected, GigabitEthernet3/1
P 10.11.0.0/20, 1 successors, FD is 3072
via 10.11.255.254 (3072/2816), GigabitEthernet3/3
P 10.8.255.252/31, 2 successors, FD is 3072
via 10.8.255.254 (3072/2816), GigabitEthernet2/6
via 10.254.0.11 (3072/2816), Vlan1
P 10.24.16.0/20, 1 successors, FD is 3072
via 10.24.255.254 (3072/2816), GigabitEthernet3/6
P 10.9.255.254/31, 1 successors, FD is 2816
via Connected, GigabitEthernet2/7
P 10.11.255.252/31, 2 successors, FD is 3072
via 10.11.255.254 (3072/2816), GigabitEthernet3/3
via 10.254.0.11 (3072/2816), Vlan 1

P means passive, this indicate a stable route. Active is a lost route which the protocol will try to find alternative path for it through quires. The first entry has FD of 28928. (this is Feasible Successor Plus the cost to that neighbor). This is considered the total cost. The second number 28672 is the Feasible Successor (the cost advertise by the neighbor to that network). For any path to become eligible as a successor, the FD should be equal or greater than FS. This ensures a loop free routing.  via are the neighbors, and which interface they connected through.

Lastly,the command that will give the summary of all routing protocols running in the router/switch.

#show ip protocols
Routing Protocol is “eigrp 10″
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 metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Default redistribution metric is 10000 100 255 1 1500
Redistributing: static, eigrp 10
Automatic network summarization is not in effect
Maximum path: 4
Routing for Networks:
10.0.0.0
172.22.0.0/30

Routing Information Sources:
Gateway         Distance      Last Update
10.11.255.254         90      05:15:10
10.10.255.254         90      05:15:10
10.9.255.254          90      05:15:10

Here, we can see which networks are advertised, any access-list filtering for routes, the K values, redistribution, and in other cases any passive-interfaces. The last column showing the neighbors connected.

Before EIGRP can update and send topology information, building relationships between EIGRP enabled routers is the first process. For two routers to become neighbors, the following conditions should be met.

• Autonomous System number should be same.
• The K values should be same. (they are same if left on default).
• The routers should be in the same subnet.

Here is the syntax for configuring EIGRP

Router> enable
Router# config terminal
Router(config)# router eigrp 1
Router(config-router)# network 10.2.9.0 ?
  A.B.C.D  EIGRP wild card bits
  <cr>
Router(config-router)# network 10.2.9.00 0.0.0.255
Router(config-router)# no auto-summary
Router(config-router)# end

EIGRP by default uses Auto-Summary for routes within certain condition. The router will summarize when it is residing between two different Networks (not subnets). For example between 192.168.1.0 and 192.168.2.0 since they are two different class C networks.

The calculation of Metrics (cost) is complicated slightly, but when using default K values =1. The equation is straight forward. Here is the Equation

Cost = [(K1 X Bandwidth + ((K2 X bandwidth)/(256-load)) +K3 X delay) X K5/(K4+reliability)] X 256

With default values K1, K3=1, and K2, K4, K5 = 0. the equation becomes.

Cost = (Bandwidth + delay) X 256

Where bandwidth is the minimum in the link, and delay is cumulative.