EIGRP General Operations

Operations

Operation information can be supplemented by looking into the ip protocol, as shown below.

R1#sho ip protocols

** omitted **

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

    Soft SIA disabled

    NSF-aware route hold timer is 240

  EIGRP NSF disabled

     NSF signal timer is 20s

     NSF converge timer is 120s

    Router-ID: 1.1.1.1

    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:

    1.1.1.1/32

    10.1.2.1/32

    10.1.3.1/32

  Routing Information Sources: MY Neighbors

    Gateway         Distance      Last Update

    10.1.3.3              90      00:49:12

    10.1.2.2              90      00:49:12

  Distance: internal 90 external 170

Best path selection

• EIGRP uses K values to calculate the metric, hence the best path.

The default K values are:

• K1=1 (Bandwidth)
• K2=0 (Reliability)
• K3=1 (Delay)
• K4=0 (Load)
• K5=0 (MTU)

• EIGRP uses Unicast and multicast to propagate information.
• Incremental and bounded updates.
• Five primary packet types (Message types) to process data. 
• Three data structures (Neighbor, topology (EIGRP RIB) Routing table (RIB))
• DUAL - how the routers make decision.
• EIGRP Router ID is preemptive.
• Default Maximum path is 4 but it can go till 32.
• The protocol number for EIGRP is 88

Equal Cost Load Balancing.

Equal-cost - EIGRP automatically load balance the traffic if multiple equal cost path exists to a destination.

To show this concept, we can once again to into the topology and routing table of R1 towards R4's loopback interface with IP 4.4.4.4/32

R1 topology table.

R1#sho ip eigrp topology | be 4.4.4.4

P 4.4.4.4/32, 2 successors, FD is 131072

        via 10.1.2.2 (131072/130816), GigabitEthernet1

        via 10.1.3.3 (131072/130816), GigabitEthernet2

Based on the output, we can see that there are two successors for 4.4.4.4. Both have the same FD and RD. Hence, both make to the routing table, as seen below.

R1#sho ip route | be 4.4.4.4

D        4.4.4.4 [90/131072] via 10.1.3.3, 00:26:08, GigabitEthernet2

                 [90/131072] via 10.1.2.2, 00:26:08, GigabitEthernet1

We can also see more details if we specify the destination subnet.

R1#sho ip route 4.4.4.4

Routing entry for 4.4.4.4/32

  Known via "eigrp 1", distance 90, metric 131072, type internal

  Redistributing via eigrp 1

  Last update from 10.1.3.3 on GigabitEthernet2, 00:27:21 ago

  Routing Descriptor Blocks:

    10.1.3.3, from 10.1.3.3, 00:27:21 ago, via GigabitEthernet2

      Route metric is 131072, traffic share count is 1

      Total delay is 5020 microseconds, minimum bandwidth is 1000000 Kbit

      Reliability 255/255, minimum MTU 1500 bytes

      Loading 1/255, Hops 2

  * 10.1.2.2, from 10.1.2.2, 00:27:21 ago, via GigabitEthernet1

      Route metric is 131072, traffic share count is 1

      Total delay is 5020 microseconds, minimum bandwidth is 1000000 Kbit

      Reliability 255/255, minimum MTU 1500 bytes

      Loading 1/255, Hops 2

Unequal Cost Load Balancing

Unequal-cost

EIGRP can load balance traffic via paths with unequal cost. This is achieved by configuring the variance.

It multiplies against the best metric.

If the Successor has an FD of 10 and I set a variance of 2, then any path between 10-20 is now considered equal. Note that variance only work for paths that are FS. If redundant path exists, but it's not an FS, the variance will not make it an FS then put it in the routing table.

To check how variance works, let's consider the route from R1 to 10.2.4.0/24, which the connection between R2 and R4.

R1#sho ip eigrp topology | be 10.2.4

P 10.2.4.0/24, 1 successors, FD is 3072

        via 10.1.2.2 (3072/2816), GigabitEthernet1

As we can seem no feasible successor exist, hence variance won't make any change here. First, let's find ways to make the path via R3 as an FS. We will do it by configuring offset-list on R2 to make increase the FD at R1 and make the RD from R3 to R1 match the feasibility condition. 

First, we will analyze all links in R1's topology table to see the alternate path values. As we can see below, the path via R3 does not meet the feasibility condition.

R1#sho ip eigrp topology all | be 10.2.4

P 10.2.4.0/24, 1 successors, FD is 3072, serno 156

        via 10.1.2.2 (3072/2816), GigabitEthernet1

        via 10.1.3.3 (3328/3072), GigabitEthernet2

Now we will login to R2 and configure an offset-list to increase the FD for 10.2.4.0 at R1.

R2#conf t

R2(config)#access-list 1 permit 10.2.4.0 0.0.0.255

R2(config)#router eigrp 1

R2(config-router)#offset-list 1 out 100 gigabitEthernet 1

Now lets see the topology table on R1. As we can see below, now the path via R3 is an FS.

1#sho ip eigrp topology | be 10.2.4

P 10.2.4.0/24, 2 successors, FD is 3172

        via 10.1.2.2 (3172/2916), GigabitEthernet1

        via 10.1.3.3 (3328/3072), GigabitEthernet2

Before changing the variance, we will see the routing table of R1 to check the routers it contains. As you can see below, only one path exists towards 10.2.4.0/24.

R1#sho ip route | be 10.2.4.0

D        10.2.4.0/24 [90/3172] via 10.1.2.2, 00:00:09, GigabitEthernet1

D        10.3.4.0/24 [90/3072] via 10.1.3.3, 00:00:09, GigabitEthernet2

Now lest change the variance and see the result. One of the simple ways to find the value for the variance is to do the best path FD/FS RD, in this case 3172/3072. If the value is not exact value, then round up by excess. Example, if the result is 1.234, it will be 2. So 3172/3072 = 1.032 ~ 2, so the variance is 2.

Now we will change the variance on R1. 

R1#conf t

R1(config)#router eigrp 1

R1(config-router)#variance 2

After changing the variance, we can see from the below output that there are now two unequal cost paths towards 10.2.4.0/24.

R1#sho ip route | be 10.2.4

D        10.2.4.0/24 [90/3328] via 10.1.3.3, 00:00:15, GigabitEthernet2

                     [90/3172] via 10.1.2.2, 00:00:15, GigabitEthernet1

Below is a more detailed output of show ip route with the destination route specified. As we can see two paths exist and the share count is also observed.

R1#sho ip route 10.2.4.0

Routing entry for 10.2.4.0/24

  Known via "eigrp 1", distance 90, metric 3172, type internal

  Redistributing via eigrp 1

  Last update from 10.1.3.3 on GigabitEthernet2, 00:00:21 ago

  Routing Descriptor Blocks:

    10.1.3.3, from 10.1.3.3, 00:00:21 ago, via GigabitEthernet2

      Route metric is 3328, traffic share count is 229

      Total delay is 30 microseconds, minimum bandwidth is 1000000 Kbit

      Reliability 255/255, minimum MTU 1500 bytes

      Loading 1/255, Hops 2

  * 10.1.2.2, from 10.1.2.2, 00:00:21 ago, via GigabitEthernet1

      Route metric is 3172, traffic share count is 240

      Total delay is 23 microseconds, minimum bandwidth is 1000000 Kbit

      Reliability 255/255, minimum MTU 1500 bytes

      Loading 1/255, Hop