EIGRP Terminology

Successor route: The current best path, with the smallest metric. The "successful" route.
Successor: The first next-hop router for the successor route.
Feasible distance (FD): Lowest metric to reach a subnet. The sum of the RD + local cost.
Reported distance (RD): The metric inside a route update from another router. The sending router included it's FD, which becomes out RD.
Feasibility condition: If another path is actually a backup, the RD will be less than the current FD.
Feasible successor: A route that satisfies the feasibility condition and is maintained as a backup route.
Split Horizon: Never advertise a network, out the same interface it was learned on.
Poison Reverse: If you must advertise a network out the same interface it was received on, advertise the delay as infinity.

Example.

R2 sends an update

10.0.0.0/24 - RD is 2000

R3 Sends an update

10.0.0.0/24 - RD is 2050

R1 calculates total path metric.

R2 is 2000 + 1000 = 3000.
R3 is 2050 + 50 = 2100. < - Successor route.

R1 sees it has an reported distance less than the current distance, so installs that route as the feasible successor.

+--------+            1000             +--------+    10.0.0.0/24      
|   R1   +-----------------------------+   R2   +---------------------                           
+-----+--+                             +-+------+      2000                                                                               
      |            +--------+            |                            
      +------------+   R3   +------------+                                                                                                        
         50        +--------+      50

Example with the EIGRP topology table

R1# show ip eigrp topology 10.0.0.0/24
EIGRP-IPv4 Topology Entry for AS(1)/ID(1.1.1.1) for 10.0.0.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2100
P 10.0.0.0/24, 1 successors, FD is 2100                <--- Feasible Distance
        via 10.0.13.3 (2100/2050), GigabitEthernet0/3  <--- Successor Route
        via 10.0.12.2 (3000/2000), GigabitEthernet0/2  <--- Feasible Successor
                       |     |
                       |     +-- Reported Distance 
                       +-------- Path Metric
        
                                                             (RD 2000 < FD 2100)

Metric calculation

metric = ([K1 * bandwidth + (K2 * bandwidth) / (256 - load) + K3 * delay] * [K5 / (reliability + K4)]) * 256

K1, set to 1 K3, set to 1

Wide metrics allow for faster links.

Unequal Cost Multi Path

EIGRP can load balance over the successor and feasible successor routes with a variance command.

Timers

Hello packets are every 5 seconds, on 60 seconds on T1 links.
- The deadtime is 3x the hold timer.

Initial Bringup

Send Hello packets, to 224.0.0.10
- Doesnt' require multicast to be on
- Unicast Init from neighbor, set Seq, Set Ack to 0
  - Neighbor Sends back Ack as prior sequence number.
  - Update Messages

Stuck in Active

The router is too busy to answer the query (generally due to high CPU utilization).
The router has memory problems and cannot allocate the memory to process the query or build the reply packet.
The circuit between the two routers is not good; there are not enough packets that get through to keep the neighbor relationship up, but some queries or replies are lost between the routers.
unidirectional links (a link on which traffic can only flow in one direction because of a failure)

Update Message

AS number
Prefixes
End-of-table Flag

Prefixes

Type (internal, etc)
Reliability
Load
MTU
Hop Count
Delay
Bandwidth
Flags
- Source Withdrawn
- Candidate Default
- Route is Active
- Route is Replicated
Next-hop
Prefix Length

Network

The CLI parser is converting the IP into binary, then comparing it to the wild mask.
The CLI parser will only save the matched bits of the IP.
The CLI parser will not save the zeroth network, anything starting with 0.
The CLI parser will only save the matched bits of an IP if if finds bits that are "on"
Using the "all" mask of 255.255.255.255 creates this statement 'network 0.0.0.0' and matches everything.
Using the "unique-ip" mask of 0.0.0.0 means "match this single address"
The wildcard mask only accepts contiguous numbers "Discontiguous mask is not supported."

192.0.2.5 127.255.255.255 - becomes 128.0.0.0, the rest of the bits get dropped.

References

https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/16406-eigrp-toc.html

Ariadne's Network Notes