EIGRP

Enhanced Interior Gateway Routing Protocol: 

• advance distance Vector routing protocol supporting large networks
• Very fast convergence time
• supports Bounded Updates where "network topology change updates" are only sent to routers affected by the change
• messages are sent using multicast
• automatically performs equal cost load balancing on up to 4 paths by default; can be increased up to 16
• EIGRP is the only protocol that can perform UnEqual Cost load balancing

Standard Configuration:
  
    R1(config)#router eigrp 100

• '100': example of an Autonomous System (AS) and all EIGRP routers need the same AS number to peer with each other

   R1(config-router)#network 10.0.0.0  0.0.255.255

• The network command uses a wildcard mask format which is the inverse of the subnet mask - subtract each octet by 255 to calculate.
• subnet mast of 255.255.0.0 equals wildcard mask of 0.0.255.255
• and 255.255.255.252 equals wildcard mask of 0.0.0.3
• If you don't specify the wildcard mask (#network 10.0.0.0), the command defaults to a classful boundary (A, B, or C)
• Network Command means:
• look for interfaces that fall within this IP range
• enable EIRGP on those interfaces, send out and listen for EIGRP hello messages and peer with adjacent EIGRP routers
• advertise the network and mask which is configured on those interfaces
• R1(config-router)#network 10.0.0.0 
• You can also enter these very specifically with:
• R1(config-router)#network 10.1.0.0  0.0.0.255
• R1(config-router)#network 10.0.1.0  0.0.0.255
• R1(config-router)#network 10.0.2.0  0.0.0.255

• Or you can also specify the interface address /32, note that the router will advertise the network on the interface which is /24, not the /32 address.
• R1(config-router)#network 10.1.0.1  0.0.0.0
• R1(config-router)#network 10.0.1.1  0.0.0.0
• R1(config-router)#network 10.0.2.1  0.0.0.0
• R1(config-router)#network 10.0.0.0  0.0.255.255 

Verification Commands:

  R1#show run | section eigrp
  R1#show ip protocols
  R1#show ip eigrp interface brief
  R1#show ip eigrp neighbor
  R1#sh ip eigrp topology  (displays Successors and Feasible Successors)
  R1#sh ip eigrp topology all-links  (displays S, FS and non Feasible Successors)
  R1#show ip route   ( (D) is the eigrp designation)

RouterID 

• EIGRP routers identify themselves to each other with an ID that looks like an IPv4 address
• This ID will default itself to the highest loopback IP address and if none configured, to the highest interface address
• Loopback interfaces never go down, therefore, best practice is to have a loopback on your router or manually set the Router ID
• R1(config)#router eigrp 100
• R1(config-router)#eigrp router-id 2.2.2.2
• if other router interfaces are already up, you must disable/enable eigrp for the ID to take effect or reboot router

Manual Summarization 

• Auto-Summary is by default off for EIGRP on all new IOS 
• Manual summary routes can be used to reduce router load as only the summarised routes are advertised, not all routes
• R2(config-router)#interface fa1/0
• R2(config-if)#ip summary-address eigrp 100 10.0.0.0 255.255.0.0
• R2(config-router)#interface fa0/0
• R2(config-if)#ip summary-address eigrp 100 10.1.0.0 255.255.0.0

Passive Interface Config 

• R2(config)#router eigrp 100
• R2(config-router)#passive-interface loopback 0
• R2(config-router)#passive-interface gi0/0

Default Route Injection

• R4(config)#ip route 0.0.0.0  0.0.0.0  203.0.113.2
• R4(config)#interface fa0/0
• R4(config-if)#ip summary-address eigrp 100 0.0.0.0 0.0.0.0

EIGRP Successors and Feasible Successors:

• EIGRP receives routes from its neighbors with the neighbors metric to a distant network. This is the Reported Distance  or Advertised Distance
• It then adds that metric to the distant network, to its own metric to reach the neighbor for the total metric. This is the Feasible Distance.
• If multiple routes are available, the best makes it into the routing table. This is the Successor Route.
• If a 'less than best' Feasible Distance route can meet certain criteria to qualify, they can be stored as a backup route, known as a Feasible Successor
• If a Successor Route fails, the Feasible Successor is immediately added to the routing table, no need to query all other neighbors.
• A route qualifies as a Feasible Successor if its Reported Distance is lower than the Feasible Distance of the current Successor Route.

         R2#sh ip eigrp topology  (displays Successors and Feasible Successors)

         R2#sh ip eigrp topology all-links (displays S, FS and non Feasible Successors)

Metric - EIGRP Calculation

• EIRGP can consider various link characteristics to calculate its metric like Bandwidth, Delay, Reliability, and Load
• The calculation is insane with five different "K" values multiplied and divided by B,D,R, and L.
• The short formula is 256* (inverted bandwidth + delay); therefore, high bandwidth + lower delay  = lower (better) metric.
• Lower metric routes are preferred, but you can manipulate the bandwidth or delay on the interfaces thus changing the metric value....NOTE - changing the delay is preferred as altering the bandwidth can affect other things in the router ...like Qos.
• R1(config)#interface fa0/0
• R1(config-if)#bandwidth 768
• R1(config-if)#delay 1000  (tens of microseconds)
• ...BW 768 Kbits/sec, DLY 10000 usec, (microseconds)
• NOTE: changing the Bandwidth or Delay does not affect the physical properties of the interface, it only affects the software policies such as this EIGRP metric