EIGRP (ENHANCED INTERIOR GATEWAY ROUTING PROTOCOL)

EIGRP is an Interior Gateway Routing Protocol.

• The Enhanced Interior Gateway Routing Protocol replaced the Interior Gateway Routing Protocol (IGRP) in 1993.
• EIGRP is an advanced routing protocol that combines many of the features of both link-state and distance-vector routing protocols

How EIGRP Becomes Enhanced?

1. No Periodic Updates in EIGRP.
   • Unlike other well-known routing protocols, such as RIP, EIGRP only sends incremental updates, reducing the workload on the router and the amount of data that needs to be transmitted.
2. Updates are Multicasted (224.0.0.10) and protocol number 88.
3. Updates are always triggered.
4. Updates are Partial.
   • The only Exception is during Startup Process, When the Routers First Form Neighborship: They Share Entire Routing Table.
5. Updates are Shared Reliably.
   • EIGRP neither use TCP or UDP, It uses its own Mechanism to Share its Messages Reliably.) RTP: Cisco’s Reliable Transport Protocol.
6. EIGRP Maintains Backup Paths along with Best Paths.
   • Improves the Convergence Time
7. Maximum Allowable Hop Counts in EIGRP is 100 by Default, and can be Increased Up to 255.
8. Equal and Unequal cost Load Balancing is Supported.
9. EIGRP uses DUAL Algorithm (Diffusion Update Algorithm)

Other Features of EIGRP:

1. By Default EIGRP is Classful, but we can make it Classless (No Auto-summary)
2. EIGRP has AD: 90
3. EIGRP was once Cisco Proprietary but has been made Open Standard in 2013.
4. Neighborship Feature.
5. DUAL Feature.
6. RTP (Reliable Transport Protocol)
7. EIGRP has Autonomous System Number Concept.
   • Autonomous System:  Collection of Network Devices with in Single Administration. Collection of Networks with Common Routing Policies.
     • Autonomous System is 16 bit Value.
     • Range is 1 – 65535
     • All the Routers within the Same AS Share Network Information with Each Other.
     • If two Routers Belongs to Different AS, Then they will not Share Network Information with Each Other by Default.
8. EIGRP Calculates Distance on the Basis of Some Realistic Parameters which must be used for Calculating Distance.

Composite Metric.

1. Bandwidth
   • Minimum Bandwidth (in kilobits per second) along the path from router to destination network
2. Load
   • Number in range 1 to 255; 255 being saturated
3. Delay
   • Delay, in 10s of microseconds, along the path from router to destination network
4. Reliability
   • Number in range 1 to 255; 255 being the most reliable

Neighborship Feature

1. The Moment you enable EIGRP on an Interface, the Router Starts transmitting hello Messages via that Interface.
2. These Hello Messages are sent Every 5 seconds By Default.
3. Hold Timer:  15 Seconds.
4. If a Neighbor Remains Silent for 15 Seconds and Does not send any EIGRP Message, then that Neighbor will be Declared Dead.
5. Hello Messages are used to form Neighborship.
6. Until or unless two Routers are Neighbors in EIGRP, they cannot Share Network Information with Each Other.

Q.  Why Neighborship is formed?

• So that a Router can track of those Routers with it is sharing Network information.  And if a Particular router does not acknowledge for the message, then the particular Router is provided that same information again and again, because in EIGRP Network Information is shared reliably.

EIGRP has few Neighbor Conditions:

• The Routers Must Belong to the Same Network.
• AS Number Must Match.
• If Authentication is Enable, then that also Must Match.
• K Metric Values Must Match.             

K  Metric Values:

• Composite Metric

Bandwidth	K1:	1	(Bandwidth Modifier)
Load	K2:	0	(Load Modifier)
Delay	K3:	1	(Delay modifier)
Reliability	K4 & K5:	0	(K4: Reliability Modifier, K5: Additional Reliability Modifier.)

• These are Default Values.
• K1 (Bandwidth) + K2 (Load) + K3 (Delay) + K4 & K5 (Reliability
• The K Metric Values are 8 bits.
• Range:  0 to 255
• With the help of these K Metric Values we can Change the Distance Calculation Formulae that EIGRP Uses.

EIGRP Distance Calculation Formulae:

• 256*[K1 [10^7/Least Bandwidth in kb/sec] + K3 [Cumulative Delay in use/ 10]      

                              K1 and K3 are 1 by default.

DUAL Feature (Diffusion Update Algorithm)

• DUAL has Two Jobs:
  1. Best Path Computation.
     • Must be Loop Free.
     • Must have the Best Distance.
  2. Helping EIGRP to Stabilize after a Topology Change.

Terms and Terminologies:

• Feasible Distance: It is the Best Metric from a Router to Reach a Network.
• Reported Distance: This Distance is reported by the Neighbor, where the Neighbor tells that’s from it how far a Network is.
• Successor: This is a Best Path from a Router to Forward data to a Network.
• Feasible Successor: This is a Backup Path to reach a Network.
  1. This Helps in Improving the Convergence Time for EIGRP.                                                
  2. Not Every other Path can Become Backup Path.
  3. There is a Condition that has to be fulfilled. And this Condition Helps EIGRP to find whether the Path is a Looped Path or a Loop free Path.
  4. Condition is Called Feasibility Condition.
  5. This Condition says:

                                RD < FD                     

Reported Distance < Feasibility Distance

In addition to the routing table, EIGRP uses the following tables to store information:

Neighbor Table: 

• The neighbor table keeps a record of the IP addresses of routers that have a direct physical connection with this router.

Topology Table: 

• Inside the Topology Table, All the Information about the Best Path’s and the Backup Path’s is Stored. The Feasible Successor and the Successors.
• Routes in the topology table are marked as “passive” or “active”.
• Passive indicates that EIGRP has determined the path for the specific route and has finished processing.
• Active indicates that EIGRP is still trying to calculate the best path for the specific route.
• Routes in the topology table are not usable by the router until they are inserted into the routing table.
• If the network changes, For example a physical link fails or is disconnected, the path will become unavailable. EIGRP is designed to detect these changes and will attempt to find a new path to the destination. The old path that is no longer available is removed from the routing table.