EIGRP - Enhanced Interior Gateway Routing Protocol

EIGRP (Enhanced Interior Gateway Routing Protocol) is a Advance distance vector routing protocol used on computer network for best routing decision and configuration. The protocol was designed by Cisco System to use only on Cisco device but nowdays its open standard.

  • Protocol number of EIGRP is 88.
  • AD value of EIGRP is 90.
  • It is default classfull but also supports classless.
  • Supports CIDR and VLSM.
  • Multicast address is 224.0.0.10
  • Has a 2 TImer: Hello (5 sec) and Hold(15 sec).
  • Max hop count is 100 can increas upto 255.
  • It's metric is composite k-Values.
  • Best path selection vis DUAL algorithm
  • Efficient neighbor Discovery
  • Support for summaries and discontiguous networks
  • Maintains 3 tables : Neighbor Table, Topology Table & Routing Table

Neighbor Discovery

Before EIGRP routers can exchange routes with each other, they must become neighbors, and there are three conditions that must be met before this can happen. And these three things will be exchanged with directly connected neighbors:
- Hello or ACK received
- AS numbers match
- Identical metrics (K values)
 Eigrp Neighbor Discovery In Eigrp all the router must have same AS number. If AS number is not matching then no neighbor table will be created. And if authentication is applied then it should be matched.
in EIGRP router sends hello packets within every 5 sec to check wheather destination router is working or not. If hello doesnot come until 5 sec then the destination router is decleared as dead.

Eigrp Hello Packet

- Version 2
- Opcode : Hello
- Checksum
- Flags
- Sequence
- Acknowledge
- Virtual Router ID
- Autonomous System (AS) 10
- Composite K parameters
- Software Version

Eigrp - DUAL Algorithm

DUAL (Diffusion Update Algorithm) is used by EIGRP. And it has a few term, lets know the term.
- SUCESSOR : The best path from source to destination with lowest metric is called Sucessor.
- FEASIBLE SUCESSOR : The secondary best path or backup path from source to destination is called Feasible Sucessor. To create a backup path router should fulfill feasible condition i.e Rd of backup path must be less then Fd of feasible Sucessor.
- FEASIBLE DISTANCE : The metric value of the path availale is called feasible Distance. If metric value is same for multiple path then eigrp will automatically load balance.
- REPORTED DISTANCE : The metric value from source's neighbour to destination is called Reported Distance.
 Eigrp DUAL

Configuring Simple EiGRP

 Eigrp Configuration

r1# config t
r1# router eigrp 10 [enables eigrp process and 10 is AS number]
r1# network 192.168.1.0
r1# network 192.168.2.0
This simply enables EIGRP process and follow same on other

Few Important troubleshooting command as steps:
#show ip eigrp neighbors - Checking neighbor table
#show ip eigrp interfaces - Checking eigrp enable interface
#show ip route eigrp- Checking routing table
#show ip eigrp topology - Checking topology table

Retransmission Time Out (RTO)

Sender router send a multicalt to all avail router in network and receives reply from all but if any particular router did not reply then a sender router will send 16 unicast to that particular router, if reply received then neighbourship will continue and if not then will declear as dead. Mainly the time difference between Multicast and Unicast is RTO.

Unequal Cost load balancing - Variance

If you want backup path along with best path when best path has a conjuction then unequal cost load balancing is performed. This is also called Variance.
Variance is always multiply with best paths FD and default is 1.
Changing Variance
#router eigrp 10
#variance [Value /2]

when we multiply FD of best path and variance, the value come will logical and can not be seen and best path will be best and backup path will be added on routing table. Max 16 and default 4 path can be added on version 12.7 but in 15.0 can be added upto 32 path.

Passive Interface

When router broadcasts the hello packet it broadcast towards all interface i.e router connected and lan connected but broadcasting towards lan is wastage and bandwidth will consume so we can make those interface passive where do not need such packet. once interface is passive broadcast packet will not go towards those interface but be sure that you are not making interface passive which is connected to another router which will cause the break on eigrp neighborship.
#router eigrp 10
# passive-interface f0/0

Router ID

It is of 32 bit as like ip address. router with same router id can communicate within same AS number but can not communicate between differet AS number with same router id.
If router has a loopback then higest loopback will be routerID but if it does not have loopback then highest physical ip address will be routers router id. but it can be change and give manual id too.

#router eigrp 10
#router-id 1.1.1.1 (1.1.1.1 is router id)

Redistribution between As numbers

 Eigrp AS redistribution
Redistribution is the precess of exchangind information from one to another. redistribution can be done on that router which consist of both AS information and in our topology R1 consist of both AS information so we will redistribute on R1.

r2#router eigrp 10
r2#redistribute eigrp 20
^z
r2#router eigrp 20
r2#redistribute eigrp 10
^Z