EIGRP Packets explained

In this lesson I want to explain the different EIGRP packets to you.




Hello packets are sent between EIGRP neighbors for neighbor discovery and recovery. If you send hello packets and receive them then EIGRP will form a neighbor relationship with the other router. As long as you receive hello packets from the other side, EIGRP will believe that the other router is still there. As soon as you don’t receive them anymore you will drop the neighbor relationship called adjacency, and EIGRP might have to look for another path for certain destinations.

EIGRP uses RTP (Reliable Transport Protocol), and its function is to deliver EIGRP packets between neighbors in a reliable and ordered way. It can use multicast or unicast and to keep things efficient, not all packets are sent reliably. Reliable means that when we send a packet we want to get an acknowledgment from the other side to make sure that they received it. So when does EIGRP use unicast or multicast? Let’s take a look at an example:

eigrp hello packet overhead

In this example, we have four routers all running EIGRP. Hello packets are sent between routers to form adjacencies. As you can see, R1 is sending three hello packets meant for R2, R3, and R4. There are two questions that we can ask ourselves here:

  • Is it really useful to send 3 different hello packets on a single link?
  • Is it necessary that a hello packet gets an acknowledgment in return?

Sending 3 packets on the same link is not very useful, so instead of doing this, EIGRP will send hello packets by using multicast on a multi-access network like Ethernet.

Hello packets don’t have to be acknowledged since EIGRP uses a hold-down time. If a router doesn’t receive hello packets in an X amount of time, it will drop the neighbor adjacency.

So which packets should be acknowledged? Think about routing information, if there’s a change in the network, you want to make sure all routers receive this routing update.

Let me show you all the different EIGRP packets:

  • Hello
  • Update
  • Query
  • Reply
  • ACK (Acknowledgement)

Hello packets are used for neighbor discovery. As soon as you send hello packets and receive them, your EIGRP routers will try to form the neighbor adjacency.

Update packets have routing information and are sent reliably to whatever router that requires this information. Update packets can be sent to a single neighbor using unicast or to a group of neighbors using multicast.

We're Sorry, Full Content Access is for Members Only...

If you like to keep on reading, Become a Member Now!

  • Learn any CCNA, CCNP and CCIE R&S Topic. Explained As Simple As Possible.
  • Try for Just $1. The Best Dollar You’ve Ever Spent on Your Cisco Career!
  • Full Access to our 800 Lessons. More Lessons Added Every Week!
  • Content created by Rene Molenaar (CCIE #41726)
539 Sign Ups in the last 30 days
satisfaction-guaranteed

  • 100% Satisfaction Guaranteed!
  • You may cancel your monthly membership at any time.
  • No Questions Asked!

Forum Replies

  1. Hello I found little mistake in text:
    As you can see router Lizzy is sending 3 hello packets meant for router Jack, John and Lizzy.
    Should be:
    As you can see router Lizzy is sending 3 hello packets meant for router Jack, John and James.

  2. it was really helpfull,
    thanks!!

  3. thank you! that was very helpful.
    additionally:
    query messages are multicast but reply messages are unicast.
    this is what i see on wireshark.

  4. Rene,
    Hi. Are query messages always multicast (even if have static neighbor)? Are Reply messages always unicast?

    Many thanks,
    Thomas

28 more replies! Ask a question or join the discussion by visiting our Community Forum