EIGRP Packets explained

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

In this lesson I want to explain the different EIGRP packets to you. Let's start with the Hello packets...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.



Let’s start with the Hello packets…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 reliable. 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 4 routers all running EIGRP. Hello packets are sent between routers in order to form adjacencies. As you can see R1 is sending 3 hello packets meant for R2, R3 and R4. There are 2 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 holddown 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 reliable 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.

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Forum Replies

  1. Hi Rene,

    I have some vague about full update and partial update ?
    Do you mean :-
    1 - The full update always send using unicast after neighbor adjacency formed ?
    2 - The partial update always send using multicast when there is a change in the topology ? and if we use static neighbors the partial update will be sent using unicast ? that’s right ?
    If there is wrong correct me please ?

    thanks

  2. Hello Clement.

    There are differing opinions as to how many and what type of EIGRP packets exist. Some refer to five, others to six. Some group Request packets and Query packets together. Some group ACK and Hello packets together (since ACK is basically an empty hello packet). Some just don’t mention Request packets. It really depends on who you ask.

    Cisco’s official documentation states that there are five packet types:

    * Hello/Acks
    * Updates
    * Queries
    * Replies
    * Requests

    Although Cisco’s official CCNA curriculum gives the following list:

    * Hello
    * Acknow

    ... Continue reading in our forum

  3. Hi
    I’m bit confuse about Acknowledgements.
    ACK packets are used to acknowledge the receipt of update, query and reply packets.
    In case it use to acknowledge the receipt of query, does the router will send ACK first or it send Reply fist or it will send ACK and Reply in the same time ?

  4. Hi Heng,

    You can see this with a quick Wireshark capture: eigrp-query-reply.pcapng

    These are two routers using IP addresses 192.168.12.1 and 192.168.12.2

    Once 192.168.12.1 loses a route (1.1.1.1/32), it sends the query to 192.168.12.2.

    192.168.12.2 first responds to the query with an ACK. Wireshark shows the ACK as Hello (5), don’t let this confuse you…this happens because the EIGRP hello and ACK packet both share the same opcode.

    192.168.12.2 responds with a reply, which is ACKed by 192.168.12.1 as well.

    I hope this helps!

    Rene

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