EIGRP Packet Types

The EIGRP packet types are as follows:

Hello, Acknowledgment

Update, Query, Reply, SIA-Query, SIA-Reply (Also called reliable packets because EIGRP makes sure they are delivered in a proper order)

Hello Packets (Opcode 5)

Hello packets are sent on the multicast link local address of 224:0.0.10 (IPv4) or FF02::A (IPv6), or they can be sent as unicast if explicitly configured. The default hello interval is 5 seconds. However on NBMA interfaces with a bandwidth of 1544Kbps or less the default is 60 seconds. EIGRP hello packets have an opcode of 5 and are not acknowledged.

Acknowledgement Packets (Opcode 5)

ACKs are sent in a response to Update, Query, Reply, SIA-Query, and SIA-Reply packets and always unicast to the sender. An ACK is a hello packet with an empty body, and thus has the same Opcode, 5.

Also each EIGRP packet type has an acknowledgement field, so it is allowed to use any EIGRP packet as an acknowledgement

Update Packets (Opcode 1)

Update packets contain information about reachability to destinations. They can be both unicast or multicast.

Normally updated packets are unicast, but when multiple neighbours are detected on a single multiaccess interface then EIGRP might choose to synchronise the updates with multicast. E.g. if a hub router in a DMVPN network detects hundreds of spoke routers.

After routers have fully synchronised, further updates are sent as multicast. If a neighbour does not acknowledge the arrival of an update packet, EIGRP will retransmit the update as unicast.

On point-to-point interfaces, and statically configured neighbours. EIGRP always uses unicast.

Query Packet (Opcode 3)

Query Packets are used to search for the best path to the destination and are delivered reliably.

Normally sent as multicast on multiaccess networks. If not acknowledged in the proper time, a query is retransmitted as a unicast. On point to point interfaces and statically configured neighbours queries are unicast.

Ack packets acknowledge query packets, but are not responses

Reply Packet (Opcode 4)

A response to a query packet, and carry senders current distance. Reply packets are always unicast to the originator.

SIA Query (Opcode 10) and SIA- Reply packets (Opcode 11)

Used to determine whether a neighbour who has not sent a reply to a query is unreachable, or still working on the query i.e. Active. If the neighbour is still working on the query it will send a SIA-reply (Stuck in Active Reply). As a result the max time a diffusing computation is allowed to run is reset, giving the neighbour more time to finish.

Both SIA Query and reply packets are sent a unicast