Update packets contain routing information updates and are used to advertise the reachability of destination subnets.
Updates can be unicast or multicast, here are the rules that must normally be followed for an UODATE packet to be sent.
- New adjacencies use unicast update messaging to synchronize the routing tables between the two new neighbors.
- if a neighbor fails to receive an update message, the originating sender will retransmit the Update message as unicast to the unresponsive neighbor.
- After routers are fully sync’d further updates are sent as multicasts
- for point to point interfaces and statically configured neighbors EIGRP uses unicast to send updates.
Update packets are sent reliably so they require an acknowledgement to be sent back and are assigned an opcode of 1