Network Sales and Services Handbook (Cisco Press Networking Technology)
EIGRP uses five specialized packets for routing table maintenance. These are as follows:
Hello Used to (re)discover and track other network EIGRP routers. Hello packets are multicast for neighbor discovery/recovery and do not require acknowledgment. Neighbor rediscovery sometimes occurs during the convergence process.
- The fixed hello interval is 60 seconds for serial interfaces less than T1 or configured for multipoint (for example, Frame Relay, ATM or X.25).
- Serial interfaces with links T1 or greater have a Hello interval of five seconds.
- Hold-timer is defaulted to three times the Hello interval, either 15- or 180-seconds, depending on the interface.
Acknowledgement Used to acknowledge receipt of any EIGRP packet that requires reliable delivery. An acknowledgment packet is a Hello packet that has no data.
Update Used to convey routing information to known destinations. Update packets are used to either provide a complete topological data dump to a new EIGRP router or to provide an update to a topological change in the network, such as a downed link or change in link cost.
Query Used whenever a router needs specific information from one or all of its neighbors. Queries are sent only when a destination becomes active. When the network is stable, all routes in the topology table are marked as PASSIVE, making it unnecessary (and a waste of bandwidth) to send EIGRP query packets.
Reply Sent in reply to EIGRP query packets. These packets instruct the originator not to recompute the route because feasible successors exist. Both query and reply packets are transmitted reliably.