EIGRP Packet Type

Describing EIGRP Packet Types (HA/U/Q/R/R)
1. Hello
– Used to recover/discover and to maintain EIGRP neighbor adjacencies.

– They do not require acknowledgement.

– EIGRP uses small hello packets to discover other EIGRP-enabled routers on
directly connected links.

– EIGRP Hello packets are sent as IPv4 or IPv6 multicast, and use RTP
unreliable delivery. This mean that the receiver does not reply with an
– The reserved EIGRP multicast address for ipv4 is
– The reserved EIGRP multicast address for IPv6 is FF02::A.

– EIGRP Hello packets are sent as multicast packet every 5 second.

However, on multipoint, nonbroadcast multiple access (NBMA) networks, such as X.25,Frame relay and Asynchronous transfer mode (ATM) interfaces with access links of T1(1.544 Mb/s) or slower. Hello packets are sent as unicast packet every 60 seconds.

EIGRP uses a Hold timer to determine the maximum time the router should wait to receive the next Hello before declaring that neighbor as unreachable.

1.a Acknowledgement
– Acks are always sent using a unicast address and contain a non-zero
acknowledgment number
– Acknowledgement is an EIGRP hello packet without data.
– EIGRP Ack packets are always sent as an unreliable unicast. (Unreliable
delivery makes sense, otherwise, there would be an endless loop of

NOTE: RTP uses reliable delivery for EIGRP update, query and reply packets.

2. Update
– Propagates routing information to EIGRP neighbors. UPdate packets are sent
only when necessary. EIGRP contain only the routing information needed and
are sent only to those routers that require it.
– Sent with reliable delivery.
– Unicast or multicast.
– Trigger updates/Periodic updates, Send only when the state of a destination changes. This can include when a new network becomes available, an existing network becomes unavailable, or a change occurs in the routing metric for an existing network.

3. Query
– Looking information to get to the prefix.
– Sent with reliable delivery. Because queries use reliable delivery, the
receiving router must return an EIGRP acknowledgement. The acknowledgment
inform the sender of the quesry that it has received the query message.
– Unicast or multicast.

4. Replies
– Sent in response to an EIGRP query.
– Always sent in response to queries to indicate to the originator that it
does not need to go into Active state because it has feasible successors.
– Sent with reliable delivery
– Unicast

5. Requests
– Request packets are used to get specific information from one or more
– Request packets are used in route server applications.
– They can be multicast or unicast.
– Requests are transmitted unreliably.

Note: Update, Query and Reply require Ack’s.Update, query and reply packets require acknowledgement, whereas hello and ack do not require ack.

Neighborship reset occurs after a 16 retry limit is reached. Slow neighbors will be sent unicast updates instead.



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