EIGRP Packets

EIGRP communicates using IP Protocol 88 and uses the following packet types:

• Hello/Ack
• Update
• Query
• Reply
• SIA-query
• SIA-reply

Hello packet
Opcode = 5
The Hello packet is used to automatically form adjacencies with neighboring routers. This is done by sending a message to the multicast address 224.0.0.10 or FF02::A. Since this packet type is unreliable the sequence number will be set to 0. The Hello packet includes the routers K values and these must match between neighbors for adjacencies to form - this ensures a consistent metric calculation throughout the network. It also includes the Holdtime, which by default is set to 15 seconds - 3 times the default Hello interval. The Hello packet is also used to send Ack messages if the acknowledge is not able to "piggyback" in another Update, Query or Reply packet.

Update packet
Opcode = 1
The Update packet is used to exchange routing information between neighbors. It is sent as unicast to new adjacencies to inform them of the full topology and afterwards as multicast to all adjacent neighbors, when a topology changes occur (such as change of metric). The first Update packet exchange between neighbors will have the Init flag set - this instructs the neighboring router to advertise all routes. The next Update packet exchange will contain the actual routes. Updates are subsequently only sent when triggered by an event and the contents of the update will be that of the changed network - not the full routing information.

Query packet
Opcode = 3
A query packet is sent in response to a route going into the active state and requests an alternate path to the affected network from the adjacent routers. This is done by sending a Query packet containing the affected network(s) with an infinite metric.
Each destination in a Query packet will be processed by DUAL on the receiving end and a reply will be sent only after the entire Query packet has been processed.

Reply packet
Opcode = 4
The Reply packet is a response to a Query packet and is acknowledged immediately, when received, and then processed by DUAL on the receiving end. If the destination network requested in a Query packet is not in the topology table, the Reply packet will state an infinite metric in its response.

SIA-Query
Opcode = 10
The SIA-query packet is sent if a Query packet has gone unanswered for 90 seconds (by default). The SIA-query requests the router to respond with whether it is still working on processing the original Query request or not. It is sent as unicast to the neighbors that have yet to reply to a Query packet. Upon receiving an SIA-query, the router must immediately send an ack, before processing the contents of the SIA-query.

SIA-reply
Opcode = 11
The SIA-reply packet is sent in response to an SIA-query, if the receiving router is still active for the destination network specified in the SIA-query. This is done by responding with the Active flag set in the response.