January 3, 2020

EIGRP uses 5 different types of packet in communication with its neighbours. EIGRP packets are sent with reliable or unreliable delivery and can be sent as a unicast, multicast, or sometimes both. EIGRP packets type are also called EIGRP packets format or EIGRP messages. The details of the EIGRP packets are the following:

EIGRP Hello Packets

EIGRP uses Hello packets to discover EIGRP-enabled neighbours on directly connected links. The router uses hello packets to perform EIGRP neighbour adjacencies, also known as neighbour relationships. The hello packet is sent as IPv4 and IPv6 multicasts. The packet is sent using RTP unreliable delivery which is not required an acknowledgement packet.

The IPv4 address 224.0.0.10 is reserved for IPv4 hello packet multicasting, and IPv6 address FF02::A is reserved for IPv6 hello packet multicasting. On most networks, EIGRP Hello packets are sent as multicast packets every five seconds.  But in some networks for example non- broadcast multi-access network (NBMA),  X.25, Frame Relay, and Asynchronous Transfer Mode (ATM) interfaces with access links of T1 (1.544 Mb/s) or slower, the timing of Hello packets are 60 seconds due to slow speed.

Hello, packets are also used to maintain previously established adjacencies. An EIGRP-enabled router assumes that as long as it receives Hello packets from a neighbour, the neighbour and its routes remain workable. EIGRP uses a hold timer to determine the maximum time the router should wait to receive the next Hello before declaring that neighbour as unreachable.

The default hold time is three times the Hello interval. Hello interval for the Ethernet and T1 lines with higher speed is 5 seconds and for slower speed networks like Non-Broadcast Multi-access networks are 60 seconds. If the hold time expires, EIGRP declares the route as down and DUAL searches for a new path by sending out queries.

EIGRP Update Packets

EIGRP Update Packets are used to send routing information. The update Messages are sent both Unicast and Multicast. Update packets are sent only when needed and the updates contain only the routing information needed and are sent only to those routers that require it. EIGRP is a distance vector routing protocol like RIP, but it does not send periodic updates like RIP and the route entries do not expire. The EIGRP sends updates only when changes occur in the network. The update occurs when a new network becomes available, an existing network becomes unavailable, or a change occurs in the routing metric for an existing network.

EIGRP also uses the terms partial and bounded for its updates. The term partial is used to for routing update which includes information about the route changes and the term bounded refers to the broadcast of partial updates that are sent only to those routers that the changes affect. The partial and bounded update minimizes the bandwidth that is required to send EIGRP updates.

EIGRP propagates routing information to EIGRP neighbours’ uses reliable delivery, which means the sending router must require an acknowledgement. An update packet is sent to a new neighbour as unicast. If this update is related to any route change, then it is sent as Multicast.

EIGRP Acknowledgment Packets

EIGRP Acknowledgment Packets are used as a feedback to the Update, Query or Reply packets as a feedback mechanism. It is not used for Hello Packets and Acknowledgment Packets. It is an empty hello messages without any data and they are sent as unreliable unicast.

EIGRP Query Packets

EIGRP Query Packets ask and request for any routing update. If a successor fails, with Query messages, a backup route is asked. EIGRP queries use reliable delivery as a unicast or multicast message. When a router lost connectivity then it sends out queries to all EIGRP neighbours searching for any possible routes to the LAN. Due to reliable delivery, the receiving router must return an EIGRP acknowledgement. The acknowledgement informs the sender of about the query reception.

EIGRP Reply Packets

EIGRP Reply Packets are used as a response to the Query Packets including the alternate routes to the requested destination. It is sent with reliable delivery as unicast messages.