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eigrp hello packet contents

EIGRP Hello Protocol – Exclusive Explanation

EIGRP Hello Protocol – Exclusive Explanation

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Hello Intervals and Hold Times – EIGRP for IPv4

EIGRP uses the Hello protocol to establish and maintain a connection with its neighbor and monitor the established connection. It sends a Hello packet periodically to its neighbor. The rate at which EIGRP sends a Hello Packet is known as the Hello interval.

The Hold timer works just like the dead interval of OSPF. It is the maximum wait time that the router should wait to receive the next Hello packet before declaring that neighbor inaccessible or dead.

Both Hello intervals and Hold times are configurable values on a per-interface basis. The range of the Hello interval “second” value is range from “1 to 65535”. The command syntax to configure a Hello interval is:

Router(config-if)# ip hello-interval eigrp <as-number> <seconds>

Changing the Hello interval equal to the Hold time value or greater than the Hello interval is important. If not, neighbor adjacency will decrease after the Hold time expires and before the next Hello interval. The range of the Hello interval “second” value also ranges from “1 to 65535”. To configure the Hold time value, use the following command to configure:

Router(config-if)# ip hold-time eigrp <as-number> <seconds>

Figure 1 illustrates the EIGRP topology. We can configure the Hello interval and Hold timer for this topology. We know that the Hello Interval and Hold timer in EIGRP is a per-interface base configurable value. Therefore, the Hello interval time and Hold time do not need to match for two routers to form an EIGRP adjacency and we can configure the different values for different interfaces on the above topology.

Hello Protocol

For a faster link, we can configure less timing (in seconds); for a slower one, we can configure larger timing (in seconds) to reduce the extra network overhead.

The default Hello Interval for 1.544 Mbps or slower link is 60 seconds and for T1 link and Ethernet link is 5 seconds. The default hold down timer for 1.544 Mbps or slower link is 180 seconds and for T1 link and Ethernet links are 15 seconds. The configuration of Hello Interval and Hold timer on the link between R1 and R2 is the following:

Router R1

R1(config)# interface gigabitEthernet 0/0

R1(config-if)# ip hello-interval eigrp 100 80

R1(config-if)# ip hold-time eigrp 100 240

Router R2

R2(config)# interface gigabitEthernet 0/0

R2(config-if)# ip hello-interval eigrp 100 80

R2(config-if)# ip hold-time eigrp 100 240

This configuration will set the hello interval to 80 seconds and hold timer to 240 seconds between R1 and R2 link. We can reset the values to default using the “no” form of the above commands.

Hello Intervals and Hold Times – EIGRP for IPv6

EIGRP for IPv6 also uses the same Hello protocol, which includes the hello interval and Holds times. The command to configure the Hello interval and Hold timers is similar commands for IPv4 with little difference. The commands syntax are the following:

Router(config-if)# ipv6 hello-interval eigrp <as-number> <seconds>

Router(config-if)# ipv6 hold-time eigrp <as-number> <seconds>

EIGRP Packet Types – Exclusive Explanation

EIGRP Packet Types – Exclusive Explanation

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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 packet types are also called EIGRP packet format or EIGRP messages. The details of the EIGRP packets are the following:

EIGRP Packets

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. It is sent using RTP unreliable delivery, which does not require an acknowledgment packet.

The IPv4 address 224.0.0.10 is reserved for IPv4 hello packet multicasting, and the 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 unreachable.

The default hold time is three times the Hello interval. The Hello interval for Ethernet and T1 lines with higher speed is 5 seconds, and for slower speed networks like non-Broadcast Multi-access networks, it is 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 to both Unicast and Multicast. Update packets are sent only when needed; the updates contain only the routing information needed and are sent only to those routers requiring 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 for routing updates, which include information about 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 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 updating packet is sent to a new neighbor as unicast. If this update is related to any route change, it is sent as Multicast.

EIGRP Acknowledgment Packets

EIGRP Acknowledgment Packets are used as 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 message without any data, and they are sent as unreliable unicast.

EIGRP Query Packets

EIGRP Query Packets ask for and request 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 loses connectivity, it sends queries to all EIGRP neighbors, searching for any possible routes to the LAN. Due to reliable delivery, the receiving router must return an EIGRP acknowledgment. The acknowledgment informs the sender 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. They are sent with reliable delivery as unicast messages.