EIGRP Packet Types and EIGRP Message Header Format

EIGRP Packet Types and EIGRP Message Header Format

In this article you will learn EIGRP Packet Types and EIGRP Message Header Format. We will start our discussion about different packets of EIGRP.

Types of EIGRP Packets

There are five different Types of EIGRP Packets.
The five types of EIGRP packages include:

1. Hello packages : they are used to discover neighbors and to maintain neighbors' adjacencies.
2. Update packages : propagate routing information to EIGRP neighbors.
3. Acknowledgment packets : used to acknowledge receipt of an EIGRP message that was sent with reliable delivery.
4. Query packages : used to consult neighbor routes.
5. Reply packets : sent in response to EIGRP queries.

It is shown in figure that EIGRP messages are normally encapsulated in IPv4 or IPv6 packets.

EIGRP messages for IPv4 use IPv4 as the network layer protocol. The IPv4 protocol field uses 88 to indicate that the data portion of the packet is an EIGRP message for IPv4.
EIGRP messages for IPv6 are encapsulated in IPv6 packets that use the following header field 88. Like the protocol field for IPv4, the next IPv6 header field indicates the type of data transported in the IPv6 packet.

EIGRP Hello Packets

EIGRP uses small hello packets to discover other routers with EIGRP enabled on directly connected links. Routers use hello packets to form adjacencies of EIGRP neighbors, also known as "neighbor relationships."
EIGRP greeting packets are sent as IPv4 or IPv6 multicast transmissions and use unreliable RTP delivery. This means that the receiver does not respond with an acknowledgment packet.

• The EIGRP multicast address reserved for IPv4 is 224.0.0.10.
• The EIGRP multicast address reserved for IPv6 is FF02 :: A.

In most networks, EIGRP greeting packets are sent as multicast packets every five seconds. However, in multipoint networks without broadcast (NBMA), such as X.25, Frame Relay, and asynchronous transfer mode (ATM) interfaces with access links of T1 (1,544 Mb / s) or slower, packets of Greetings are sent as unicast packets every 60 seconds.

EIGRP also uses greeting packages to maintain established adjacencies. An EIGRP router assumes that, as long as it receives hello packets from a neighbor, the neighbor and its routes remain viable.
EIGRP uses a wait timer to determine the maximum time the router must wait to receive the next greeting before declaring that the neighbor is unreachable.
By default, the wait time is three times the greeting interval, that is, 15 seconds on most networks and 180 seconds on low-speed NBMA networks. If the timeout expires, EIGRP declares the route as inactive and DUAL searches for a new route by sending queries.

EIGRP Update packets

EIGRP sends update packets to propagate routing information. Update packets are sent only when necessary.
Unlike RIP, EIGRP (another distance vector routing protocol) does not send periodic updates, and route entries do not expire.
Instead, EIGRP sends incremental updates only when the status of a destination is modified. This may include when a new network is available, when an existing network becomes unavailable, or when a change in the routing metric of an existing network occurs.
Regarding your updates, the terms partial and limited are used in EIGRP. The partial term means that the update only sends information about route changes. The term "limited" refers to the propagation of partial updates that are sent only to those routers that are affected by the change.
By sending only the necessary routing information only to the routers that need it, EIGRP minimizes the bandwidth that is required to send EIGRP updates.
EIGRP update packets use reliable delivery , which means that the sending router requires an acknowledgment. Update packets are sent as multicast when required by multiple routers, or as unicast when required by only one router.
In Image, because the links are point to point, the updates are sent as unicast.

EIGRP Acknowledgment Packets

EIGRP sends acknowledgment (ACK) packets when the reliable delivery method is used. An EIGRP acknowledgment is an EIGRP Hello packets without any data.
RTP uses reliable delivery for EIGRP update, query and response packets . EIGRP acknowledgment packets are always sent as unreliable unicast transmissions. The unreliable sense of delivery is that, otherwise, there would be an endless loop of acknowledgments.
In above Image, R2 lost connectivity to the LAN connected to its Gigabit Ethernet interface. R2 immediately sends an update to R1 and R3, where the out of service route is signaled. R1 and R3 respond with an acknowledgment so that R2 knows they received the update.
Note : in some documents, the greeting and the acknowledgment are referred to as a single type of EIGRP package.

EIGRP Query packets

DUAL uses query and response packages when searching for networks and when performing other tasks. Query and reply packets use reliable delivery. Queries use multicast or unicast, while reply are always sent as unicast.
In Image, R2 has lost LAN connectivity and sends queries to all EIGRP neighbors and searches for any possible route to the LAN.

Because the queries use reliable delivery, the receiving router must return an EIGRP acknowledgment packet. The acknowledgment informs the sender of the query that the inquiry message was received. To make the example simpler, the acknowledgments in the graphic were omitted.

EIGRP Reply Packets 

All neighbors must send a response, regardless of whether or not they have a route to the network out of order. Because responses also use reliable delivery, routers such as R2 must send an acknowledgment.
It may not be obvious why R2 should send a query to a network that knows it is inactive. In reality, only the R2 interface that is connected to the network is inactive. Another router could be connected to the same LAN and have an alternative route to the same network. Therefore, the R2 queries for such a router before completely removing the network from its topology table.

EIGRP Message Encapsulation

The data portion of an EIGRP message is encapsulated in a package. This data field is called " type, length, value " (TLV). The types of TLVs relevant to this course are the EIGRP parameters, internal IP routes and external IP routes.

The EIGRP package header is included with each EIGRP package, regardless of its type. Then, the EIGRP packet header and the TLV are encapsulated in an IPv4 packet.
In the IPv4 packet header, the protocol field is set to 88 to indicate EIGRP, and the destination IPv4 address is set to multicast 224.0.0.10. If the EIGRP packet is encapsulated in an Ethernet frame, the destination MAC address is also a multicast address, 01-00-5E-00-00-0A.
EIGRP for IPv6 uses a similar type of encapsulation. EIGRP for IPv6 is encapsulated with an IPv6 header. The destination IPv6 address is the multicast address FF02 :: A, and the following header field is set to 88.

Header of EIGRP Packets

All EIGRP Packets include the header, as shown in Image.

  

Important fields include the operation code field and the autonomous system number field. The operation code specifies the type of EIGRP package as follows:

• To update
• Query
• Reply
• Hello

The autonomous system number specifies the EIGRP routing process. Unlike RIP, several instances of EIGRP can be run on a network, and the autonomous system number is used to track each running EIGRP process.

TLV of EIGRP parameters

The EIGRP parameter message includes the weights that EIGRP uses for its composite metric. Only bandwidth and delay are weighted by default. Both are weighted equally, therefore, both the K1 field for bandwidth and the K3 field for delay are set to one (1). The other K values ​​are set to zero (0).

The Timeout is the amount of time that the EIGRP neighbor who receives this message should wait before considering that the router that performs the notification is disabled.

TLV of internal IP routes

The internal IP message is used to announce EIGRP routes within a stand-alone system. Important fields include the metric fields (delay and bandwidth), the subnet mask field (prefix length) and the destination field.

The delay is calculated as the sum of delays from the origin to the destination in units of 10 microseconds. The bandwidth is the one with the lowest configuration in all the interfaces of the route.

The subnet mask is specified as the prefix duration or the number of network bits in the subnet mask. For example, the prefix length for subnet mask 255.255.255.0 is 24, because 24 is the number of network bits.

The Destination field stores the address of the destination network. Although only 24 bits are shown in this figure, this field varies depending on the value of the network portion of the 32-bit network address.

For example, the network portion of 10.1.0.0/16 is 10.1; therefore, the destination field stores the first 16 bits. Since the minimum length of this field is 24 bits, the rest of the field is filled with zeros. If a network address is longer than 24 bits (192.168.1.32/27, for example), then the Destination field extends another 32 bits (with a total of 56 bits) and unused bits are completed with zeros.

TLV of external IP routes

The external IP message is used when external routes are imported into the EIGRP routing process. Note that the lower half of the TLV of external IP routes includes all fields used by the internal IP TLV.

Note : The maximum transmission unit (MTU) is not a metric used by EIGRP. The MTU is included in routing updates, but is not used to determine routing metrics.