The Eigrp Packet Header

Figure 8.28 shows the EIGRP header, which begins every EIGRP packet.

Figure 8.28. The EIGRP packet header.

Figure 8.28. The EIGRP packet header.

Packet Format

Version specifies the particular version of the originating EIGRP process. Although two software releases of EIGRP are currently available,[11] the version of the EIGRP process itself has not changed since its release.

[11] Because of the improvements to its stability beginning with IOS 10.3(11), 11.0(8), and 11.1(3) use of the later version of EIGRP is highly recommended.

Opcode specifies the EIGRP packet type, as shown in Table 8.2. Although the IPX SAP packet type is included in the table, a discussion of IPX EIGRP is outside the scope of this book.

Table 8.2. EIGRP packet types.

Opcode

Type

1

Update

3

Query

4

Reply

5

Hello

6

IPX SAP

Checksum is a standard IP checksum. It is calculated for the entire EIGRP packet, excluding the IP header.

Flags currently include just two flags. The right-most bit is Init, which when set (0x00000001) indicates that the enclosed route entries are the first in a new neighbor relationship. The second bit (0x00000002) is the Conditional Receive bit, used in the proprietary Reliable Multicasting algorithm.

Sequenceis the 32-bit sequence number used by the RTP.

ACK is the 32-bit sequence number last heard from the neighbor to which the packet is being sent. A Hello packet with a nonzero ACK field will be treated as an ACK packet rather than as a Hello. Note that an ACK field will only be nonzero if the packet itself is unicast because acknowledgments are never multicast.

Autonomous System Number is the identification number of the EIGRP domain.

Following the header are the TLVs, whose various types are listed in Table 8.3. IPX and AppleTalk types are included, although they are not discussed in this book. Each TLV includes one of the two-octet type numbers listed in Table 8.3, a two-octet field specifying the length of the TLV, and a variable field whose format is determined by the type.

Table 8.3. Type/length/value (TLV) types.

Number

TLV Type

General TL V Types

0x0001

EIGRP Parameters

0x0003

Sequence

0x0004

Software Version[12]

0x0005

Next Multicast Sequence

IP-Specific TL V Types

0x0102

IP Internal Routes

0x0103

IP External Routes

AppleTalk-Specific TLV Types

0x0202

AppleTalk Internal Routes

0X0203

AppleTalk External Routes

0x0204

AppleTalk Cable Configuration

IPX-Specific TLV Types

0x0302 0x0303

IPX Internal Routes IPX External Routes

[12] This packet indicates whether the older software release is running (software version 0) or the newer release, as of IOS 10.3(11), 11.0(8), and 11.1(3), is running (version 1).

[12] This packet indicates whether the older software release is running (software version 0) or the newer release, as of IOS 10.3(11), 11.0(8), and 11.1(3), is running (version 1).

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