Category: EIGRP

EIGRP packets use IP protocol 88. Each packet has a 20 byte header, followed by a variably sized body containing TLVs (Type-Length-Value).

The maximum size EIGRP packet is the IP MTU minus the header. So for a 1500byte IP MTU this would be 1480 bytes.

With respect to RTP this is essentially the entire EIGRP packet header. The flags, sequence number, and acknowledgement fields provide the RTP functionality.

The following explains all of the fields:

Version (4 bits) – set to 2 (this has not changed since EIGRP was created

Opcode (4 bits) – specifies the EIGRP packet type, and is one of the following:

1 = update
3 = query
4 = reply
5 = hello/ack
10 = SIA query
11 = SIA reply

Checksum (24 bits) – used to run a sanity check on the EIGRP packet

Flags (32 bit), as outlined below:

0x1 = init (used during initial adjacency build)
0x2 = conditional receive (used by RTP to allow only s subset of receivers)
0x4 = restart (router has restarted)
0x8 = End of table (transmission of entire EIGRP database is complete)

Sequence (32 bits) – contains sequence number used by RTP

Acknowledgement (32 bits) – Used by RTP. Sequence number of last packet heard from neighbour. If an hello packet is sent and the ack field is no zero, then the packet is determined to be an acknowledgement and not a hello packet.

Virtual Router ID (16bits) – identifies the virtual router:

0x1 = unicast address family
0x2 = multicast address family
0x8000 = unicast address family (in Service Advertisement Framework)

Autonomous System Number (16 bits) – specifies the EIGRP domin

TLV (Type Length Value) – carries route entries, and DUAL information, as per the below:

0x0001 = EIGRP parameters
0x0002 = Authentication Type
0x0003 = Sequence
0x0004 = Software version
0x0005 = Next Multicast Sequence
0x0102 = IPv4 Internal routes
0x0103 = IPv4 External routes
0x0402 = IPv6 Internal routes
0x0403 = IPv6 External routes
0x0602 = Multi Protocol Internal Routes (AFI)
0x0603 = Multi Protocol External Routes (AFI)

More on TLVs (Type, Length, Value)

TLVs are also found in other protocols. e.g. IS-IS, CDP, and LLDP as well. They are a structured way to send information in a packet. The type and length fields are fixed (typically between 1 and 4 bytes). The value is of variable size.

Type: states kind of information in Value field
Length: states total length (in EIGRP this is the whole TLV)
Value: contains actual information