Hello Sarah,
Nice to be in touch with you again! How are you and how are things going?
Regarding the Version field in EIGRP packet header and the Version TLV, I have no definitive answer for that, but my assumption is as follows.
The Version field in the EIGRP packet header that is currently set to 2, and it will always be set to 2 regardless of the true EIGRP version. I assume this is an additional check for the IOS code to distinguish between IGRP and EIGRP messages. The IGRP used the Version field set to 1 and Cisco has always considered the EIGRP to be the next generation of the former IGRP, hence the value of 2 for EIGRP messages.
Please see the following document about IGRP:
http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a00800c8ae1.shtml#ipimplement
Specifically, it states: "The version number is currently 1. Packets having other version numbers are ignored."
Regarding EIGRP, the encoding of its messages and the details about RTP itself, I strongly recommend reading the patent application No. 5,519,704 "Reliable Transport Protocol for Internetwork Routing" at
http://www.google.com/patents?id=XSMdAAAAEBAJ&printsec=frontcover&dq=5,519,704&hl=en&ei=437zTeHTJo_KtAabu7m1Bg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCkQ6AEwAA
The page 20 of the PDF document in the section "Example Headers and Data for Protocol Packets" specifically states: "In a preferred embodiment, a fixed header for EIGRP packets may comprise a version number 1201 (always 2) [... cut ...]". The 1201 in this statement only refers to a diagram element in Figure 12.
It is true that the IGRP was sent as IP protocol 9 messages while the EIGRP is using IP protocol 88, so the IP protocol number alone could be indicative of the IGRP/EIGRP. However, this is very similar to the IPv4 and IPv6 having their Version field set to 4 and 6, respectively, despite IPv4 using the EtherType 0x0800 and IPv6 using the EtherType 0x86DD. Strictly taken, because of EtherType or similar protocol-demultiplexing fields in L2 frames, the Version field in the IP header is not necessary, but then again, it is an additional useful check. I consider the Version field in IGRP and EIGRP packet headers to serve the same purpose.
Regarding the EIGRP version itself, the following document puts it quite nicely:
http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml#majorrevisions
"There are two major revisions of EIGRP, versions 0 and 1. Cisco IOS versions earlier than 10.3(11), 11.0(8), and 11.1(3) run the earlier version of EIGRP; some explanations in this paper may not apply to that earlier version. We highly recommend using the later version of EIGRP, as it includes many performance and stability enhancements."
Thus the Version TLV contains the true version of the EIGRP being used, and so far, we seem to be running the EIGRP version 1.2 on most IOS platforms.
So to sum it up, the Version field in EIGRP packet headers is actually a protocol discriminator: a value of 1 would mean IGRP, the value 2 means EIGRP. The true EIGRP version, however, is indicated in the Version TLV.
My two cents about this...
Best regards,
Peter
