Now that we have covered the different EIGRP message and packet types,
let's move into a final presentation section on some of the common TLVs
that are sent within these different types of packet.
Let's start with the hello packet.
A normal hello packet with no special configuration will
include two different TLVs every time it is sent as a hello
and not as an acknowledgement.
These include the parameter type and software version TLVs.
The parameter type TLV is used to relay the local
device's K value configuration,
as well as its currently configured hold timer value.
These are used to determine which neighbors have matching
K values for neighborship establishment and the hold timer
to use should they match.
The software version TLV is used to ensure that the devices are
using a supported EIGRP version and TLV format.
Since the TLV format is different from the former 1.2
version and the current 2.0 version,
it is important that devices are aware of this.
Version 2.0 devices are able to communicate with version 1.2 devices and have
the ability to use both TLV versions in mixed environments.
A third TLV is also commonly seen in hello packets,
but only when using authentication.
EIGRP, like most routing protocols,
supports the ability to configure authentication on the
interfaces going between neighbors.
Specifically,
it supports both MD5 and SHA- 256 with specific support
defined by the software being used.
The authentication type TLV is used to relay the
information related to authenticating a neighbor,
including the type of authentication, the key ID,
and hash among other information.
When authentication is configured, this TLV is included with all packet types,
except acknowledgement.
The only other TLVs that we will cover for this course include those that
are used to advertise IPv4 and IPv6 network information.
These are used within update, query, reply, SIA-query, and SIA-reply packets.
There are actually a few different TLVs that can be used
to advertise the same types of network depending on the
specific EIGRP implementation.
Traditionally,
IPv4 networks were advertised in two different IPv4 specific
TLVs depending on whether the route originated inside EIGRP or
whether it originated outside EIGRP.
These TLVs include types 0102 and 0103.
The same was true of IPv6 networks where the networks were advertised using
the 0402 and 0403 TLV types that were each IPv6 specific.
In testing, it appears that at least on Cisco equipment,
these older specific TLVs were used in iOS versions previous to 15.
On modern iOS versions, however,
these specific TLVs have been depreciated with the use of a
multi-protocol TLV that is not specific to the protocol type.
When using these TLVs,
the 0602 type is used for both IPv4 and IPv6 internal networks. And the 0603 type is used for both IPv4 and IPv6 external networks.