2008). these new TLVs enables the protocol to

TLVs are the extensible parameter portions of the IS–IS PDUs that are used to
carry different kinds of information. The protocol also supports hierarchical
networking allowing a larger network domain to be separated into logical
divisions called areas. Each intermediate system resides in at least one area.
The hierarchy defines two levels that are used to organize routers in larger
routing domains. These are Level 1 and Level 2. Level 1 routers are the same as
IRs in OSPF. Level 2 routers are used to connect two or more areas in the
routing domain (Lemma et al., 2009).

2.2.8 IS–IS Changes for IPv6

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ability of the IS–IS packets to be modified to include additional TLVs has
given the protocol an added advantage. The addition of these new TLVs enables
the protocol to support routing in newer network addressing schemes without
changing the protocol operation. This in contrast to OSPF is different. In
order to support routing in IPv6, the OSPF protocol was completely upgraded to
a newer version. This has changed the protocol structure and makes its
operation slightly different from the version supported by IPv4. In IS–IS, only
two new TLVs are added to the protocol structure to support routing in IPv6.
These TLVs are:

IPv6 Reachability

IPv6 Interface Address


an IPv6 protocol identifier was included in the protocol structure. However,
the extended metrics and up or down semantics of RFC 5305 are still used in the
new TLVs (Hopps, 2008).


2.2.9 IS–IS features IS–IS Packets


System to Intermediate System packets are called Protocol Data Units (PDUs).
There are four PDUs defined for IS–IS routers. These include:


IS–IS Hello (IIH) packet:IIH is
used for detecting neighbor routers, establishing and maintaining adjacency
between them.

Link State Packet (LSP): LSP is the main packet the IS–IS protocol uses
to transmit routing information in its routing domain. Each IS–IS router sends
an LSP containing information about itself and the links that are connected to
it. The content of an LSP 


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