IS-IS Terminology

IS-IS Terminology
This section discusses some of the terms that are used when referring to IS-IS that you may not
be familiar with.
ES
An end system (ES) is a non-routing network device, such as a host.
IS
An intermediate system (IS) is a routing device, in our case a router.
ES-IS
End System to Intermediate System (ES-IS) is the protocol that is used to enable end systems
to discover intermediate systems and vice versa.
SNPA
The subnetwork point of attachment (SNPA) is the point at which subnetwork services
are provided.
PDUs
Protocol data units (PDUs) are the data passed between an OSI layer of one node to the
peer OSI layer of another node.
DLPDU
A data link frame is referred to as a data link PDU (DLPDU).
NPDU
A packet is referred to as a network PDU (NPDU).
LSP
The
link-state PDU (LSP)
is the IS-IS equivalent of the OSPF LSA. The main difference
between the two is that the LSA is encapsulated behind the OSPF header and the IP packet,
whereas the LSP is a packet all its own.
Level 1 intermediate systems
Level 1 intermediate systems route within an area. When the
destination is outside an area, they route toward a Level 2 system.
Level 2 intermediate systems
Level 2 intermediate systems route between areas and toward
other ASs.
NET
The
network entity title (NET)
uniquely defines each router on the network. The NET
is a network address, which contains a system ID and an area ID.
All of these terms play an important part in understanding the operation of IS-IS. You must
come to know and understand each of these terms. As you read through this chapter, you’ll be
able to place the terms in their proper context.