In recent years, the Intermediate
System-to-Intermediate System (IS-IS) routing protocol has become
increasingly popular, with widespread usage among Service
Providers. IS-IS enables very fast convergence and is very
scalable. It is also a very flexible protocol and has been
extended to incorporate leading edge features such as
Multiprotocol Label Switching Traffic Engineering (MPLS/TE).
The features of IS-IS include the
following:
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Hierarchical routing
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Classless behavior
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Rapid flooding of new information
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Fast Convergence
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Very scalable
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Flexible timer tuning
The Cisco IOS implementation of IS-IS also supports the
following features:
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Multi-area routing
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Route-leaking
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Overload-bit
IS-IS is an Open System Interconnection (OSI) routing protocol
originally specified by International Organization for
Standardization (ISO) 10589. IS-IS is a dynamic, link-state,
intradomain, interior gateway protocol (IGP). The protocol is
designed to operate in an OSI Connectionless Network Service (CLNS)
environment. IS-IS selects routes based upon a cost metric
assigned to links in the IS-IS network. The cost is an arbitrary
value assigned by a network engineer as the value of the path to
a neighbor router.
A two-level hierarchy is used to support large routing domains.
A large domain can be administratively divided into areas. Each
system resides in exactly one area. Routing within an area is
referred to as Level 1 routing. Routing between areas is
referred to as Level 2 routing. A Level 2 Intermediate System
(IS) keeps track of the paths to destination areas. A Level 1 IS
keeps track of the routing within its own area. For a packet
destined for another area, a Level 1 IS sends the packet to the
nearest Level 2 IS in its own area, regardless of the level of the
destination area. Then the packet travels by way of Level 2
routing to the destination area, where it may travel by way of Level 1
routing to the destination. It should be noted that selecting an
exit from an area based on Level 1 routing to the closest Level
2 IS might result in suboptimal routing.
On broadcast multi-access media, a Designated Intermediate
System (DIS) is elected and conducts the flooding over the
media. The DIS is analogous to the designated router in OSPF,
even though the details including the election process and
adjacencies within a multi-access media differ significantly.
The DIS is elected by priority. The highest priority becomes the
DIS. This is configurable on an interface basis using the
isis priority command. In
the case of a tie, the router with the highest subnetwork
point-of-attachment address (SNPA) will become the DIS. In the
case of Ethernet, the SNPA address is just the MAC address.
All of these concepts are discussed in this module, beginning
with an introduction to OSI protocols.
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