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An IS-IS network is termed a domain,
analogous to an autonomous system (AS) in TCP/IP. The following shows
IS-IS within the domain as a
two-level hierarchy:
- Level 1 (L1) ISs, closely equivalent to OSPF internal non-backbone
routers, are responsible for routing to ESs inside an area. L1 ISs
enable communication between ESs in an area.
- Level 2 (L2) ISs, closely equivalent to backbone routers in OSPF,
route between areas only.
- Level 1 and Level 2 (L1L2) Intermediate ISs, closely equivalent to area
border routers (ABRs) in OSPF, route between areas and the backbone.
They participate in the L1 intra-area routing and the L2 inter-area
routing.
L1 routers are also referred to as station routers because they enable
stations to communicate with each other and the rest of the
network. A contiguous group of L1 routers defines an area. The L1
routers maintain the L1 link-state PDU database (LSPD), which defines
the picture of the area itself and the exit points to neighboring areas.
L2 routers are also referred to as area routers because they
interconnect the L1 areas. L2 routers store a separate LSPD, which
contains only the inter-area topology information.
L1L2 routers act as if they were two IS-IS routers.
Physically, an L1L2
router connects to L1 routers inside its area and to L2 routers in the
backbone. Notice that the boundary between areas in IS-IS exists on a
link between routers and not on an ABR itself, as in OSPF. Logically,
the L1L2 router acts, for the purposes of IS-IS routing, as if it were
two logical routers. An L1L2 router operates an L1 routing process, with
its own L1 topology table and adjacency table, to handle its association
with other L1 routers and ESs. An L1L2 router also operates an L2
routing process, with a separate L2 topology table and a separate L2
adjacency table, to handle its association with its neighbor backbone
routers.
L1L2 routers support an L1 function to communicate with the other L1
routers in their area and maintain the L1 LSP information in an L1 LSPD.
They inform other L1 routers that they are an exit point for the area.
They also support an L2 function to communicate with the rest of the
backbone and maintain an L2 topology database separately from their L1
LSPD. IS-IS does not share the concept of an Area 0 with OSPF. An IS-IS domain appears as a set of distinct areas interconnected by a
chain of L2 routers, weaving their way through and between the Level 1
areas.
An L1 area is a collection of L1 and L1L2 routers. The backbone area is
a collection of L2 and L1L2 routers and has to be contiguous. IS-IS does
not have a backbone area like OSPF Area 0. The IS-IS backbone is a
contiguous collection of L2 capable routers, each of which can be in a
different area.

Note: By default, all Cisco routers behave as L1L2 routers.
This default behavior can be overridden by using
one or both of the is-type,
router global config mode, and
isis circuit-type, interface config mode, commands.
Useful examples of the L1, L2, and L1L2 concepts are discussed in this
section. Area 1 contains two
routers. One router borders Area 2 and therefore is an L1L2 IS. The
other router is L1 only.
Area 2 has many routers. Some routers are specified as L1 only and can
route internally to that area only and to the exit points. L1L2 routers
form a chain across the area linking to the neighbor areas. Even though
the middle of these three L1L2 routers does not link directly to another
area, it must support Level 2 routing so the backbone is contiguous. If
that middle router fails then the other L1 only routers, though providing a
physical path across the area, could not perform the L2 function. This
will cause the backbone to fail.
Area 3 contains one router that borders Area 2 and Area 4, but has no
intra-area neighbors, so this router is an L2 only router. In the event
that another router is added to Area 3, the border router would revert
to L1L2.
Note: Previously, each Cisco router could participate in only
one area. The router would perform L1, intra-area, routing locally.
The router would perform L2, inter-area, routing to other areas in the
network. This limitation meant that when the network was divided into
a large number of L1 areas, a correspondingly large number of L1L2
routers were required to route between all areas. The need for
redundancy increases the number of routers needed. With IOS release
12.0(5)T, support was added for configuration of multiple L1 areas
within a single router. A single Cisco router can now participate in
routing in up to 29 areas, as well as perform L2 routing in the
backbone. Restated, expansion of an IS-IS network consisting of many
small areas is simpler than before because multiple L1 areas can now
be configured on the same Cisco router. This is possible without
needing to add and configure physical units for each additional local
area. This feature provides connectivity between L1 areas local to the
router. Previously, L1 areas could only be connected using the L2
backbone. However, it should be emphasized that multiarea support per
IS was introduced in later implementations of IS-IS to accommodate OSI
telecommunications management networks needs. This functionality is
generally not useful or recommended for IP network design.
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Interactive Media Activity
Matching: IS-IS Level 1 Adjacencies
Upon completion of this activity, the student will be able to
identify the Level 1 adjacencies between routers in an IS-IS
network.

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