System information

Troubleshooting ISO CLNS
Book Title
12-278
ISO CLNS: Host Cannot Access Hosts in Same Area
Symptom: Hosts cannot access other hosts in the same area. The hosts might be on the same
network or they might be in a different network in the same area.
Table 12-4 outlines the problems that might cause this symptom and describes solutions to those
problems.
Table 12-4 ISO CLNS: Host Cannot Access Hosts in Same Area
Possible Problem Solution
Area address is configured
incorrectly on the host
Step 1 Check all Level 1 routing tables and link-state databases.
Step 2 Verify that the hosts are in the same area.
Step 3 Check that the NSAP address is entered correctly on the hosts.
Different area addresses are
merged into a single area, but the
router is configured incorrectly
Step 1 Use the show running-config privileged exec command to see router
configurations. Check whether multiple area addresses are configured.
Step 2 If multiple network addresses are configured, verify that the router is
configured to support a multihomed area
(a single area that has more than one area address; see
Figure 12-3).
Step 3 To communicate, routers must establish a Level 1 adjacency. Therefore,
area addresses in a multihomed area must overlap across routers.
For example, in the multihomed area shown in Figure 12-3, to
configure Area 1 and Area 2 as a multihomed area, both Router A and
Router B must be configured to be in both areas. IS-IS routing supports
the assignment of multiple area addresses on the same router. This
concept is referred to as multihoming. Multihoming provides a
mechanism for smoothly migrating network addresses, as follows:
• Splitting up an area—Nodes within a given area can accumulate until
they are difficult to manage, cause excessive traffic, or threaten to
exceed the usable address space for an area. Multiple area addresses
can be assigned so that you can smoothly partition a network into
separate areas without disrupting service.
Different area addresses are
merged into a single area, but the
router is configured incorrectly
• Merging areas—Use transitional area addresses to merge as many as
three separate areas that have a common area address into a single
area.
• Transition to a different address—You may need to change an area
address for a particular group of nodes. Use multiple area addresses
to allow incoming traffic intended for an old area address to continue
being routed to associated nodes.
You must statically assign the multiple area addresses on the router.
Cisco currently supports assignment of up to three area addresses on a
router. The number of areas allowed in a domain is unlimited.
All the addresses must have the same system ID. For example, you can
assign one address (area1 plus system ID) and two additional addresses
in different areas (area2 plus system ID and area3 plus system ID)
where the system ID is the same.
Step 4 Alternatively, one router can be configured in both areas, while the
other router remains configured for a single area. Provided that the area
numbers on routers overlap, the routers will establish a Level 1
adjacency, allowing them to communicate.