An array of useful IS-IS commands in the
context of the pictured topology will be introduced and analyzed.

The
show isis topology command
will be explored first.
The
show isis topology
command displays the least-cost paths to the destination NETs. The
system ID shows the NET of the destination. IOS uses Dynamic Hostname
Mapping, see RFC 2763, to map this system ID to a hostname, when that
hostname is available to the router. The hostname of the router is
included in its outgoing LSP.
The metric shows the sum of the metrics on the least-cost path to the
destination. The next hop router, IS, is shown. Also shown is the
interface through which that next hop is reached and the SNPA of that
next hop. HDLC is shown as the next hop across a serial line. The
output for Router R2 shows that separate topology databases exist for
L1 and L2.
Recall that the SNPA is taken from the
following:
-
MAC address on a LAN interface
-
Virtual Circuit ID for X.25 or ATM
-
DLCI
for Frame Relay
-
HDLC for High-Level Data Link Control
interfaces
The
show clns route and
show isis route commands are
the next commands to be looked at. There is a common source of confusion for those
learning Integrated IS-IS. That confusion comes from the frequent use of commands directly
referencing CLNS, but used for the purpose of verifying and troubleshooting
IP routing.
The
show clns route command displays the CLNS destinations to
which this router can route packets.
R1 shows only its local NET
entry, because it is an L1 only router and therefore has no L2 area
routes to display. The
show isis route command shows the L1
routes to IS-IS neighbors. R1 has visibility of the other L1 routers
in its area. The L1L2 routers appear in the L1 routing table by
virtue of their L1 connection. There is a note at the end of their entry
to show that they also act as L2. The closest L1L2 router also appears
as the default route out of area. Again, the next-hop IS, its SNPA,
is the interface over which that next hop is reached. The cumulative
metric to that destination is shown for all IS routes. The neighbors
show that their state is ’up’ and the Hello process has established an
adjacency.
The command
show clns route shows the local NET entry.
This command also shows the
L2 routes to its own area and the neighbor areas. Notice that L2
regards the route to the area of R2 as being through itself. This
further emphasizes that the L1 and L2 processes operate separately.
The command
show isis routes shows the IS-IS neighbors.
Next, the which-route
command in the context of L1 and L2 will be explored. The
which-route command is
an alternative method of finding the route to a destination NET or
NSAP. The command is entered
on the L1 only router, R1.The command returns the next hop to the
destination and states whether the destination is reachable by L1 or
by the default exit point to L2. Executing the
which-route command on
an L2 router specifies the next hop. It also states that the route was
matched by an entry from the CLNS L2 routing table.

Building the IP Forwarding Table
So far, the process and outputs have referred to the OSI part of the
IS-IS process. These are the same as for pure OSI IS-IS routing.
However, in the IP world, when running Integrated IS-IS, IP
information is included in the LSPs. IP reachability behaves in IS-IS
as if it were ES information. IP information takes no part in the
calculation of the SPF tree. It is simply information about leaf
connections to the tree. Therefore, updating the IP reachability is
only a PRC. This is similar to ES reachability. IP routes are generated by the PRC and offered to the
routing table. Here they will be accepted based on routing table
rules comparing, for example, administrative distance. When entered in
the routing table, IP IS-IS routes are shown as being by way of Level 1
or Level 2, as appropriate. The separation of IP reachability from the
core IS-IS network architecture gives Integrated IS-IS better
scalability than, for example, OSPF. OSPF sends LSAs for individual IP
subnets. If an IP subnet fails, then the LSA is flooded through the
network. In all circumstances, all routers must run a full SPF
calculation. In Integrated IS-IS, the SPF tree is built from CLNS
information. If an IP subnet fails in Integrated IS-IS, the LSP is
flooded as it is for OSPF. However, if this is a leaf IP subnet,
meaning the loss of the subnet has not affected the underlying CLNS
architecture, then the SPF tree is unaffected. Only a PRC will take
place.
The IP routing table, a pure entity, will
now be looked at. The output for the
show ip route
command shows the IS-IS routes chosen by the SPF algorithm, from the
IS-IS LSP database, to populate the IP routing table.
The “i” indicates that the
route was sourced from IS-IS. "L1" and "L2" show whether the IS-IS
path to these destination IP networks is by way of IS-IS L1 or L2
routing. The next-hop IP addresses are matched from the corresponding
next-hop IS-IS neighbor routers. Notice the metric is ten (10) for
each route because ten is the Cisco default for the IS-IS metric over a
link.
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Lab Activity
e-Lab Activity: IS-IS Show Commands
This lab is to analyze the running
IS-IS router process, using IS-IS show commands.
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