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Look at the example of a router
network connected over Frame Relay using point-to-point subinterfaces.
Each Frame Relay permanent
virtual circuit (PVC) is treated as its own point-to-point network,
with its own IP addresses. The example is of a hub-and-spoke
network topology. It is important to note that the spoke routers
are also configured with point-to-point subinterfaces, even
though, unlike the hub router, they use only one VC. This is the best
practice in general for all routing protocols. It allows further VCs
to be added without affecting the existing VC. This practice is
imperative for IS-IS. A main interface is a multipoint
interface, even if it happens to have only one VC configured. If the
single VC were configured under a main interface, IS-IS would treat
this as a broadcast network and attempt to elect a DIS. Also, an
adjacency would not be established because the multipoint end would
send broadcast-network-style Hello PDUs, but the point-to-point end
would send point-to-point Hello PDUs.
The hub router configuration is shown in
Figure
.
The encapsulation type, Frame Relay, is set under the main interface
of Serial0/0. No IP or IS-IS configuration is included under the main
interface. Three subinterfaces are defined, one for each VC. Each
subinterface specifies the following:
- The IP address for that
point-to-point link, which is a different subnet for each
subinterface.
- Integrated IS-IS, ip
router isis, as the routing
protocol over that subinterface.
- The VC to use for that
point-to-point subinterface, using the
frame-relay interface-dlci
command. This is the only command needed to enable both IP and CLNS
across this VC. The router automatically enables, across this VC,
all the protocols that are enabled on the point-to-point
subinterface. With this configuration, the subinterface will
automatically forward routing updates, as is the case when using the
broadcast
keyword with a Frame Relay map.
Notice that no Frame Relay maps are used in this scenario, since
point-to-point interfaces are used.
The show frame-relay map
command displays the status of each Frame Relay VC as follows:

- Status defined means it has been
configured on the Frame Relay switch, and active indicates that this
VC is operational.
- Type is point-to-point, meaning it
has been assigned to a point-to-point subinterface.
- Assigned subinterface, for example,
Serial0/0.1
- VC identification, for example, DLCI
100.
- Whether it supports broadcast
packets, such as RIP Version 1 routing packets.
The debug isis adj-packet
command shows neighbor relationship establishment across one of the
subinterfaces, Serial0/0.1, sending and receiving point-to-point IIH
PDUs and declaring the adjacency "up". Ongoing Hello conversations for
the other subinterfaces are also shown.
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Lab Activity
e-Lab Activity: Frame Relay Point-to-Point Scenario with
Integrated IS-IS
This lab is to use point-to-point
subinterfaces in a hub-and-spoke network with IS-IS running.
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