You can configure
subinterfaces to support the following connection types:
- Point to point - A single
subinterface is used to establish one PVC connection to another
physical or subinterface on a remote router. In this case, the
interfaces would be in the same subnet, and each interface would
have a single DLCI. Each point-to-point connection is its own
subnet. In this environment, broadcasts are not a problem
because the routers are point-to-point and act like a leased
line.
- Multipoint - A single
subinterface is used to establish multiple PVC connections to
multiple physical interfaces or subinterfaces on remote routers.
In this case, all the participating interfaces would be in the
same subnet, and each interface would have its own local DLCI.
In this environment, because the subinterface is acting like a
regular NBMA Frame Relay network, broadcast traffic is subject
to the split-horizon rule.
Figure
provides the visual representation for example
of subinterface configuration.
To configure subinterfaces on a
physical interface, perform the following steps:
1. Select the interface that you
want to create subinterfaces on, and enter the interface
configuration mode.
2. Remove any network-layer address that is assigned to the
physical interface. If the physical interface has an address,
frames will not be received by the local subinterfaces.
3. Configure Frame Relay encapsulation, as discussed in the “Configuring
Basic Frame Relay” section.
4. Select the
subinterface you want to configure :
Router(config-if)#interface
serial number subinterface-number
{multipoint | point-to-point}
5. Configure a network-layer
address on the subinterface. If the subinterface is point to point
and you are using IP, you can use the ip unnumbered
command:
Router(config-if)#ip
unnumbered interface
If you use this command, it is
recommended that the interface be the loopback interface because the Frame Relay link will not work if this command is
pointing to an interface that is not fully operational, and a
loopback interface is less likely to fail. When using ip
unnumbered, it should have two ends on the same major network.
6. If you configured the
subinterface as multipoint or point-to-point, you must configure
the local DLCI for the subinterface to distinguish it from the
physical interface :
Router(config-if)#frame-relay
interface-dlci dlci-number
This command is required for all
point-to-point subinterfaces. It is also required for multipoint
subinterfaces for which inverse ARP is enabled. It is not required
for multipoint subinterfaces that are configured with static route
maps. Do not use this command on physical interfaces.
If you defined a subinterface for
point-to-point communication, you cannot reassign the same
subinterface number to be used for multipoint communication without
first rebooting the router. Instead, you must avoid using that
subinterface number and use a different subinterface number.
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Lab
Activity |
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You
have been contracted by a small bank to install a
Frame Relay network to connect their offices in
Atlanta, Orlando, and Houston. Each office needs to
connect to the other two. You have ordered Frame
Relay service from a national Frame Relay
service provider and have received the DLCIs in the
above diagram. Routing should be handled using IGRP. |
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