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Inter-Switch Link Routing

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The external router can be configured to have one Ethernet interface for

each VLAN, or you can use trunking protocols like ISL or 802.1q to config-
ure one FastEthernet or Gigabit Ethernet for all the VLANs that use sub-
interfaces. These subinterfaces give you an extremely flexible solution for
providing routing between VLANs. To perform ISL routing on a single inter-
face, the interface must be at least a FastEthernet interface that supports ISL
routing. The Cisco 2600 is the least expensive router that can perform this
function.

To configure ISL routing on a single interface, you must configure sub-

interfaces. These are configured by using the type int.subinterface_
number

command. Here is an example on a 2600 router with a FastEthernet

interface:

Router#config t

Enter configuration commands, one per line. End with
CNTL/Z.

Router(config)#int f0/0.?

<0-4294967295> FastEthernet interface number

Router(config)#int f0/0.1

Router(config-subif)#

Notice the amount of subinterfaces available (4.2 billion). You can

choose any number that feels good because they are only locally significant
to the router. However, we usually like to choose the VLAN number for ease
of administration. Notice that the prompt on the router is now telling you
that you are configuring a subinterface (config-subif).

Once you configure the subinterface number you want, you then need to

define the type of encapsulation you are going to use. Here is an example of
the different types of trunking protocols you can use:

Router(config-subif)#encap ?

dot1Q IEEE 802.1Q Virtual LAN

isl Inter Switch Link - Virtual LAN encapsulation

sde IEEE 802.10 Virtual LAN - Secure Data Exchange

tr-isl Token Ring Inter Switch Link - Virtual LAN encapsulation

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