Using ISL and 802.1Q Routing

Using ISL and 802.1Q Routing
The best solution to inter-VLAN routing might be to provide a Gigabit Ethernet router interface
for each VLAN. Obviously this can be cost prohibitive, as well as stretching the physical limitations
of router options. What if you have 200 VLANs? Can you really afford a router with
200 Gigabit Ethernet ports? That would be an interesting configuration.
Well, there are some other options open to you, because you can use just one interface for
all your VLANs. Using either the Cisco proprietary Inter-Switch Link (ISL) or the standardsbased
802.1Q protocol, you can configure routing between VLANs with only one FastEthernet
or one Gigabit Ethernet interface. To run either ISL or 802.1Q, you need to have two VLANcapable
FastEthernet or Gigabit Ethernet devices such as a Cisco 4000 or 6500 switch and a
7000 (or larger) series router. (We will be using a 2600 router in the hands-on lab, but that is
a little low-powered for larger networks.)
Remember from Chapter 14 that both ISL and 802.1Q are trunking protocols, ways of
explicitly tagging VLAN information onto an Ethernet frame. This tagging information enables
VLANs to be multiplexed over a trunk link through an external encapsulation method. By running
a trunking protocol on the switch and router interfaces, you can interconnect both devices
and maintain VLAN information end to end.
Using ISL and 802.1Q Routing
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You can configure inter-VLAN routing with either an external router or an internal route
processor that can be placed in a slot of a modular Catalyst switch such as the 4000 and 6500
series (as well as the old 5000 series). In this section, we take a look at both options.