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Chapter 6

Inter-VLAN Routing

trunked and configured as one load-balanced link running EtherChannel
and ISL or 802.1q. Subinterfaces are then used to configure each VLAN.

Inter-Switch Link Routing

he best solution to inter-VLAN routing is to provide a Gigabit Ether-

net router interface for each VLAN. However, we have found that this can
be cost prohibitive. What if you have 500 VLANs? Can you really afford a
router with 500 Gigabit Ethernet ports? That would be an interesting
configuration.

Cisco to the rescue! You can use either one FastEthernet or one Gigabit

Ethernet interface for all your VLANs. Cisco has created the proprietary
protocol Inter-Switch Link (ISL) to allow routing between VLANs with only
one Ethernet interface. To run ISL, you need to have two VLAN-capable
FastEthernet or Gigabit Ethernet devices, such as a Cisco 5000 switch and a
7000 series router.

Remember from Chapter 3 that ISL is a way of explicitly tagging VLAN

information onto an Ethernet frame? This tagging information allows
VLANs to be multiplexed over a trunk link through an external encapsula-
tion method. By running ISL, you can interconnect multiple switches and
still maintain VLAN information as traffic travels between switches on trunk
links.

Configuring ISL

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 Catalyst switch. In
this section, we'll take a look at both options.

External

An external layer 3 device can be used to provide routing between VLANs.
You can use almost any router to perform this function, but FastEthernet or
Gigabit Ethernet is suggested. If you have many small VLANs that perform
at least 80 percent or more of their network function on the local VLAN,
then you can probably get away with a 10Mbps Ethernet connection into
each VLAN. Still, you should get FastEthernet if you can.

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