Configuring ISL/802.1Q with an External Router

Configuring ISL/802.1Q with an External Router
An external layer 3 device can be used to provide routing between VLANs. You can use almost
any router to perform the function of external routing between VLANs, but if trunking is being
used, the selected router must support the VLAN tagging method used, whether it’s ISL or
802.1Q; then the FastEthernet or Gigabit Ethernet interface would be your choice.
If you have a few small VLANs that perform 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. Just remember that 10Mb interfaces do
not support trunking, so the configuration would be one VLAN per interface.
You should get FastEthernet if you can.
The external router interface needs to be configured with a trunking protocol encapsulation such
as ISL or 802.1Q, thus allowing different VLANs to be assigned to different subinterfaces. These
subinterfaces give you an extremely flexible solution for providing routing between VLANs. To perform
ISL routing on a single interface, the interface must be at least a FastEthernet interface that supports
ISL routing. The Cisco 1750 is the least expensive router that can perform this function.
To configure ISL/802.1Q routing on a single interface, you must first configure the subinterfaces.
These are configured by using the
int
.
subinterface_number
global command. Here is
an example on a 2600 router with a FastEthernet interface:
Terry_2620#
configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Terry_2620(config)#
interface fa0/0.?
<0-4294967295> FastEthernet interface number
Terry_2620(config)#
interface fa0/0.1
Terry_2620(config-subif)#
Notice the number of subinterfaces available (4.2 billion). You can choose any number that
feels good because the subinterfaces 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
).
After 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:
Terry_2620(config-subif)#
encapsulation ?
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
You’re not done yet. You need to tell the subinterface which VLAN it is a member of, and you
provide this information on the same line as the encapsulation command. Here is an example:
Terry_2620(config-subif)#
encapsulation isl ?
<1-1000> Virtual LAN Identifier.
Notice that you can configure the subinterface to be a part of any VLAN up to 1000. The
dot1Q
encapsulation is for the IEEE standard 802.1Q trunking, and
isl
is for ISL encapsulation.
After you choose the interface and encapsulation type and VLAN number, configure the IP
address that this subinterface is a member of. The complete configuration looks like this:
Terry_2620#
configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Terry_2620(config)#
interface fa0/0.1
Terry_2620(config-subif)#
encapsulation isl 1
Terry_2620(config-subif)#
ip address 172.16.10.1 255.255.255.0
The preceding configuration is for subinterface fa0/0.1 to VLAN 1. You would create a
subinterface for each VLAN. You can verify your configuration with the
show runningconfig
command:
!
interface FastEthernet0/0.1
encapsulation isl 1
ip address 172.16.10.1 255.255.255.0
If you had elected the 802.1Q encapsulation, the complete router configuration would look
like this:
Terry_2620#
configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Terry_2620(config)#
interface fa0/0.1
Terry_2620(config-subif)#
encapsulation dot1Q 1
Terry_2620(config-subif)#
ip address 172.16.10.1 255.255.255.0
Once again, you can verify your configuration with the show running-config command:
!
interface FastEthernet0/0.1
encapsulation dot1Q 1
ip address 172.16.10.1 255.255.255.0
!