Specifications

Configuring Routing Between VLANs
How to Configure Routing Between VLANS
15
Configuring Routing Between VLANs with Inter-Switch Link Encapsulation
This section describes the Inter-Switch Link (ISL) protocol and provides guidelines for configuring ISL
and Token Ring ISL (TRISL) features. This section contains the following:
• Frame Tagging in ISL, page 15
• Configuring AppleTalk Routing over ISL, page 16
• Configuring Banyan VINES Routing over ISL, page 18
• Configuring DECnet Routing over ISL, page 19
• Configuring the Hot Standby Router Protocol over ISL, page 20
• Configuring IP Routing over TRISL, page 22
• Configuring IPX Routing on 802.10 VLANs over ISL, page 23
• Configuring IPX Routing over TRISL, page 25
• Configuring VIP Distributed Switching over ISL, page 26
• Configuring XNS Routing over ISL, page 28
• Configuring CLNS Routing over ISL, page 29
• Configuring IS-IS Routing over ISL, page 30
Frame Tagging in ISL
ISL is a Cisco protocol for interconnecting multiple switches and maintaining VLAN information as
traffic goes between switches. ISL provides VLAN capabilities while maintaining full wire speed
performance on Fast Ethernet links in full- or half-duplex mode. ISL operates in a point-to-point
environment and will support up to 1000 VLANs. You can define virtually as many logical networks as
are necessary for your environment.
With ISL, an Ethernet frame is encapsulated with a header that transports VLAN IDs between switches
and routers. A 26-byte header that contains a 10-bit VLAN ID is propounded to the Ethernet frame.
A VLAN ID is added to the frame only when the frame is prepended for a nonlocal network. Figure 78
shows VLAN packets traversing the shared backbone. Each VLAN packet carries the VLAN ID within
the packet header.
Step 7
show running-config
Example:
Router# show running-config
Verifies subinterface configuration.
Step 8
show interfaces
Example:
Router# show interfaces
Verifies that subinterfaces have been created.
Command Purpose