Inter VLAN routing using ISL By Eric and Holly

Overview of ISL Trunking Trunking is a way to carry traffic from several VLANs over a point-to-point link between the two devices. Trunking is a way to carry traffic from several VLANs over a point-to-point link between the two devices. Router to switch Router to switch Switch to switch Switch to switch Two ways in which Ethernet trunking can be implemented are: Two ways in which Ethernet trunking can be implemented are: ISL (Cisco proprietary protocol) ISL (Cisco proprietary protocol) 802.1Q (Institute of Electrical and Electronics Engineers (IEEE) standard) 802.1Q (Institute of Electrical and Electronics Engineers (IEEE) standard) ISL is a Cisco proprietary protocol for interconnecting multiple switches and maintaining VLAN information as traffic goes between switches. ISL is a Cisco proprietary 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 Ethernet links in full-duplex or half-duplex mode. ISL provides VLAN capabilities while maintaining full wire speed performance on Ethernet links in full-duplex or half-duplex mode. ISL operates in a point-to-point environment and will support up to 1000 VLANs. ISL operates in a point-to-point environment and will support up to 1000 VLANs.

Differences of ISL/802.1q ISL the original packet is encapsulated and an additional header is added before the frame is carried over a trunk link. the original packet is encapsulated and an additional header is added before the frame is carried over a trunk link. At the receiving end, the header is removed and the packet is forwarded to the assigned VLAN At the receiving end, the header is removed and the packet is forwarded to the assigned VLAN802.1q The trunking device inserts a four-byte tag into the original packet and recomputes the Frame Check Sequence (FCS) before sending the frame over the trunk link. The trunking device inserts a four-byte tag into the original packet and recomputes the Frame Check Sequence (FCS) before sending the frame over the trunk link. When the destination is reached, the tag is removed and the packet is forwarded to the assigned VLAN When the destination is reached, the tag is removed and the packet is forwarded to the assigned VLAN

ISL Routing

The ISL Frame Field Descriptions DA - Destination Address TYPE - Frame Type USER - User Defined Bits (TYPE Extension) SA - Source Address LEN - Length AAAA03 - Subnetwork Access Protocol (SNAP) and Logical Link Control (LLC) HSA - High Bits of Source Address VLAN - Destination Virtual LAN ID BPDU - Bridge Protocol Data Unit (BPDU) and Cisco Discovery Protocol (CDP) Indicator INDX - Index RES - Reserved for Token Ring and Fiber Distributed Data Interface (FDDI) ENCAP FRAME - Encapsulated Frame FCS - Frame Check Sequence

Configuring Trunking Port Switch1#configure terminal Switch1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Enter configuration commands, one per line. End with CNTL/Z. Switch1(config)#int fastEthernet 0/1 Switch1(config)#int fastEthernet 0/1 Switch1(config-if)#switchport mode trunk Switch1(config-if)#switchport mode trunk Enter the trunking encapsulation as either isl or dot1q Enter the trunking encapsulation as either isl or dot1q Switch1(config-if)#switchport trunk encapsulation isl Switch1(config-if)#switchport trunk encapsulation isl Switch1(config-if)#switchport trunk encapsulation dot1q Switch1(config-if)#switchport trunk encapsulation dot1q In the case of 2950 switches, the above two switchport commands are not used. ! switches only support 802.1q encapsulation, which is the configured automatically !--- when trunking is enabled on the interface by using the switchport mode trunk command. In the case of 2950 switches, the above two switchport commands are not used. ! switches only support 802.1q encapsulation, which is the configured automatically !--- when trunking is enabled on the interface by using the switchport mode trunk command. Switch1(config-if)#switchport trunk allowed vlan all Switch1(config-if)#switchport trunk allowed vlan all Switch1(config-if)#exit Switch1(config-if)#exit

Router configuration c2600(config)#int fastEthernet 0/0.1 c2600(config)#int fastEthernet 0/0.1 c2600(config-subif)#encapsulation isl 1 c2600(config-subif)#encapsulation isl 1 c2600(config-subif)#ip address c2600(config-subif)#ip address c2600(config-subif)#exit c2600(config-subif)#exit c2600(config)#int fastEthernet 0/0.2 c2600(config)#int fastEthernet 0/0.2 c2600(config-subif)#encapsulation isl 2 c2600(config-subif)#encapsulation isl 2 c2600(config-subif)#ip address c2600(config-subif)#ip address c2600(config-subif)#exit c2600(config-subif)#exit

Debug and Show Commands Certain show commands are supported by the Output Interpreter tool, which allows you to view an analysis of show command output. Certain show commands are supported by the Output Interpreter tool, which allows you to view an analysis of show command output. On the Catalyst 2900XL/3500XL/2950 switches: On the Catalyst 2900XL/3500XL/2950 switches: show int {FastEthernet | GigabitEthernet} switchport show int {FastEthernet | GigabitEthernet} switchport show vlan show vlan show vtp status show vtp status