Inter Vlan Routing Using Isl

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.  

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Two ways in which Ethernet trunking can be implemented are:  

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Router to switch Switch to switch

ISL (Cisco proprietary protocol) 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 provides VLAN capabilities while maintaining full wire speed performance on Ethernet links in full-duplex or halfduplex mode. ISL operates in a point-to-point environment and will support up to 1000 VLANs.

Differences of ISL/802.1q 

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ISL 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 802.1q 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. When the destination is reached, the tag is removed and the packet is forwarded to the

ISL Routing

The ISL Frame

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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 Enter configuration commands, one per line. End with CNTL/Z. Switch1(config)#int fastEthernet 0/1 Switch1(config-if)#switchport mode trunk 

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Switch1(config-if)#switchport trunk encapsulation isl Switch1(config-if)#switchport trunk encapsulation dot1q 

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Enter the trunking encapsulation as either isl or dot1q

In the case of 2950 switches, the above two switchport commands are not used. !--- 2950 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)#exit

Router configuration        

c2600(config)#int fastEthernet 0/0.1 c2600(config-subif)#encapsulation isl 1 c2600(config-subif)#ip address 10.10.10.1 255.255.255.0 c2600(config-subif)#exit c2600(config)#int fastEthernet 0/0.2 c2600(config-subif)#encapsulation isl 2 c2600(config-subif)#ip address 10.10.11.1 255.255.255.0 c2600(config-subif)#exit

Debug and Show Commands 

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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: 

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show int {FastEthernet | GigabitEthernet} <module/port> switchport show vlan show vtp status