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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. Router to switch Switch to switch Two ways in which Ethernet trunking can be implemented are:

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Inter VLAN routing using ISL

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  1. Inter VLAN routing using ISL By Eric and Holly

  2. Overview of ISL Trunking • Trunking is a way to carry traffic from several VLANs over a point-to-point link between the two devices. • Router to switch • Switch to switch • Two ways in which Ethernet trunking can be implemented are: • 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 half-duplex mode. • ISL operates in a point-to-point environment and will support up to 1000 VLANs.

  3. 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. • 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 assigned VLAN

  4. ISL Routing

  5. 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

  6. 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 • Enter the trunking encapsulation as either isl or dot1q • Switch1(config-if)#switchport trunk encapsulation isl • Switch1(config-if)#switchport trunk encapsulation 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

  7. 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

  8. 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. • On the Catalyst 2900XL/3500XL/2950 switches: • show int {FastEthernet | GigabitEthernet} <module/port> switchport • show vlan • show vtp status

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