Core issue
Inter-Switch Link (ISL) is a Cisco proprietary trunking protocol for the encapsulation of Virtual LAN (VLAN) traffic. 802.1q is an Institute of Electrical and Electronics Engineers (IEEE) standard trunking protocol. Most Cisco LAN switches support both, but non-Cisco LAN switches that support VLAN trunking use 802.1q.
Most non-Cisco 802.1q switches maintain only a single instance of spanning tree, known as Mono Spanning Tree (MST), that defines the spanning-tree topology for all VLANs. When a Cisco switch is connected to a non-Cisco switch through an 802.1q trunk, the MST of the non-Cisco switch and the native VLAN spanning tree of the Cisco switch combine in order to form a single spanning tree topology, known as the Common Spanning Tree (CST).
Because the Cisco switches transmit Bridge Protocol Data Units (BPDUs) to the Spanning Tree Protocol (STP) multicast MAC address on VLANs other than the native VLAN of the trunk, the non-Cisco switches do not recognize these frames as BPDUs and flood them on all of the ports in the related VLAN. Other Cisco switches connected to the non-Cisco 802.1q cloud receive these flooded BPDUs. This allows the Cisco switches to maintain a spanning tree topology for each VLAN across a cloud of non-Cisco 802.1q switches. The non-Cisco 802.1q cloud that separates the Cisco switches is treated as a single broadcast segment between all of the switches that are connected to the non-Cisco 802.1q cloud through 802.1q trunks.
Resolution
Verify that the trunking mode and encapsulation are configured correctly. Make certain that the native VLAN is the same on all of the 802.1q trunks that connect the Cisco switches to the non-Cisco 802.1q cloud.
If multiple Cisco switches are connected to a non-Cisco 802.1q cloud, all of the connections must be through 802.1q trunks. Do not connect Cisco switches to a non-Cisco 802.1q cloud through ISL trunks or through access ports. If this is done, it causes the switch to place the ISL trunk port or access port into the spanning tree port inconsistent state and no traffic passes through the port.
If the trunk does not pass specific VLAN traffic, the issue can potentially be that the VLAN has not been added in the database. Issue the vlan database privileged EXEC command in order to enter VLAN configuration mode. From this mode, it is possible to add, delete, and modify VLAN configurations for normal-range VLANs.
Note: The configurations of the VLAN IDs 1 to 1005 are saved in the VLAN database if the VLAN Trunking Protocol (VTP) is in the transparent or server modes. The extended-range VLAN configurations are not saved in the VLAN database.
Refer to System Requirements to Implement Trunking for more information.
