2.5 Configuring connected switches

Trunking- The process of creating a logical connection between two physically connected switches allowing frames to flow between them.

• A tag is added to the frame by the sending switch. The receiving switch uses this tag to determine what VLan should receive the frame. This allows VLans to span switches.
• Trunk ports belong to all VLans

ISL- Inter Switch Protocol- Encapsulation type- Cisco Proprietary

• Cisco propiteary protocol- Other switch brands can not use it
• Many Cisco Switch models do not support this either
• Encaspulates the entire frame before it sends it over the trunk. This adds a lot of overhead
• Does not allow for native Vlan

DOT1q (IEEE 802.1q)- Encapsulation Type- IEEE standard

• Industry standard trunking protocol- can be used on all switching models/brands
• Does not encapsulate the frame
• Inserts a 4 byte value into the Ethernet header to represent the VLan ID
• Supports Native Vlans
• If it is destined for the native VLan it will not even include the 4 byte header.

Native Vlan- Default Vlan. 1 is the default on Cisco Switches.

Dynamic port configuation- Allows switch to negotiate the status of the port Trunk.

• Auto- Will form a trunk but the other side has to initiate this. IE Set to Desirable or On. Both sides in auto and the trunk will not form.
• Desirable- Actively attempts to trunk. If the remote side is set to On, Desirable or auto a trunk will form.

Commands-

Switchport mode trunk Unconditional Trunking-

Switchport mode access Unconditional Non trunking-

Switchport mode trunk auto- Forms only if other side initiates trunk

Switchport mode trunk desirable- Actively initiates a trunk

Switchport nonegotiate- Puts port into permanent trunking and DTP frames are not sent.