Switching-VLANs

Ethernet Port Types:

Access Ports – Single VLAN, Untagged
Trunk – Multiple VLANs, tagged
Tunnel – Layer 2 Tunnel
Dynamic – Dynamic Trunking Protocol

Switched Virtual Interfaces – Virtual Layer 3 interface (VLAN interface)
Routed Interface – NO switchport

DTP Negotiation:

• Sometimes enabled by default on all trunk links.
• DTP Desirable Mode:
  • Initiates Trunking
  • ‘Switchport mode dynamic desirable’ on trunk interface
  • ‘Switchport mode trunk’ on trunk interface
• DTP Auto Mode:
  • Sits waiting for trunking negotiation
  • Does not initiate DTP.
  • Switchport mode dynamic auto
• Disabling:
  • ‘Switchport nonegotiate’ – can only be used if dynamic auto is turned off.
    • set as access or trunk port manually, then use switchport nonegotiate.
  • ‘Switchport mode access’ – due to being access port.
  • ‘Switchport mode dot1q-tunnel’ – due to being tunnel interface
• Can be seen with:
  • ‘show interface switchport’

VLANs:

• Standard Range 1 – 1005
  • VLAN1
    • Default access and native (on trunk links)
    • Cannot be pruned by VTP
    • Cannot be deleted, can be manually pruned from trunks
    • Should not be used for actual traffic.
  • VLANs 1002 – 1005
    • Legacy FDDI and Token Ring
    • Cannot be deleted, can be manually pruned from trunks.
    • Cannot be pruned by VTP
    • Should not be used for actual traffic.
• When assigning a port to a VLAN in access mode, if the VLAN does not exist in the database, the switch will create the VLAN automatically IF VTP mode is ‘Server’.

• Extended Range 1006-4094
  • Can normally only be used in VTP Version3 and VTP Transparent.
  • Not all of these IDs can be used. Some are reserved for internal resources.
    • ‘show vlan internal usage’

• The above image shows that when setting up a layer 3 interface with ‘no switchport’, internally one of these reserved IDs are used.
• vlan internal allocation policy <ascending/descending>
  • Not all platforms do the same by default for reserved internal ID usage. Some go ascending, some go descending by default.
  • Corner case – Change ascending to descending, or vice versa, if a VLAN ID like 4094 needs to be used and there are routed interfaces on switch.

Manual Trunk Pruning:

• Commands:
  • switchport trunk allowed vlan add <#>
  • switchport trunk allowed vlan remove<#>
  • switchport trunk allowed vlan <#> – This will allow only the number(s) that are added to the command.