notes: layer 2 tunneling

802.1q Tunnel
- Used to provide transparent layer2 VPN over a switched ethernet network, to carry unicast, broadcast, multicast, CDP, VTP or STP.
- Uses dot1q inside dot1q, to tunnel layer2 traffic.
- Cannot be dynamically negotiated, and traffic is not encrypted.
NOTE: Confirm prior to configuration that underlying end-to-end connectivity is established.
- When using dot1q tunneling CDP, STP & VTP are NOT carried across the tunnel by default.
- Additionally dot1q also supports etherchannels between customer sites.
- Dot1q-Tunnel requires:
- 802.1q trunking end-to-end
- System MTU should be a minimum of 1504, to support the additional 4-byte metro tag.

PITFALL: Careful when running OSPF to a switch with a system MTU of 1504, the adjacency won’t come up, due to a MTU mismatch.
Disable the MTU check on the routers OSPF interface with "ip ospf mtu-ignore"

Note: you can configure l2 protocol tunneling on ports configured as access or tunnel ports. You cant enable l2 tunneling on ports that is configured
switchport dynamic mode auto
switchport dynamic desirable

from Ruhan's CCIE notes