9-10
Ethernet Card Software Feature and Configuration Guide, R7.2
January 2009
Chapter 9 Configuring IEEE 802.1Q Tunneling and Layer 2 Protocol Tunneling
Configuring Layer 2 Protocol Tunneling
• CDP discovers and shows information about the other Cisco devices connected through the
service-provider network.
• VTP provides consistent VLAN configuration throughout the customer network, propagating
through the service provider to all switches.
Layer 2 protocol tunneling can be used independently or to enhance I EEE 802.1Q tunneling. If protocol
tunneling is not enabled on IEEE 802.1Q tunneling ports or on specific VLANs, remote switches at the
receiving end of the service-provider network do not receive the PDUs and cannot properly run STP,
CDP, and VTP. When protocol tunneling is enabled, Layer 2 protocols within each customer’s network
are totally separate from those running within the service-provider network. Customer switches on
different sites that send traffic through the service-provider network with IEEE 802.1Q tunneling
achieve complete knowledge of the customer’s VLAN. If IEEE 802.1Q tunneling is not used, you can
still enable Layer 2 protocol tunneling by connecting to the customer switch through access ports and
enabling tunneling on the service-provider access port.
Configuring Layer 2 Protocol Tunneling
Layer 2 protocol tunneling (by protocol) is enabled on the tunn el ports or on specific tunnel VLANs that
are connected to the customer by the edge switches of the service-provider network. ML-Series card
tunnel ports are connected to customer IEEE 802.1Q trunk ports. The ML-Series card supports Layer 2
protocol tunneling for CDP, STP, and VTP at the interface and subinterface level. Multiple STP (MSTP)
Tunneling support is achieved through subinterface protocol tunneling. The ML-Series cards connected
to the customer switch perform the tunneling process.
When the Layer 2 PDUs that entered the inbound ML-Series switch through the tunnel port exit the
switch through the trunk port into the service-provider network, the swit ch overwrites the customer
PDU-destination MAC address with a well-known Cisco proprietary multicast address
(01-00-0c-cd-cd-d0). If IEEE 802.1Q tunneling is enabled, pa ckets are also double-tagged; the outer tag
is the customer metro tag and the inner tag is the customer VLAN tag. The core switches ignore the inner
tags and forward the packet to all trunk ports in the same metro VLAN. The ML-Series switches on the
outbound side restore the proper Layer 2 protocol and MAC address information and forward the
packets. Therefore, the Layer 2 PDUs are kept intact and delivered across the service-provider
infrastructure to the other side of the customer network.
This section contains the following information about configuring Laye r 2 protocol tunneling:
• Default Layer 2 Protocol Tunneling Configuration, page 9-10
• Layer 2 Protocol Tunneling Configuration Guidelines, page 9-11
• Configuring Layer 2 Tunneling on a Port, page 9-11
• Configuring Layer 2 Tunneling Per-VLAN, page 9-12
• Monitoring and Verifying Tunneling Status, page 9-12

Default Layer 2 Protocol Tunneling Configuration

Table 9-2 shows the default Layer 2 protocol tunneling configuration.