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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter13 Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling
Understanding Layer 2 Protocol Tunneling
Understanding Layer 2 Protocol Tunneling
Customers at different sites connected across a service-provider network need to use various Layer 2
protocols to scale their topologies to include all remote sites, as well as the local sites. STP must run
properly, and every VLAN should build a proper spanning tree that includes the local site and all remote
sites across the service-provider network. Cisco Discovery Protocol (CDP) must d iscover nei ghbori n g
Cisco devices from local and remote sites. VLAN Trunking Protocol (VTP) must provide con si sten t
VLAN configuration throughout all sites in the customer network that are participating in VTP.
Note The Cisco ME 3400 switch does not support VTP; CDP and STP are supported o nly on NNIs. However,
Layer 2 protocol tunneling is supported on all ports on the switch.
When protocol tunneling is enabled, edge switches on the inbound side of the service-provider network
encapsulate Layer 2 protocol packets with a special MAC address and send them across the
service-provider network. Core switches in the network do not proces s t hes e pa ckets but f or ward th em
as normal packets. Layer 2 protocol data units (PDUs) for CDP, STP, or VTP cross the service-provider
network and are delivered to customer switches on the outbound side of the service-provider network.
Identical packets are received by all customer ports on the same VLANs with these results:
•Users on each of a customer’s sites can properly run STP, and every VLAN can build a correct
spanning tree based on parameters from all sites and not just from the local site.
•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, pr opagating to all
switches through the service provider that support VTP.
Layer 2 protocol tunneling can be used independently or can enhance IEE E 802.1Q tunneling. If protocol
tunneling is not enabled on IEEE 802.1Q tunneling ports, remote switches at the receiving end of the
service-provider network do not receive the PDUs and cannot properly run ST P, CDP, and VTP. When
protocol tunneling is enabled, Layer 2 protocols within each cust omer ’s network are to tal ly se para te
from those running within the service-provider network. Customer switch es on different sites that send
traffic through the service-provider network with IEEE 802.1Q tunneling ac hieve comple te k nowledg e
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 by enabling tunneling on the
service-provider access port.
For example, in Figure 13-4, Customer X has four switches in the same VLAN, that are connected
through the service-provider network. If the network does not tunnel PDUs, switches on the far ends of
the network cannot properly run STP, CDP, and VTP. For example, STP for a VLAN on a switch in
Customer X, Site 1, will build a spanning tree on the switches at that site without considering
convergence parameters based on Customer X’s switch in Site 2. This could result in the topology shown
in Figure13-5.