Cisco 7604-RSP720C-R Configuration Manual page 281

Chapter 9 Configuring Q-in-Q VLAN Tunnels
Information About Layer 2 Protocol Tunneling
S e n d d o c u m e n t c o m m e n t s t o n e x u s 7 k - d o c f e e d b a c k @ c i s c o . c o m
local site and all remote sites across the service-provider infrastructure. Cisco Discovery Protocol (CDP)
must be able to discover neighboring Cisco devices from local and remote sites, and the VLAN Trunking
Protocol (VTP) must provide consistent VLAN configuration throughout all sites in the customer
network.
When protocol tunneling is enabled, edge switches on the inbound side of the service-provider
infrastructure 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 process these packets, but forward
them as normal packets. Bridge protocol data units (BPDUs) for CDP, STP, or VTP cross the
service-provider infrastructure 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.
If protocol tunneling is not enabled on 802.1Q tunneling ports, remote switches at the receiving end of
the service-provider network do not receive the BPDUs and cannot properly run STP, CDP, 802.1X, 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 802.1Q tunneling achieve complete
knowledge of the customer's VLAN.
Note Layer 2 protocol tunneling works by tunneling BPDUs in software. A large number of BPDUs coming
into the SUP will cause the CPU load to go up. You may need to make use of hardware rate limiters to
reduce the load on the SUP CPU. See the "Configuring the Rate Limit for Layer 2 Protocol Tunnel Ports"
section on page 9-13.
For example, in Figure 9-4, Customer X has four switches in the same VLAN that are connected through
the service-provider network. If the network does not tunnel BPDUs, switches on the far ends of the
network cannot properly run the STP, CDP, 802.1X, and VTP protocols.
Cisco Nexus 7000 Series NX-OS Interfaces Configuration Guide, Release 5.x
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