Cisco Systems 3750E manual 17-3

Chapter 17 Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling

Understanding IEEE 802.1Q Tunneling

Figure 17-2 Original (Normal), IEEE 802.1Q, and Double-Tagged Ethernet Packet Formats

	Source
	address
Destination			Length/		Frame Check
address			EtherType			Sequence
							Original Ethernet frame
	DA	SA	Len/Etype	Data		FCS

DA

SA

Etype

Tag

Len/Etype

Data

FCS

IEE 802.1Q frame from customer network

DA

SA

Etype

Tag

Etype

Tag

Len/Etype

Data

FCS

74072

Double-tagged frame in service provider infrastructure

When the packet enters the trunk port of the service-provider egress switch, the outer tag is again stripped as the switch internally processes the packet. However, the metro tag is not added when the packet is sent out the tunnel port on the edge switch into the customer network. The packet is sent as a normal IEEE 802.1Q-tagged frame to preserve the original VLAN numbers in the customer network.

In Figure 17-1, Customer A was assigned VLAN 30, and Customer B was assigned VLAN 40. Packets entering the edge switch tunnel ports with IEEE 802.1Q tags are double-tagged when they enter the service-provider network, with the outer tag containing VLAN ID 30 or 40, appropriately, and the inner tag containing the original VLAN number, for example, VLAN 100. Even if both Customers A and B have VLAN 100 in their networks, the traffic remains segregated within the service-provider network because the outer tag is different. Each customer controls its own VLAN numbering space, which is independent of the VLAN numbering space used by other customers and the VLAN numbering space used by the service-provider network.

At the outbound tunnel port, the original VLAN numbers on the customer’s network are recovered. It is possible to have multiple levels of tunneling and tagging, but the switch supports only one level in this release.

If traffic coming from a customer network is not tagged (native VLAN frames), these packets are bridged or routed as normal packets. All packets entering the service-provider network through a tunnel port on an edge switch are treated as untagged packets, whether they are untagged or already tagged with IEEE 802.1Q headers. The packets are encapsulated with the metro tag VLAN ID (set to the access VLAN of the tunnel port) when they are sent through the service-provider network on an IEEE 802.1Q trunk port. The priority field on the metro tag is set to the interface class of service (CoS) priority configured on the tunnel port. (The default is zero if none is configured.)

On Catalyst 3750-E switches, because 802.1Q tunneling is configured on a per-port basis, it does not matter whether the switch is a standalone switch or a stack member. All configuration is done on the stack master.

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

OL-9775-02 17-3