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15-9
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-09
Chapter 15 Configuring 802.1Q and Layer 2 Protocol Tunneling
Configuring Layer 2 Protocol Tunneling
In a SP network, you can use Layer 2 protocol tunneling to enhance the creation of EtherChannels by
emulating a point-to-point network topology. When you enable protocol tunneling (PAgP or LACP) on
the SP switch, remote customer switches receive the PDUs and can negotiate the automatic creation of
EtherChannels.
, Customer A has two switches in the same VLAN that are connected
through the SP network. When the network tunnels PDUs, switches on the far ends of the network can
negotiate the automatic creation of EtherChannels without needing dedicated lines. See the
Layer 2 Tunneling for EtherChannels” section on page 15-13
for instructions on configuring Layer 2
protocol tunneling for EtherChannels.
Figure 15-6 Layer 2 Protocol Tunneling for EtherChannels
Configuring Layer 2 Protocol Tunneling
You can enable Layer 2 protocol tunneling (by protocol) on the access ports or tunnel ports that are
connected to the customer in the edge switches of the SP network. The SP edge switches connected to
the customer switch perform the tunneling process. Edge-switch tunnel ports are connected to customer
802.1Q trunk ports. Edge-switch access ports are connected to customer access ports.
The Catalyst 3550 switch supports Layer 2 protocol tunneling for CDP, STP, and VTP. For emulated
point-to-point network topologies, it also supports PAgP, LACP, and UDLD protocols.
Caution
PAgP, LACP, and UDLD protocol tunneling is only intended to emulate a point-to-point topology. An
erroneous configuration that sends tunneled packets to many ports could lead to a network failure.
When the Layer 2 PDUs that entered the SP inbound edge switch through the tunnel port or the access
port exit through its the trunk port into the SP network, the switch overwrites the customer
PDU-destination MAC address with a well-known Cisco proprietary multicast address
(01-00-0c-cd-cd-d0). If 802.1Q tunneling is enabled, packets are also double-tagged; the outer tag is the
customer metro tag, and the inner tag is the customer’s VLAN tag. The core switches ignore the inner
tags and forward the packet to all trunk ports in the same metro VLAN. The edge switches on the
outbound side restore the proper Layer 2 protocol and MAC address information and forward the packets
to all tunnel or access ports in the same metro VLAN. Therefore, the Layer 2 PDUs remain intact and
are delivered across the SP network to the other side of the customer network.
Swich 1
VLAN 17
VLAN 18
VLAN 19
VLAN 20
VLAN 17
VLAN 18
VLAN 19
VLAN 20
Swich 2
Swich 3
Service
Provider
EtherChannel 1
Customer A
Site 1
Customer A
Site 2
98678
Swich 4
EtherChannel 1
Trunk
Asymmetric link