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Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
OL-23826-09
Chapter 35 Layer 2 Control Protocol Peering, Forwarding, and Tunneling
Layer 2 Control Protocol Forwarding
Restrictions
If you want to peer Operation, Administration, and Maintenance (OAM) packets when l2proto-forward
tagged command is configured at the interface level, you should also configure the l2protocol peer lacp
command.
Layer 2 Control Protocol Forwarding
The ASR 901 forwards Layer 2 Control Protocol (L2CP) packets between customer-edge (CE) devices.
Figure 35-1 depicts an end-to-end layer 2 forwarding. The layer 2 traffic is sent through the S-network,
and the S-network switches the traffic from end to end. The Cisco ASR 901 router forwards frames from
the user network interface (UNI) to the network-to-network Interface (NNI) after appending S-tag. The
third party provider edge (PE) router forwards the S-tagged frames. The PE peers the untagged Link
Layer Discovery Protocol (LLDP) and Link Aggregation Control Protocol (LACP) frames. On the
reverse path (from NNI to UNI), the S-tag is removed.
Figure 35-1 Layer 2 Forwarding
Layer 2 Control Protocol Tunneling
Layer 2 Control Protocol Tunneling (L2PT) is a Cisco proprietary protocol for tunneling Ethernet
protocol frames across layer 2 switching domains. The following tunnel protocols are supported:
• Cisco Discovery Protocol (CDP)
• Dynamic Trunking Protocol (DTP)
• Link Aggregation Control Protocol (LACP)
• Link Layer Discovery Protocol (LLDP)
1 2 3 2 1
334051
CE CE
ASR901
ASR901
PE
4 4
1 L2CP packets are forwarded between CE
devices.
3 Third party PE forwards S-tagged frames and
peers untagged frames.
2 Frames are forwarded from UNI to NNI after
appending the S-tag. On the reverse path (NNI
to UNI), S-tag is removed.
4 Untagged LLDP and LACP is peered.