Specifications

15-44
Cisco MWR 2941 Mobile Wireless Edge Router Release 3.5 Software Configuration Guide, Cisco IOS Release 15.1(3)MR
OL-26895-01
Chapter 15 Configuring Ethernet OAM, CFM, and E-LMI
Enabling Ethernet Loopback
Per-port facility loopback puts the port into a loopback state where the link is up, but the line
protocol is down for regular traffic. The switch loops back all received traffic.
When you configure per-port, per-VLAN loopback by entering the vlan vlan-list keywords, the
other VLANs on the port continue to switch traffic normally, allowing nondisruptive loopback
testing.
• Terminal loopback allows testing of full-path QoS in both directions. Terminal loopback puts the
port into a state where it appears to be up but the link is actually down externally, and no packets
are sent. Configuration changes on the port immediately affect the traffic being looped back.
With terminal loopback, traffic that is looped back goes through the forwarding path a second time.
If MAC swap is not configured. looped-back multicast or broadcast traffic is flooded on that VLAN.
The packet then goes out the other ports twice, once from the ingress packet and once from the
looped-back packet. See Figure 15-5.
You can configure only one terminal loopback per switch.
Figure 15-5 Terminal Loopback Packet Flow
By default, no loopbacks are configured.
Ethernet loopback has these characteristics:
• You can configure Ethernet loopback only on physical ports, not on VLANs or port channels.
• You can configure one loopback per port and a maximum of two loopbacks per switch.
• You can configure only one terminal loopback per switch.
• The port ends the loopback after a port event, such as a shutdown or change from a switch port to a
routed port.
• When you configure VLAN loopback by entering the vlan vlan-list keywords, the VLANs are
tunneled into an internal VLAN that is not forwarded to any ports. The tunnel ends at the egress, so
it is transparent to the user.
• VLAN loopback is not supported on nontrunk interfaces.
• Terminal loopback is not supported on routed interfaces.
• You cannot configure SPAN and loopback on the switch at the same time. If you try to configure
SPAN on any port while loopback is configured, you receive an error message.
• If a port is a Flex Link port or belongs to an EtherChannel, it cannot be put into a loopback state. If
loopback is active, you cannot add a port to a Flex Link or EtherChannel.
Ingress
port
Ingress
VLAN
mapping
Policer
Shaper
Egress
VLAN
mapping
MAC swap
Egress
port
MAC
loopback
Port(s)
Egress
VLAN
mapping
Shaper
Policer
Ingress
VLAN
mapping
skip
Egress
port
205654