Specifications

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System Management Software Configuration Guide for Cisco IE 2000U and Connected Grid Switches
Chapter 17 Configuring Ethernet OAM, CFM, and E-LMI
Enabling Ethernet Loopback
Enabling Ethernet Loopback
Service providers can use per-port and per VLAN Ethernet loopback to test connectivity at initial startup,
to test throughput, and to test quality of service (QoS) in both directions. The switch supports two types
of loopback:
• Facility loopback allows per-port or per-port, per-VLAN loopback of traffic. It provides an alternate
method to Ethernet OAM remote loopback (see the “Enabling Ethernet OAM Remote Loopback”
section on page 17-38) to test connectivity across multiple switches. You can exchange (swap) MAC
destination and source addresses to allow a packet to cross multiple switches between the test head
and a test switch.
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 17-3.
You can configure only one terminal loopback per switch.
Figure 17-3 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.
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
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