Specifications

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Cisco MWR 2941 Mobile Wireless Edge Router Software Configuration Guide, Release 12.2(33)MRB
OL-21227-02
Chapter 1 Cisco MWR 2941 Router Overview
Features
Remote Loopback
An OAM entity can put its remote peer into loopback mode using the loopback control OAM PDU.
Loopback mode helps an administrator ensure the quality of links during installation or when
troubleshooting. In loopback mode, every frame received is transmitted back on the same port except for
OAM PDUs and pause frames. The periodic exchange of OAM PDUs must continue during the loopback
state to maintain the OAM session.
The loopback command is acknowledged by responding with an information OAM PDU with the
loopback state indicated in the state field. This acknowledgement allows an administrator, for example,
to estimate if a network segment can satisfy a service-level agreement. Acknowledgement makes it
possible to test delay, jitter, and throughput.
When an interface is set to the remote loopback mode the interface no longer participates in any other
Layer 2 or Layer 3 protocols such as Spanning Tree Protocol (STP) or Open Shortest Path First (OSPF).
The reason is that when two connected ports are in a loopback session, no frames other than the OAM
PDUs are sent to the CPU for software processing. The non-OAM PDU frames are either looped back
at the MAC level or discarded at the MAC level.
From a user's perspective, an interface in loopback mode is in a link-up state.
Cisco Vendor-Specific Extensions
Ethernet OAM allows vendors to extend the protocol by allowing them to create their own
type-length-value (TLV) fields.
OAM Messages
Ethernet OAM messages or OAM PDUs are standard length, untagged Ethernet frames within the
normal frame length bounds of 64 to 1518 bytes. The maximum OAM PDU frame size exchanged
between two peers is negotiated during the discovery phase.
OAM PDUs always have the destination address of slow protocols (0180.c200.0002) and an Ethertype
of 8809. OAM PDUs do not go beyond a single hop and have a hard-set maximum transmission rate of
10 OAM PDUs per second. Some OAM PDU types may be transmitted multiple times to increase the
likelihood that they are successfully received on a deteriorating link.
Four types of OAM messages are supported:
• Information OAM PDU—A variable-length OAM PDU that is used for discovery. This OAM PDU
includes local, remote, and organization-specific information.
• Event notification OAM PDU—A variable-length OAM PDU that is used for link monitoring. This
type of OAM PDU may be transmitted multiple times to increase the chance of a successful receipt;
for example, in the case of high-bit errors. Event notification OAM PDUs also may include a time
stamp when generated.
• Loopback control OAM PDU—An OAM PDU fixed at 64 bytes in length that is used to enable or
disable the remote loopback command.
• Vendor-specific OAM PDU—A variable-length OAM PDU that allows the addition of
vendor-specific extensions to OAM.
For instructions on how to configure Ethernet Link OAM, see the “Configuring Ethernet Link
Operations, Administration, and Maintenance (OAM)” section on page 4-33.