Loopback (Ieee 802.1Ag And Itu-T Y.1731); Linktrace (Ieee 802.1Ag And Itu-T Y.1731); Interface And Hardware Component Configuration Guide For Cisco Ncs 5500 Series Routers, Ios Xr Release - Cisco NCS 5500 Series Configuration Manual

Ethernet CFM

Loopback (IEEE 802.1ag and ITU-T Y.1731)

Loopback Messages (LBM) and Loopback Replies (LBR) are used to verify connectivity between a local
MEP and a particular remote MP. At the request of the administrator, a local MEP sends unicast LBMs to the
remote MP. On receiving each LBM, the target maintenance point sends an LBR back to the originating MEP.
Loopback indicates whether the destination is reachable or not—it does not allow hop-by-hop discovery of
the path. It is similar in concept to an ICMP Echo (ping). Since loopback messages are destined for unicast
addresses, they are forwarded like normal data traffic, while observing the maintenance levels. At each device
that the loopback reaches, if the outgoing interface is known (in the bridge's forwarding database), then the
frame is sent out on that interface. If the outgoing interface is not known, then the message is flooded on all
interfaces.
This figure shows an example of CFM loopback message flow between a MEP and MIP.
Figure 5: Loopback Messages
Loopback messages can be padded with user-specified data. This allows data corruption to be detected in the
network. They also carry a sequence number which allows for out-of-order frames to be detected.

Linktrace (IEEE 802.1ag and ITU-T Y.1731)

Linktrace Messages (LTM) and Linktrace Replies (LTR) are used to track the path (hop-by-hop) to a unicast
destination MAC address. At the request of the operator, a local MEP sends an LTM. Each hop where there
is a maintenance point sends an LTR back to the originating MEP. This allows the administrator to discover
connectivity data about the path. It is similar in concept to IP traceroute, although the mechanism is different.
In IP traceroute, successive probes are sent, whereas CFM Linktrace uses a single LTM which is forwarded
by each MP in the path. LTMs are multicast, and carry the unicast target MAC address as data within the
frame. They are intercepted at each hop where there is a maintenance point, and either retransmitted or dropped
to discover the unicast path to the target MAC address.

Interface and Hardware Component Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release

6.1.x
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Configuring Ethernet OAM