Cfm Protocol Messages; Continuity Check (Ieee 802.1Ag And Itu-T Y.1731) - Cisco NCS 5500 Series Configuration Manual

Configuring Ethernet OAM
MIPs also continue to receive CFM frames at their level if the interface is STP blocked, and can respond to
any received frames. However, MIPs do not allow CFM frames at the level of the MIP to be forwarded if the
interface is blocked.
A separate set of CFM maintenance levels is created every time a VLAN tag is pushed onto the frame.
Note
Therefore, if CFM frames are received on an interface which pushes an additional tag, so as to "tunnel"
the frames over part of the network, the CFM frames will not be processed by any MPs within the tunnel,
even if they are at the same level. For example, if a CFM MP is created on an interface with an encapsulation
that matches a single VLAN tag, any CFM frames that are received at the interface that have two VLAN
tags will be forwarded transparently, regardless of the CFM level.

CFM Protocol Messages

The CFM protocol consists of a number of different message types, with different purposes. All CFM messages
use the CFM EtherType, and carry the CFM maintenance level for the domain to which they apply.
This section describes the following CFM messages:

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

Continuity Check Messages (CCMs) are "heartbeat" messages exchanged periodically between all the MEPs
in a service. Each MEP sends out multicast CCMs, and receives CCMs from all the other MEPs in the
service—these are referred to as peer MEPs. This allows each MEP to discover its peer MEPs, and to verify
that there is connectivity between them.
MIPs also receive CCMs. MIPs use the information to build a MAC learning database that is used when
responding to Linktrace. For more information about Linktrace, see the
Y.1731).
Figure 4: Continuity Check Message Flow
Interface and Hardware Component Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.1.x
Ethernet CFM
Linktrace (IEEE 802.1ag and ITU-T
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