DGS-3700-12/DGS-3700-12G Series Layer 2 Gigabit Ethernet User Manual
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Maintenance Point
A maintenance point in CFM is a point of demarcation on a port within a maintenance domain. Maintenance points
filter CFM frames within the boundries of an MD by dropping frames that do not belong to the correct maintenance
level. There are two types of maintenance points,
Maintenance Endpoints
(MEPs) and
Maintenance Intermediate
Points (MIPs). MEPS and MIP are manually configured by a network administrator.
An MEP exists at the edge of a maintenance domain, defining the boundry of the MD. MEP functions include filtering
CFM messages so that they are confined to the MD. A MEP can be configured to transmit Connectivity Check
Messgaes (CCMs) and will transmit treacroute and loopback messages if configured to do so. A MEP can be
Inward
facing or
Outward
facing.
An Inward facing MEP source CFM frames toward the bridge relay function, not through the bridge port on which the
MEP is configured. An Inward facing MEP drops all CFM frames at its level or lower that are received from the Inward
side; and forwards all CFM frames at a higher level regardless of the origin of the frame, Inward or Outward. If the port
on which the inward MEP is configured is blocked by Spanning-Tree Protocol, the MEP can no longer transmit or
receive CFM messages.
An Outward facing MEP source frames toward the bridge port and can only be configured on routed ports. An
Outward facing port drops all CFM frames at it level or lower coming from the bridge relay function side. It processes
all CFM frames at its level, and drops all CFM frames at a lower level, coming from the bridge port. An Outward facing
port forwards all CFM frames at higher levels regardless of which direction the frames come in. If the port on which the
outward MEP is configured is blocked by Spanning-Tree Protocol, the MEP can still transmit and receive CFM
messages through the bridge port.
An MIP is a maintenance point that is internal to an MD, not at the boundry. A MIP receives CFM frames from other
MIPs and from MEPs. These frames are cataloged and forwarded using the bridge relay function and bridge port. All
CFM frames at a lower level than the MIP are blocked and dropped regardless of the origin. All CFM frames at a
higher level are forwarded regardless of the origin. If the port on which a MIP is configured is blocked by Spanning-
Tree Protocol, the MIP cannot receive CFM messages or relay them toward the bridge relay function side. The MIP
can, however, receive and respond to CFM messages from the bridge port.
CFM messages include Continuity Check Messages (CCMs), Loopback messages (LBMs) and Link Trace Messages
(LTMs). CFM uses standard Ethernet frames that can be sourced, terminated, processed, and relayed by bridges.
Routers support limited CFM functions.
Continuity Check Messages (CCM)
CCMs are multicast messages exchanged among MEPs. CCMs allow discovery of MEPs for other MEPs within a
domain and allow MIPs to discover MEPs. CCMs are confined to a maintenance domain. CCMs are cataloged by
MIPs are the same maintenance level and terminated by remote MEPs at the same maintenance level. They are
unidirectional (no response soliticitation) and carry the status of the port on which the MEP is configured.
Loopback Messages (LBM)
LBMs are unicast frames that the MEP transmits to successive MIPs to locate a fault. The concept of LBMs is similar
to Internet Control Message Protocol (ICMP). In a similar way to a flood ping, a high volume of Loopback Messages
can be sent to test the bandwidth, reliability, or jitter of a service. An LBM can be sent by an MEP to any other MEPs
or MIPs that are part of the same service. LBMs are different to CCMs as they can be initated and stopped by an
administrator.
Link Trace Messages (LTM)
LTMs are multicast CFM frames sent by MEPs to identify adjacency relationships with remote MEPs and MIPs at the
same maintenance level. The message body of an LTM includes a destination MAC address of a target MEP that
terminates the linktrace. When a MIP or MEP receives an LTM, it generates a unicast Link Trace Reply (LTR) to the
initiating MEP. It also forwards the LTM to the target MEP destination MAC address. An LTM effectively traces the
path to the target MEP or MIP.
Alarm Indication System (AIS)
The OAM feature uses the Alarm Indication System (AIS) so that a network node can inform other nodes of any faults
that have occurred, regardless of the intermediary network connection types.
Ethernet Lock System (LCK)
The Ethernet Lock System (LCK) is used by a server (sub) layer MEP to inform the MEP that it is administratively
locked. When an MEP is administratively locked, the LCK feature adds the ETH-LCK information to a frame. The
ETH-LCK information enables the MEP to distinguish between traffic that has been administratively locked by the
server (sub) layer MEP and faulty behavior
