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Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 32 Configuring Ethernet OAM, CFM, and E-LMI
Understanding Ethernet CFM
CFM 802.1ag supports up and down per-VLAN MEPs, as well as port MEPs, which are untagged
down MEPs that are not associated with a VLAN. Port MEPs are configured to protect a single hop
and used to monitor link state through CFM. If a port MEP is not receiving continuity check
messages from its peer (static remote MEP), for a specified interval, the port is put into an
operational down state in which only CFM and OAM packets pass through, and all other data and
control packets are dropped.
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An up MEP sends and receives CFM frames through the relay function. It drops all CFM frames
at its level or lower that come from the wire side, except traffic going to the down MEP. For
CFM frames from the relay side, it processes the frames at its level and drops frames at a lower
level. The MEP transparently forwards all CFM frames at a higher level, regardless of whether
they are received from the relay or wire side. If the port on which MEP is configured is blocked
by STP, the MEP can still send or receive CFM messages through the relay function. CFM runs
at the provider maintenance level (UPE-to-UPE), specifically with up MEPs at the user network
interface (UNI).
Note
The switch rate-limits all incoming CFM messages at a fixed rate of 500 frames per second.
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A down MEP sends and receives CFM frames through the wire connected to the port on which
the MEP is configured. It drops all CFM frames at its level or lower that come from the relay
side. For CFM frames from the wire side, it processes all CFM frames at its level and drops
CFM frames at lower levels except traffic going to the other lower-level down MEP. The MEP
transparently forwards all CFM frames at a higher level, regardless of whether they are received
from the relay or through the wire.
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Maintenance intermediate points (MIPs) are internal to a domain, not at the boundary, and respond
to CFM only when triggered by traceroute and loopback messages. They forward CFM frames
received from MEPs and other MIPs, drop all CFM frames at a lower level (unless MIP filtering is
enabled), and forward all CFM frames at a higher level and at a lower level and regardless of whether
they are received from the relay or wire side. When MIP filtering is enabled, the MIP drops CFM
frames at a lower level. MIPs also catalog and forward continuity check messages (CCMs), but do
not respond to them.
MIP filtering is disabled by default, and you can configure it to be enabled or disabled. When MIP
filtering is disabled, all CFM frames are forwarded.
You can manually configure a MIP or configure the switch to automatically create a MIP. You can
configure a MEP without a MIP. In case of a configuration conflict, manually created MIPs take
precedence over automatically created MIPs.
If port on which the MEP is configured is blocked by Spanning-Tree Protocol (STP), the MIP can
receive and might respond to CFM messages from both the wire and relay side, but cannot forward
any CFM messages.
CFM Messages
CFM uses standard Ethernet frames distinguished by EtherType or (for multicast messages) by MAC
address. All CFM messages are confined to a maintenance domain and to a service-provider VLAN
(S-VLAN). These CFM messages are supported:
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Continuity Check (CC) messages—multicast heartbeat messages exchanged periodically between
MEPs that allow MEPs to discover other MEPs within a domain and allow MIPs to discover MEPs.
CC messages are configured to a domain or VLAN. Enter the continuity-check Ethernet service
configuration command to enable CCM.
