Specifications

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Cisco MWR 2941 Mobile Wireless Edge Router Release 3.5 Software Configuration Guide, Cisco IOS Release 15.1(3)MR
OL-26895-01
Chapter 15 Configuring Ethernet OAM, CFM, and E-LMI
Understanding Ethernet CFM
Note CFM draft 1 referred to inward and outward-facing MEPs. CFM draft 8.1 refers to up and down MEPs,
respectively. This document uses the CFM 8.1 terminology for direction.
CFM draft 1 supported only up MEPs on a per-port or per-VLAN basis. 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.
– 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).
– 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.
• 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.
In the first draft of CFM, MIP filtering was always enabled. In draft 8.1, 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 router 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. This differs from CFM draft 1, where STP blocked ports could not send or
receive CFM messages.