Cisco Catalyst 4500 Series, Software Configuration Manual

Page: 1878 / 2086

75-28
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 75 Configuring Ethernet OAM and CFM
Understanding CFM ITU-T Y.1731 Fault Management
Alarm Indication Signals
The Ethernet Alarm Signal function (ETH-AIS) is used to suppress alarms after defects are detected at
the
server (sub) layer, which is a virtual MEP layer capable of detecting fault conditions. A fault
condition could be a signal fail condition, an AIS condition, or a LCK condition.
Note Although the configuration is allowed, you should not configure AIS in networks running STP. An STP
configuration might cause AIS interruption or redirection.
When a MEP or a service MEP (SMEP) detects a connectivity fault at a specific maintenance association
level, it multicasts AIS frames in the direction away from the detected failure at the client maintenance
association level. The frequency of AIS frame transmission is based on the AIS transmission period. The
first AIS frame is always sent immediately following the detection of the defect condition. We
recommend a transition period of 1 second in a network of only a few VLANs to ensure that the first AIS
frame is sent immediately following error detection. We recommend a 60-second interval in a network
of multiple (up to 4094) VLANs to prevent stressing the network with 1-second transmissions.
A MEP that receives a frame with ETH-AIS information cannot determine the specific server with the
defect condition or the set of peer MEPs for which it should suppress alarms. Therefore, it suppresses
alarms for all peer MEPs, whether or not they are connected.
When a MEP receives an AIS frame, it examines it to be sure that the Maintenance Entity Group (MEG)
level matches its own MEG and then detects the AIS default condition. (A MEG is Y.1731 terminology
for maintenance association in 802.1ag.) After this detection, if no AIS frames are received for an
interval of 3.5 times the AIS transmission period, the MEP clears the AIS defect condition. For example,
if the AIS timer is set for 60 seconds, the AIS timer period expires after 3.5 times 60, or 210 seconds.
The AIS condition is terminated when a valid CCM is received with all error conditions cleared or when
the AIS period timer expires (the default time is 60 seconds).
Ethernet Remote Defect Indication
When Ethernet OAM continuity check (ETH-CC) transmission is enabled, the Ethernet Remote Defect
Indication (ETH-RDI) function uses a bit in the CFM CC message to communicate defect conditions to
the MEP peers. For ETH-RDI functionality, you must configure the MEP MEG level, the ETH-CC
transmission period, and the ETH-CC frame priority. ETH-RDI does not require any MIP configuration.
When a MEP receives frames with ETH-RDI information, it determines that its peer MEP has
encountered a defect condition and sets the RDI files in the CCM frames for the duration of the defect
condition. When the defect condition clears, the MEP clears the RDI field.
When a MEP receives a CCM frame, it examines it to ensure that its MEG level is the same and if the
RDI field is set, it detects an RDI condition. For point-to-point Ethernet connections, a MEP can clear
the RDI condition when it receives the first frame from its peer MEP with the RDI field cleared.
However, for multipoint Ethernet connectivity, the MEP cannot determine the associated subset of peer
MEPs with which the sending MEP has seen the defect condition. It can clear the RDI condition after it
receives CCM frames with the RDI field cleared from its entire list of peer MEPs.