C H A P T E R
4
Using Ethernet Fault Detection
Different interfaces on the routers nowadays support layer 1 failure detection where the signal loss between
two peers is detected and the information is communicated to the control plane to allow a corrective action.
POS interfaces detect complex failures in the routers at layer 2 level and use higher level protocols such as
Point-to-Point Protocol (PPP) to negotiate the state between peers before the interface is brought up at an
layer 2 level in the control plane.
With the proliferation of the pure layer 2 network, ethernet must also detect failures and perform corrective
actions. Hence, the Ethernet Fault Detection (EFD) mechanism is used that allows Ethernet OAM protocols,
such as CFM, to control the "line protocol" state of an interface so that if a CFM defect is detected, the
corrective actions can be performed.
Previously, EFD was used as an event notifier for CFM only. Protocols like G8032 and REP could register
to CFM events and could update the protocol (could bring it down or up) accordingly. But now, EFD allows
CFM events and the OAM protocols to be used as an layer 2 BFD or line-protocol for Ethernet interfaces.
•
Information about Ethernet Fault Detection, page 59
Information about Ethernet Fault Detection
Ethernet Fault Detection (EFD) is a mechanism that allows Ethernet OAM protocols, such as CFM, to control
the
“
line protocol
”
state of an interface.Unlike many other interface types, Ethernet interfaces do not have a
line protocol, whose state is independent from that of the interface. For Ethernet interfaces, this role is handled
by the physical-layer Ethernet protocol itself, and therefore if the interface is physically up, then it is available
and traffic can flow.
EFD changes this to allow CFM to act as the line protocol for Ethernet interfaces. This allows CFM to control
the interface state so that if a CFM defect (such as AIS or loss of continuity) is detected with an expected peer
MEP, the interface can be shut down. This not only stops any traffic flowing, but also triggers actions in any
higher-level protocols to route around the problem. For example, in the case of Layer 2 interfaces, the MAC
table would be cleared and MSTP would reconverge. For Layer 3 interfaces, the ARP cache would be cleared
and potentially the IGP would reconverge.
Carrier Ethernet Configuration Guide (Cisco ASR 920 Series)
59
