Introduction to OAM Connectivity Fault Management (CFM)

The major features of CFM are:

• Fault monitoring using the continuity check protocol. This is a neighbor discovery and health check protocol that discovers and maintains adjacencies at the VLAN or link level.

• Path discovery and fault verification using the linktrace protocol. Similar to IP traceroute, this protocol maps the path taken to a destination MAC address through one or more bridged networks between the source and destination.

• Fault isolation using the loopback protocol. Similar to IP ping, this protocol works with the continuity check protocol during troubleshooting.

CFM partitions the service network into various administrative domains. For example, operators, providers, and customers might be part of different administrative domains.

Each administrative domain is mapped into one maintenance domain providing enough information to perform its own management, thus avoiding security breaches and making end-to-end monitoring possible. Each maintenance domain is associated with a maintenance domain level from 0 through 7. Level allocation is based on the network hierarchy, where outermost domains are assigned a higher level than the innermost domains.

Customer end points have the highest maintenance domain level. In a CFM maintenance domain, each service instance is called a maintenance association. A maintenance association can be thought as a full mesh of maintenance endpoints (MEPs) having similar characteristics. MEPs are active CFM entities generating and responding to CFM protocol messages.

There is also a maintenance intermediate point (MIP), which is a CFM entity similar to the MEP, but more passive (MIPs only respond to CFM messages).

MEPs can be up MEPs or down MEPs. A link can connect a MEP at level 5 to a MEP at level 7. The interface at level 5 is an up MEP (because the other end of the link is at MEP level 7), and the interface at level 7 is a down MEP (because the other end of the link is at MEP level 5).

In a Metro Ethernet network, CFM is commonly used at two levels:

• By the service provider to check the connectivity among its provider edge (PE) routers

• By the customer to check the connectivity among its customer edge (CE) routers

  Note:

  The configured customer CFM level must be greater than service provider CFM level.

In many Metro Ethernet networks, CFM is used to monitor connectivity over a VPLS and bridge network.

CFM support on PTX10001-36MR, PTX10004, PTX10008, and PTX10016 devices includes the following limitations:

• Maintenance End Point (MEP) and maintenance Intermediate Point (MIP) related limitations—You cannot configure:

  • Up MEP and down MEP at same level on an interface.

  • Combination of up MEP, down MEP or MIP at different level over same interface.

• CFM packets take on the default queue. There is no forwarding class to queue (fc-to-queue) mapping, in the following instances:

  • Egress traffic, if cos-rewrite is not configured

  • Untagged traffic

• CFM session goes down, if you configure the VLAN identifier list on a non-trunk interfaces.

• CFM packets that are host-bound and host-generated, don’t bypass the configured firewall filters for both ingress and egress direction.

Ethernet interfaces on M7i and M10i routers with the Enhanced CFEB (CFEB-E) and on M120, M320, MX Series, T Series, and PTX Series routers support the IEEE 802.1ag standard for Operation, Administration, and Management (OAM). The IEEE 802.1ag specification provides for Ethernet connectivity fault management (CFM). The goal of CFM is to monitor an Ethernet network that may comprise one or more service instances. Junos OS supports IEEE 802.1ag connectivity fault management.

• OAM Connectivity Fault Management Overview
• Connectivity Fault Management Key Elements
• Best Practices for Configuring 802.1ag Ethernet OAM for VPLS

Connectivity Fault Management Key Elements

Figure 1 shows the relationships among the customer, provider, and operator Ethernet bridges, maintenance domains, maintenance association end points (MEPs), and maintenance intermediate points (MIPs).

Figure 1: Relationship Among MEPs, MIPs, and Maintenance Domain Levels

Note:

On ACX Series routers, the maintenance intermediate points (MIP) are supported only on the ACX5048 and ACX5096 routers.

A maintenance association is a set of MEPs configured with the same maintenance association identifier and maintenance domain level. Figure 2 shows the hierarchical relationships between the Ethernet bridge, maintenance domains, maintenance associations, and MEPs.

Figure 2: Relationship Among Bridges, Maintenance Domains, Maintenance Associations, and MEPs