Introduction to OAM Connectivity Fault Management (CFM)

The major features of CFM are:

• Fault monitoring using the continuity check protocol. This protocol serves as a neighbor discovery and health check protocol that identifies 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 divides the service network into different administrative domains, such as operators, providers, and customers. These domains might belong to separate administrative domains.

Every administrative domain is linked with one maintenance domain that contains sufficient information for self-management, enable end-to-end monitoring, and prevent security breaches. Each maintenance domain is associated with a maintenance domain level ranging from 0 to 7, based on the network hierarchy. The outermost domains are allocated a higher level than the innermost domains. Customer end points have the highest maintenance domain level.

Each service instance in a CFM maintenance domain is called a maintenance association. A maintenance association consists of a full mesh of maintenance endpoints (MEPs) that share similar characteristics. MEPs are active CFM entities that generate and respond to CFM protocol messages.

There is also a maintenance intermediate point (MIP), which is a CFM entity similar to the MEP. However, MIP is relatively passive and only responds 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.

• If the up MEP is higher than the down MEP, the system does not drop the CCM PDUs selectively and allows them to pass through without interruption.

• DM related timestamping on AE with child links across multiple PFEs is not supported.

• 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

• The configuration of vlan-id-list on OAM enabled IFLs can impact CFM scaling.

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

Junos OS supports IEEE 802.1ag connectivity fault management. 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 standard facilitates Ethernet connectivity fault management (CFM) that helps to monitor an Ethernet network comprising one or more service instances.

In Junos OS Release 9.3 and later, CFM also supports aggregated Ethernet interfaces. CFM sessions operate in distributed mode on the Flexible PIC Concentrator (FPC) on aggregated Ethernet interfaces. As a result, graceful Routing Engine switchover (GRES) is supported on aggregated Ethernet interfaces. In releases before Junos OS Release 13.3, CFM sessions operate in centralized mode on the Routing Engine. However, CFM sessions are not supported on aggregated Ethernet interfaces if the interfaces that form the aggregated Ethernet bundle are in mixed mode. Additionally, CFM sessions with a continuity check message (CCM) interval of 10 milliseconds are not supported over aggregated Ethernet interfaces.

CFM sessions are distributed by default. All CFM sessions must operate in either only distributed or only centralized mode. A mixed operation of distributed and centralized modes for CFM sessions is not supported. To disable the distribution of CFM sessions on aggregated Ethernet interfaces and make the sessions operate in centralized mode, include the no-aggregate-delegate-processing statement at the [edit protocols oam ethernet connectivity-fault-management] hierarchy level.

Starting in Junos OS Release 10.3, CFM is not supported on untagged aggregated Ethernet member links on interfaces configured on Modular Port Concentrators (MPCs) and Modular Interface Cards (MICs) on MX Series routers. However, CFM is supported on both untagged and tagged aggregated Ethernet logical interfaces configured on MPCs and MICs.Starting in Junos OS Release 12.3, CFM does not support Multichassis Link Aggregation (MC-LAG). We recommend not to configure the mc-ae statement when you configure CFM.

Starting in Junos OS Release 11.3, on T Series and M320 routers, CFM is not supported on interfaces configured with CCC encapsulation. If you configure CFM, the system displays the following message: “MEPs cannot be configured on ccc interface on this platform”.

Starting in Junos OS Release 17.4, you can enable support for IEEE 802.1ag CFM on pseudowire service interfaces by configuring maintenance intermediate points (MIPs) on the pseudowire service interfaces. Pseudowire service interfaces support configuring of subscriber interfaces over MPLS pseudowire termination. Termination of subscriber interfaces over PW enables network operators to extend their MPLS domain from the Access/Aggregation network to the service edge and use uniform MPLS label provisioning for a larger portion of their network.​

IEEE 802.1ag OAM supports graceful Routing Engine switchover (GRES). IEEE 802.1ag OAM is supported on untagged, single tagged, and stacked VLAN interfaces.

On EX Series switches, to use the CFM feature, you must first add the CFM to basic Junos OS by installing an enhanced feature license (EFL). See Licenses for EX Series for more details.

• 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