The Flexible VLAN Tagging for CFM feature ensures that CFM
packets are sent with the right VLAN tags so that they are appropriately
handled as a CFM packet by the remote device. When packets are received by an
edge router, they are treated as either CFM packets or data packets, depending
on the number of tags in the header. The system differentiates between CFM
packets and data packets based on the number of tags in the packet, and
forwards the packets to the appropriate paths based on the number of tags in
CFM frames are normally sent with the same VLAN tags as the
corresponding customer data traffic on the interface, as defined by the
configured encapsulation and tag rewrite operations. Likewise, received frames
are treated as CFM frames if they have the correct number of tags as defined by
the configured encapsulation and tag rewrite configuration, and are treated as
data frames (that is, they are forwarded transparently) if they have more than
this number of tags.
In most cases, this behavior is as desired, since the CFM frames
are then treated in exactly the same way as the data traffic flowing through
the same service. However, in a scenario where multiple customer VLANs are
multiplexed over a single multipoint provider service (for example, N:1
bundling), a different behavior might be desirable.
This figure shows an example of a network with multiple VLANS
Figure 8. Service Provider Network With Multiple VLANs and
This figure shows a provider's access network, where the S-VLAN
tag is used as the service delimiter. PE1 faces the customer, and PE2 is at the
edge of the access network facing the core. N:1 bundling is used, so the
interface encapsulation matches a range of C-VLAN tags. This could potentially
be the full range, resulting in all:1 bundling. There is also a use case where
only a single C-VLAN is matched, but the S-VLAN is nevertheless used as the
service delimiter—this is more in keeping with the IEEE model, but limits the
provider to 4094 services.
CFM is used in this network with a MEP at each end of the access
network, and MIPs on the boxes within the network (if it is native Ethernet).
In the normal case, CFM frames are sent by the up MEP on PE1 with two VLAN
tags, matching the customer data traffic. This means that at the core
interfaces and at the MEP on PE2, the CFM frames are forwarded as if they were
customer data traffic, since these interfaces match only on the S-VLAN tag. So,
the CFM frames sent by the MEP on PE1 are not seen by any of the other MPs.
Flexible VLAN tagging changes the encapsulation for CFM frames
that are sent and received at Up MEPs. Flexible VLAN tagging allows the frames
to be sent from the MEP on PE1 with just the S-VLAN tag that represents the
provider service. If this is done, the core interfaces will treat the frames as
CFM frames and they will be seen by the MIPs and by the MEP on PE2. Likewise,
the MEP on PE1 should handle received frames with only one tag, as this is what
it will receive from the MEP on PE2.
To ensure that CFM packets from Up MEPs are routed to the
appropriate paths successfully, tags may be set to a specific number in a
domain service, using the
Currently, tags can only be set to one (1).