Chapter 43: BSoD L2VPN
STANDARD Revision 1.0
C4® CMTS Release 8.3 User Guide
© 2016 ARRIS Enterprises LLC. All Rights Reserved.
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Overview
The BSoD L2VPN feature provides point-to-point transparent Layer-2 forwarding between a CM and a network-side Layer-2
device using one or two provider-imposed Q-tags to multiplex the packets belonging to each L2VPN instance on the
designated L2VPN network interface between the C4 CMTS and the network-side device. These provider Q-tags are
imposed or deposed by the CMTS and are not transmitted to or received from CPE devices on the cable interfaces. Any
customer Q-tags imposed by CPE devices are considered to be part of the L2VPN payload and are not considered to be
service delimiting to the CMTS. BSoD does not require the CMTS to learn MAC addresses. MAC address learning is not
required in the downstream direction because of the one-to-one mapping between the Q-tags and the downstream CM. It
is not required in the upstream direction because all traffic for all L2VPN instances arrives on known L2VPN upstream
service flows (SFs) and is vectored to a single active egress L2VPN network interface.
Since CPE-imposed Q-tags are not trusted as L2VPN membership tokens, all L2VPN traffic from CPE must arrive on one or
more upstream SFs designated during CM registration as belonging to the L2VPN instance. This is done with an L2VPN TLV
encoding in the CM configuration file that provides the upstream Q-tags and Priority values to be used when transmitting
these packets on the active L2VPN egress network interface. An L2VPN instance may be configured with either one or two
12-bit Q-tags. Single Q-tag encapsulated L2VPN instances provide a numbering space of up to 4,000 values; dual Q-tag
encapsulated L2VPN instances provide up to 16 million values.1
However, the CMTS supports a maximum of 16,000 L2VPN instances identified by either single or dual Q-tags that may be
taken from anywhere within the Q-tag number space. An outer Q-tag may have a Q-tag only in the range of 2-4094
because Q-tags 0, 1, and 4095 are reserved by IEEE 802.1Q while inner Q-tags may be numbered from 1-4095.
Within a given CM one or more SFs may share the same Q-tags, so the CM configuration file may contain either a single
L2VPN TLV encoding that applies to all SFs or it may contain individual L2VPN TLV encodings that apply only to a single SF.
Either way, individual SFs are configured for L2VPN treatment in the CMTS during CM registration. The CMTS does not
support L2VPNs that are defined in dynamic SFs.
When a downstream Q-tagged packet arrives on the active L2VPN ingress network interface in the RCM, the Q-tags
uniquely identify the CM belonging to the L2VPN instance and hence the downstream cable port. Only individual values of
the Q-tags identify L2VPN instances because there is no service type encoding in the Q-tags. This means that the Q-tag
number space for single (or outer) Q-tags on a given physical network interface is shared between the existing L3VPN VRF
Q-tag subinterface feature and this L2VPN Q-tag feature. Since the L3VPN VRF Q-tag feature only uses a single Q-tag, only
the outer Q-tag of a Q-tag pair must be inspected to determine if the packet is to receive L2VPN or L3VPN forwarding
