Chapter 15: Interface IP Configuration
STANDARD Revision 1.0
C4® CMTS Release 8.3 User Guide
© 2016 ARRIS Enterprises LLC. All Rights Reserved.
436
Provider (P) router. The C4/c CMTS does this using a layer 2 virtual circuit (VC) mechanism with 802.1Q Virtual LAN (VLAN)
tags embedded in the traffic. This allows a single physical network interface to host multiple logical subinterfaces identified
by Q-tags, thereby multiplexing traffic from multiple VPNs over a single physical link.
Normally subinterfaces in the C4/c CMTS segregate packets by source IP address (SIP) prefix only. This works well on the
cable side, but not on the network side. Network subinterfaces typically have incoming SIPs that belong to remote subnets
not hosted by the C4/c CMTS.
The Q-tag feature extends the existing network subinterface function to include layer 2 VCs based on the presence of a Q-
tag containing a VLANid in the ethernet header, as in the figure below:
Figure 77:
Difference between Standard IP and Q-tag Encapsulation
In this case subinterface traffic that arrives or leaves the RCM port is encapsulated in an ethernet frame that has a Q-tag
ethertype (0x8100, as in the figure below) positioned in front of a native ARP or IP ethertype (0x0806 or 0x0800