S-Vlan Overview; Figure 13: Use Of Vlans To Multiplex Different Protocols Over A Single Physical - Juniper JUNOSE SOFTWARE FOR E SERIES 11.3.X - LINK LAYER CONFIGURATION GUIDE 2010-10-13 Configuration Manual

JunosE 11.3.x Link Layer Configuration Guide
Figure 13: Use of VLANs to Multiplex Different Protocols over a Single Physical Link
ERX14xx models (rear view)

S-VLAN Overview

168
As shown in Figure 13 on page 168, VLANs are similar to ATM VCs, with the VLAN ID serving
the same function as the virtual path identifier (VPI) and virtual channel identifier (VCI)
to multiplex the different channels over the physical link. The Ethernet protocol type
serves the same function within a VLAN as the logical link control (LLC) subnetwork
attachment point (SNAP) within a VC, to multiplex the different protocols over the
channel.
In a VLAN configuration, the router can send VLAN 0 tagged or untagged frames.
All VLAN subinterfaces use the MAC address of the Ethernet interface over which they
are configured. However, some configurations, such as multiple IP over VLAN
subinterfaces, require that you connect many VLAN subinterfaces to a single device. In
these cases, the device uses the MAC address to identify and select the correct VLAN to
use. When the MAC address is the same for all VLANs, uneven load balancing of traffic
occurs. To ensure proper load balancing, you must assign unique MAC addresses to the
individual VLAN subinterfaces that are connected to the device. Any ARP requests and
responses generated for the IP address assigned to a VLAN subinterface use this MAC
address.
You must assign the MAC address when you configure the VLAN ID. If you change the
MAC address of the VLAN subinterface after you configure it, system errors can occur.
To change the MAC address, you must first remove the VLAN subinterface and then
reconfigure it.
For more information, see:
Chapter Configuring IP in JunosE IP, IPv6, and IGP Configuration Guide
"Configuring Point-to-Point Protocol over Ethernet" on page 371
As described in "VLAN Overview" on page 167, VLANs permit multiplexing multiple IP
interfaces and PPPoE interfaces over a single physical Ethernet port by creating VLAN
subinterfaces. As specified in IEEE Standard 802.1q, the 12-bit VLAN identifier's tagged
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