Specifications

Configuring Routing Between VLANs
How to Configure Routing Between VLANS
41
Restrictions
The following restrictions apply to the Cisco 10000 series Internet router:
• Supported on Ethernet, FastEthernet, or Gigabit Ethernet interfaces.
• Supports only Point-to-Point Protocol over Ethernet (PPPoE) packets that are double-tagged for
Q-in-Q VLAN tag termination.
• IP and Multiprotocol Label Switching (MPLS) packets are not supported.
• Modular QoS can be applied to unambiguous subinterfaces only.
• Limited ACL support.
IEEE 802.1Q-in-Q VLAN Tag Termination on Subinterfaces
IEEE 802.1Q-in-Q VLAN Tag Termination simply adds another layer of IEEE 802.1Q tag (called “metro
tag” or “PE-VLAN”) to the 802.1Q tagged packets that enter the network. The purpose is to expand the
VLAN space by tagging the tagged packets, thus producing a “double-tagged” frame. The expanded
VLAN space allows the service provider to provide certain services, such as Internet access on specific
VLANs for specific customers, and yet still allows the service provider to provide other types of services
for their other customers on other VLANs.
Generally the service provider’s customers require a range of VLANs to handle multiple applications.
Service providers can allow their customers to use this feature to safely assign their own VLAN IDs on
subinterfaces because these subinterface VLAN IDs are encapsulated within a service-provider
designated VLAN ID for that customer. Therefore there is no overlap of VLAN IDs among customers,
nor does traffic from different customers become mixed. The double-tagged frame is “terminated” or
assigned on a subinterface with an expanded encapsulation dot1q command that specifies the two
VLAN ID tags (outer VLAN ID and inner VLAN ID) terminated on the subinterface. See Figure 81 on
page 42.
IEEE 802.1Q-in-Q VLAN Tag Termination is generally supported on whichever Cisco IOS features or
protocols are supported on the subinterface; the exception is that Cisco 10000 series Internet router only
supports PPPoE. For example if you can run PPPoE on the subinterface, you can configure a
double-tagged frame for PPPoE. The only restriction is whether you assign ambiguous or unambiguous
subinterfaces for the inner VLAN ID. See the “Unambiguous and Ambiguous Subinterfaces” section on
page 44.
Note The Cisco 10000 series Internet router only supports PPPoE over Q-in-Q (PPPoEQinQ).
The primary benefit for the service provider is reduced number of VLANs supported for the same
number of customers. Other benefits of this feature include:
• PPPoE scalability. By expanding the available VLAN space from 4096 to approximately 16.8
million (4096 times 4096), the number of PPPoE sessions that can be terminated on a given interface
is multiplied.
• When deploying Gigabyte Ethernet DSL Access Multiplexer (DSLAM) in wholesale model, you can
assign the inner VLAN ID to represent the end-customer virtual circuit (VC) and assign the outer
VLAN ID to represent the service provider ID.