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7
C H A P T E R
Configuring IP Unnumbered on IEEE 802.1Q
VLANs
Service providers continuously seek ways in which they can make their networks less complex and less
expensive, and reduce the cost of provisioning subscribers. One way in which service providers can
achieve these results is to migrate their ATM networks to IP networks and upgrade their DSLAM to use
a Gigabit Ethernet uplink, instead of an ATM uplink, to connect their DSLAM to an aggregation router,
such as the Cisco 10000 series router.
In the Digital Subscriber Line (DSL) environment, service providers use a service model that configures
ATM Routed Bridge Encapsulation (RBE) on an unnumbered interface of the aggregation router. This
configuration associates an IP route with a subscriber, and uses a virtual path identifier/virtual
connection identifier (VPI/VCI) pair to identify the IP route. In this way, all subscribers can securely
share the same subnet, which enables the service provider to save IP address space. When the DHCP
server provides an IP address to the subscriber, the aggregation router dynamically configures the IP
route.
The Cisco 10000 series router builds on the RBE on an unnumbered interface service model to enable
you to configure IP unnumbered on IEEE 802.1Q VLANs. Instead of using a VPI/VCI pair to identify a
subscriber route, the Cisco 10000 series router maps a VLAN identifier to the subscriber on an Ethernet
interface.
The Cisco 10000 series router supports the IP Unnumbered on IEEE 802.1Q VLANs feature. Prior to
Cisco IOS Release 12.3(7)XI1, IP support for VLAN subinterfaces required that you configure separate
IP subnets for each of the subinterfaces that terminate the VLAN. This resulted in inefficient use of the
IP address space because an entire IP subnet is often not needed for the hosts assigned to a VLAN. The
IP Unnumbered on VLANs feature helps to conserve IP address space for service provider
configurations that include Ethernet VLAN subinterfaces.
VLAN subinterfaces with IP unnumbered configured support DHCP for IP address allocation. The
DHCP server uses the information in DHCP Option 82 to assign IP addresses to the hosts on a VLAN.
The routing table is dynamically updated to insert an IP route for the IP address assigned on each of the
subinterfaces. These IP host routes exist until the DHCP lease time expires or the host releases the leased
address.
For more information about Option 82, see the
"DHCP Relay Agent Information Option—Option 82"
Note
section on page
3-9.
Cisco 10000 Series Router Software Configuration Guide
7-1
OL-2226-23
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