The specific configuration settings for a DSL installation will depend on the type of router used
and the features desired, but there are common elements.
The key element of a DSL installation is that the technology is fundamentally a physical
transport of ATM cells. As such, we will configure a Cisco 3810 router to terminate multiple
DSL connections (ADSL, in this case). The head end is a T-1 ATM connection. You might realize
that the T-1 is a poor termination choice for ADSL services; however, for this application it
is an appropriate solution. A DS-3 or other ATM connection could provide the termination just
as well.
Configuration of the DSLAM is beyond the scope of the test and this book, but
functionally it is PVC configuration and other parameters. Stated another way,
it is not complicated.
In addition to the typical configuration parameters you might include (such as routing,
logging, security, and management), the DSL configuration requires very little additional
configuration. In this excerpt, we configure the T-1 physical interface with Extended Super
Frame and B8ZS encoding, in addition to setting it for ATM cells. The ATM interface has
no configuration, but is subinterfaced for multiple connections. (Recall that this is a headend,
non-DSLAM connection.) We configure a PVC with unspecified bit rate (UBR) ATM,
and, as an extra service, we configure Operation, Administration, and Maintenance (OAM)
cells to the PVC. OAM provides link monitoring; if any part of the PVC fails, OAM will
detect it and shut down the interface until corrected.
The following configuration also specifies AAL5SNAP, or AAL5 with SNAP headers, for the
encapsulation type. So long as this matches on each side, there is no issue in most cases. For
those not familiar with PVC configurations, interface ATM0.1 has a VPI (virtual path identifier)
of 5 and a VCI (virtual circuit identifier) of 51.
!
controller T1 0
framing esf
linecode b8zs
mode atm
fdl both
description T1 to DSL Cloud
!
interface ATM0
description DSL Headend
no ip address
no ip directed-broadcast
!
interface ATM0.1 point-to-point
description DSL link to Gryffendor
ip address 10.1.1.25 255.255.255.252
no ip directed-broadcast
pvc 5/51
ubr 1500
oam-pvc manage
oam retry 3 5 1
encapsulation aal5snap
!
interface ATM0.2 point-to-point
description DSL Link to Ravenclaw
ip address 10.1.1.33 255.255.255.252
no ip directed-broadcast
pvc 4/51
ubr 1500
oam-pvc manage
oam retry 3 5 1
encapsulation aal5snap
!
NOTE:If there were only one PVC for this circuit, it would be acceptable to use the major
interface and not a subinterface. However, if an installation
might
use more than
one PVC in the future, then the use of a subinterface is recommended.
Other routers might limit various options. The Cisco 827, for example, uses a Bridge Group
Virtual Interface (BVI), which is part of Integrated Routing and Bridging (IRB) services for connectivity
instead of routing in most installations. This bridging solution negates layer 3 and leverages
Network Address Translation (NAT) for those services that are layer 3. The configuration is
not DSL-specific however, because the use of IRB is primarily used to negate the need for remote
configuration. A standard router configuration file can service all end points because DHCP and
NAT hide the Ethernet network, and the DSL side is assigned its address dynamically.
NOTE:IRB, BVI, NAT, and DHCP in this context are beyond the scope of this chapter and
of the exam. Chapter 31, “Network Address Translation (NAT) and Port Address
Translation (PAT),” provides information regarding NAT, and Chapter 25, “Using
Microsoft Windows 95/98/2000/XP,” describes DHCP. If you are interested in learning
more about the 827 router (a common remote DSL platform) and IRB/BVI,
please refer to Cisco’s documentation at