User`s guide

XSR User’s Guide 193
Chapter 9 Understanding ISDN
Configuring Integrated Services Digital Network (ISDN)
Bandwidth Optimization
The XSR offers features which reduce call connection time and prevent
network overhead from triggering ISDN calls.
Dial-on-Demand (DoD) processes data calls strictly as needed, when
interesting packets must be passed to specific destinations.
Bandwidth-on-Demand (BoD) allocates ISDN bandwidth as efficiently as
possible to accommodate varying traffic loads. The first element of this
feature set is short-hold mode, which prevents links from forming in the
absence of data traffic, while simulating continuous connections.
For instance, suppose a remote workstation was connected to the corporate
LAN via ISDN, but no data was being sent because a user’s PC was idle. With
short-hold mode, in the absence of any data traffic the ISDN call would be
brought down, although from the user's perspective the link/route would
still be active, since any data transfer would automatically (and
transparently) bring up an ISDN call.
The second element of BoD directs that as traffic requirements increase or
decrease, B-channels can be added or subtracted to best accommodate the
load. This dynamic form of channel aggregation is often used by Multilink
PPP which aggregates channels across multiple B channels of one or more
BRI/PRI ports. The XSR implements this element of BoD with the
multilink
load-threshold
, multilink min-links, and bap set of commands.
To further make BoD work properly, the XSR also implements filtering and
protocol spoofing in order to prevent network overhead such as RIP updates
from needlessly bringing up the ISDN link. Although some of these frames
can be discarded without any negative consequences, most are required to
keep workstations and servers across the entire enterprise network
synchronized with one another.
The XSR filters unnecessary overhead by the use of Access Control Lists
specifying interesting packets, and by spoofing protocol overhead packets to
maintain the routes while keeping ISDN connection costs under control.
The XSR performs LAN spoofing where on demand calls spoof RIP or OSPF
updates - RIP updates are sent over the WAN only when changes to the
network occur and are piggy-backed with data traffic. The
dialer map
command
is used to enable spoofing.