Traffic Shaping on a Frame Relay Network
Router 1
Router 4
Router 3
Router 2
1544k (T1) Port
256k Port
64k Port
64k Port
VC 1
16k CIR
VC 2
16k CIR
VC 3
64k CIR
With this configuration, we have a separate virtual circuit going to each
remote site in a point-to-multipoint configuration. Because of the unequal data
rates, without traffic shaping, it is possible that the traffic flowing out of Router 1
might overload any one of the three other routers.Traffic could potentially travel
across the majority of the Frame Relay network, only to be dropped at the egress
of the network, right before the remote router.This does not make very efficient
use of the network.You might consider simply lowering the port speed of the
hub router to 64 Kbps to prevent this; however, not only would Router 4 then
have the potential to overwhelm the hub router, but if Routers 2, 3, and 4 all
transmitted at their port speed simultaneously (64 + 64 + 256 = 384), they definitely
would.
FRTS can solve this problem.What is typically done is enable FRTS at the hub
location and set the FRTS CIR parameter (not the carrier CIR) equal to the port
speed at the far end of the VC.Thus, for the first VC from Router 1 to Router 2,
we would have a CIR set to 64 Kbps.The same configuration would apply to the
second VC.We would set the CIR of the third VC to 256 Kbps.This overcomes
the data-rate mismatch, the traffic becomes well-behaved, and unnecessary packet
www.syngress.com
Figure 8.5 Traffic Shaping on a Frame Relay Network
Router 1
Router 4
Router 3
Router 2
1544k (T1) Port
256k Port
64k Port
64k Port
VC 1
16k CIR
VC 2
16k CIR
VC 3
64k CIR
Frame Relay
258 Chapter 8 • Advanced QoS for AVVID Environments
drops are eliminated. Enabling FRTS on the remote ends might be helpful if you
wanted the routers to heed BECNs and throttle down when the network is congested,
but by enabling FRTS on the hub site alone, we have eliminated the datarate
mismatch problem. FRTS will heed congestion notifications from both BECN
and Cisco Foresight messages.