A router is connected to an HDLC circuit via a T1 physical interface. The SLA for this link only
allows for a sustained rate of 768 kb/s. Bursts are allowed for up to 30 seconds at up to line rate,
with a window Tc of 125 ms. What should the Bc and Be setting be when using generic traffic
shaping?
A.
Be = 46320000 , Bc = 96000
B.
Be = , 768000 Be = 32000
C.
Be = , 128000 Be = 7680
D.
Be = , 0 Be = 96000
Explanation:
Tc= 125
CIR = 768
What is the BeT1 = 1.544 Mbps
Bursts are allowed for 30 seconds
Seconds * Bandwidth in bps = Be
30 * 1544000 = Be
30 * 1544000 = 46320000
Be = 46320000
What is Bc?
Bc = Tc * CIR
Bc = 125 * 768
Bc = 96000
Traffic Shaping Parameters
We can use the following traffic shaping parameters:
CIR = committed information rate (= mean time)
EIR = excess information rate
TB = token bucket (= Bc + Be)
Bc = committed burst size (= sustained burst size)
Be = excess burst size
DE = discard eligibility
Tc = measurement interval
AR = access rate corresponding to the rate of the physical interface (so if you use a T1, the AR is
approximately 1.5 Mbps).
Committed Burst Size (Bc)
The maximum committed amount of data you can offer to the network is defined as Bc. Bc is a
measure for the volume of data for which the network guarantees message delivery under normal
conditions. It is measured during the committed rate Tc.
Excess Burst Size (Be)
The number of noncommitted bits (outside of CIR) that are still accepted by the Frame Relay
switch but are marked as eligible to be discarded (DE). The token bucket is a ‘virtual’ buffer. It
contains a number of tokens, enabling you to send a limited amount of data per time interval. The
token bucket is filled with Bc bits per Tc.
The maximum size of the bucket is Bc + Be. If the Be is very big and, if at T0 the bucket is filled
with Bc + Be tokens, you can send Bc + Be bits at the access rate. This is not limited by Tc but by
the time it takes to send the Be. This is a function of the access rate.
Committed Information Rate (CIR)
The CIR is the allowed amount of data which the network is committed to transfer under normal
conditions. The rate is averaged over a increment of time Tc. The CIR is also referred to as the
minimum acceptable throughput. Bc and Be are expressed in bits, Tc in seconds, and the access
rate and CIR in bits per second.
Bc, Be, Tc and CIR are defined per datalink connection identifier (DLCI). Due to this, the tokenbucket filter controls the rate per DLCI. The access rate is valid per usernetwork interface. For Bc,
Be and CIR incoming and outgoing values can be distinguished. If the connection is symmetrical,
the values in both directions are the same. For permanent virtual circuits, we define incoming and
outgoing Bc, Be and CIR at subscription time.
Peak = DLCI’s maximum speed. The bandwidth for that particular DLCI.
Tc = Bc / CIR
Peak = CIR + Be/Tc = CIR (1 + Be/Bc)
If the Tc is one second then:
Peak = CIR + Be = Bc + Behttp://www.cisco.com/warp/public/125/21.pdf