Riverstone Networks WICT1-12 - Manual (Page 624)

30-20 Riverstone Networks RS Switch Router User Guide Release 8.0
WAN Rate Shaping WAN Configuration
2. The number of bits sent through the WAN port exceeds Bc. If Be is not defined or if there is no extra
bandwidth available on the WAN interface, packets begin to be queued.
3. The number of bits sent through the WAN port exceeds Bc. This occurs only if Be is defined and
there is extra bandwidth available.
4. The number of bits sent through the WAN port has exceeds Bc + Be, and packets are queued.
5. The number of bits sent through the WAN port drops below Bc + Be; as long as excess bandwidth
is available, the queue begins to empty.
6. The number of bits sent through the WAN port drops below Bc, the buffer empties and packets are
once again passed directly through the WAN port.
7. One second has elapsed (the sum of the sampling intervals), and the rate shaping algorithm has
controlled the Ethernet flow so that the number of bits sent through the WAN port is approximately
equal to CIR.
Notice that if Be is defined and excess bandwidth is available, the amount of WAN port bandwidth utilized by an
Ethernet flow can exceed its specified CIR. In the best possible case, where there is sustained excess bandwidth, a
flow’s bandwidth can attain a maximum equal to ((Be + Bc) / Bc)
*
CIR. For example, if CIR = 128 Kbps, Bc = 4000,
and Be = 2000, the calculated maximum bandwidth that the Ethernet flow could potentially reach is:
Max Bandwidth = ((2000 + 4000) / 4000)
*
128000 = 192000 or 192 Kbps
30.13.3 WAN Rate Shaping Example
In this example, computers on three different floors are connected to R1 through Ethernet switches. R1 connects to the
WAN through a Clear Channel T3 line. Rate shaping is applied, and limits each switch to 150 Kbps of bandwidth on
the Clear Channel T3 line. Flows from each switch are identified by the physical port to which they connect on R1;
these ports are
et.2.1
,
et.2.7
, and
et.3.8
. At the other end of the WAN connection, R2 passes Ethernet flows onto
the Metro backbone. Conversely, R2 rate shapes Ethernet flows from the Metro backbone that originate on subnet
124.141.77.0/24 and sends them through its own Clear Channel T3 line to R1 (see
Figure 30-2 ).