25-50
ATM Switch Router Software Configuration Guide
OL-7396-01
Chapter 25
Configuring ATM Router Module Interfaces
Configuring VC Bundling with IP and ATM QoS
Also, notice that the weights shown for the ATM connections in
Figure 25-12
are one sixteenth of the
weights shown in
Table 25-1
. For example, in
Figure 25-12
, the CBR output VC weight is shown as 15,
but in
Table 25-1
the scheduler-class weight is shown as 240. This is because the weights maintained in
the Cisco IOS are in the range 1-15, whereas the weights to be installed in the fabric are in the range
16-240. This means the weights are multiplied by 16 before being installed in the switch fabric.
Congestion Control
Congestion Control is the second part of per hop behavior. It is configured using output policy. The
output policy operates only if the enhanced ATM router module is congested. Without congestion, all of
the traffic entering the enhanced ATM router module is switched without drops. If congestion occurs,
dropping can occur in two places. In the first case, when the enhanced ATM router module is congested
from other Layer 3 interfaces, traffic going to the scheduler class with the lowest weight is dropped first.
The traffic being dropped depends on the IP QoS output policy configured and if the class has higher
bandwidth than the the other traffic. These classes experience fewer drops than other classes.
In the second case, when the ATM output is congested with excess traffic from the enhanced ATM router
module, traffic is dropped based on the characteristics of the ATM PVCs and not on the IP QoS
configuration.
If no drop policy is configured in the output policy for each class, the default is tail drop. Tail drop simply
means that if there is congestion, the last packet received is the first packet dropped. This continues until
congestion is alleviated.
The other option is to configure the XPIF based Random Early Detect (xRed). The xRED algorithm
drops packets intelligently based on some probability. This helps bursty applications like TCP achieve
optimum performance. xRED can be configured for each class-map in the output policy so each queue
has xRED running individually.
Troubleshooting and Verifying the VC Bundling with IP and ATM QoS
To troubleshoot and verify the bundled VCs with IP and ATM QoS, use the following privileged EXEC
commands:
Command
Purpose
show epc ipqos database interface
interface-type
card
/
subcard
/
port
input
Displays the IP QoS manager database
configuration.
show epc ipqos output
interface-type
card
/
subcard
/
port
Displays the output QoS configuration.
show epc ip-atm-qos interface atm
card
/
subcard
/
port
Displays bandwidth and weights of the
scheduler classes.
show epc vc-bundle {
bundle-name
|
interface atm
card
/
subcard
/
port
}
Displays the bundle-ID to bundle-name
mapping and precedence to VC mapping for a
VC bundle.
show running-config
Displays the configuration information
currently running.
