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Combining hardware filters and QoS
The switch compares the packet with every hardware filter before it compares the packet
with any QoS flow group. If the packet matches a hardware filter, the switch takes the action
specified by that hardware filter and stops the comparison process. If a packet matches both
a hardware filter and a QoS flow group, the packet only gets matched against the hardware
filter. It bypasses the QoS process.
If the hardware filter actions include discard, then this is not a problem, because the packet
was never going to get into the QoS system anyway (given that it was being discarded). But, if
the hardware filter actions include forward, and the packet would also be matched by a QoS
flow group, then this is a problem. The packet will not be matched by the QoS flow group, so
the switch will not apply any intended QoS-based filtering, metering, queue redirection, etc
to the packet. Instead the switch will forward the packet as if it belongs to the default traffic
class for the port's QoS policy.
For this reason, we only recommend combining hardware filters and QoS if all your hardware
filters result in traffic being dropped. For traffic that you want forwarded with QoS control,
use QoS for both the filtering and the QoS functionality. Of course, you can also use QoS
flow groups to drop traffic.
How many filters can you create?
The total number of filters that can be created is not an exact number, but depends on which
fields the various filters are matching on. So, to understand how to work out whether the set
of filters you are creating might run out of space, it is necessary to understand the way in
which the filters operate in the switch hardware.
There are two items within the switch hardware which set limits on the number of filters that
can be created: the filter rules table and the profile (mask).
Hardware filters and QoS share the same filter rules table and mask.
1. The filter rules table
One item that sets a limit on the number of filters is the table that contains the list of filter
rules. This has a strict limit of 1024 entries. Entries get made when:
You create a hardware filter.
You use QoS to apply a classifier to a port.
Page 7 | AlliedWare™ OS How To Note: Hardware Filters
Combining hardware filters and QoS