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Extra rules used when combining QoS and hardware filters

In fact, QoS can cause the limit on the number of hardware filters to be reduced rather more

radically than might be initially evident. To see why this is, we have to understand a bit more

about how the rule table is used. When a packet is to be compared against rules in the rule

table, the comparison does not have to start at the top of the table—it can start at other

points in the table. The decision as to the starting point for any particular packet is made on

the basis of the packet's ingress port. When no QoS policies have been configured on the

switch, and only hardware filters have been configured, it is convenient and simple to have the

rule comparison process for all packets start at the top of the rule table and run to the last

non-null entry in the table, regardless of the packet's ingress port. This is because hardware

filters on the AT-9900 and x900 series switches are not ingress-port specific.

Therefore, when only hardware filters have been configured on the switch, all rule

comparisons start at the first rule in the rule table, irrespective of the packet's ingress port.

Port

...

Table that maps ingress port

to the starting point of the

rule comparison process

However, QoS policies are ingress-port specific. Different policies can be configured on

different ports. So, the rules for allocating packet to flow groups can differ from port to port.

Hence, QoS can result in the rule table containing different sets of rules for different ports.

This means that for the purposes of QoS, the decision that dictates the starting point of the

rule comparison process, depending on ingress port, must result in different start values for

different ingress ports. But, the problem is that the hardware filtering must use the exact

same decision process. So we end up with a conflict of interests—the hardware filter process

wants to run every packet through the same set of rules, but QoS wants to use different sets

of rules for different packets, depending on the packet's ingress port. But it is not possible to

make a single ingress-port-to-rule-table-starting-point decision process fulfil these two

desires both at the same time.

The solution to this problem is as follows. As soon as a QoS policy is configured, which

requires the creation of a set of rules specific just to the ports in that policy, then a full copy

of the hardware filter rules is also added to this set of QoS rules. So, for packets entering the

switch via one of the ports in the QoS policy, the hardware filter rule lookup process is

actually carried out on this new copy of the hardware filter rules.

Page 8 | AlliedWare™ OS How To Note: Hardware Filters

Start

...

How many filters can you create?

Rule 1

Rule 2

Rule 3

Rule 4

Empty

Rule table