Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring HQoS (Rev.03)
QoS/HQoS Implementation
© 2008 Foundry Networks, Inc.
Page 13 of 98
Figure 8: Internal Architecture and Main Functional Blocks
The main functions of the Packet Processor are:
•
Classification
is responsible to determine the flow, to which each packet belongs, so that other
functions (filtering, policing, mapping, queuing, etc.) can be performed based on dedicated
configuration per flow. Classification is implemented using enhanced Access Control Lists
(ACLs), which may include rules based on various Ethernet/VLAN, IP/UDP and other
protocol fields. Overall, ACLs form a very robust and flexible classification tool.
•
Filtering
prevents unwanted packet flows (e.g. packets from untrusted sources) from entering
the device.
•
Mapping to FC and Color
maps the incoming packet to one of 8 defined Forwarding Classes,
thus deciding the egress queue into which the packet will be en-queued and the Color (Green
or Yellow), which is used for the packet dropping mechanisms Tail-drop/WRED) to prioritize
dropping of packets. The mapping can be done in two ways:
o
Trusted
– when the service provider trusts the subscriber to classify the incoming packets
and appropriately mark either 802.1p (VPT) or DSCP field. In this case the mapping is
done according to 802.1p/DSCP only.
o
Un-trusted
– when the service provider doesn’t trust the subscriber’s equipment to
perform packet CoS classification. In this case the mapping is done according to packet
flow as classified by the ACLs.
•
Single-Rate Policing
implements an SRTCM (Single-Rate Three-Color Marker) and allows
rate-limiting of the incoming traffic flows. The rate and bursts of the incoming flows are
measured in order to detect packets that don’t conform to the committed rate/burst for that
flow, as defined in the SLA. The non-conforming packets can be marked as Yellow, so they’re
more likely to be dropped if congestion occurs. Please note that by enabling policing on traffic
arriving from the ES Processor, the device is able to perform egress policing.
•
Marking/remarking
is supported as follows:
o
Marking of VPT field according to the internal FC and Color. Egress VPT is not affected
by the results of policing.
o
Remarking of DSCP field according to the internal FC and Color. Egress DSCP is affected
by the results of policing (if Color was changed), allowing ‘Egress Policing’ of the
outgoing packets.
•
Switching/Bridging, Replication and VC De/encapsulation
are the main functions of the
Packet Processor. It performs switching of packets arriving from the User side and sending