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Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
OL-23826-09
Chapter 24 Configuring QoS
Understanding QoS
Default QoS for Traffic from External Ethernet Ports
The Cisco ASR 901 router allows complete configuration of QoS via policy-maps for the external
ethernet ports. However, the default case when no policy-map is configured is described below:
By default, the qos-group (internal-priority) applied to every packet from an external port is zero.
In cases where Cisco ASR 901 router configuration causes fields to be generated that were not present
on the incoming packet, (for example, if a VLAN tag or an MPLS label is added by Cisco ASR 901 that
was not present on the incoming packet) the router uses the following default procedures to propagate
the priority from the received frame as described below:
a. In the absence of a policy-map, when adding an 802.1Q VLAN outer tag (service tag) when a service
tag was not previously present, the priority value in outer tag is zero. The priority of the inner tag
(if present) is not modified from its original value.
b. When adding an 802.1Q VLAN inner tag (customer tag), the default priority value for the inner tag
is zero.
c. The default QoS-group, used for internal prioritization, output queuing and shaping, and for
propagating QoS information to MPLS EXP, is zero.
d. For tunneling technologies, such as EoMPLS pseudowires and L3VPN, additional defaults are in
place to propagate QoS. These are described below:
Default QoS for Traffic from Internal Ports
The Cisco ASR 901 router does not allow policy maps to be applied to internal ports, such as the
Ethernet or PCI ports to the CPU, nor the Ethernet ports to the timing CPU or the Winpath.
Cisco ASR 901 router generally treats these internal ports as trusted. The Cisco ASR 901 Series
Aggregation Services Router defaults to propagate the priority from the received frame as described
below:
a. By default, the QoS-group (internal-priority) applied to every packet from an internal port is equal
to the priority received in the 802.1Q VLAN tag received on that packet.
b. If a packet is received on one of these internal interfaces which does not have a VLAN tag attached,
a VLAN tag is added internally, with the priority value copied from the ip-precedence field (in case
of IP packets), and zero (in case on non-ip packets).
c. The default QoS-group, (internal priority) for internal queue assignment and for propagating QoS
information to MPLS EXP, is set equal to the priority of the outer VLAN tag (either the original or
the default value) on the received frame.
d. For tunneling technologies, such as EoMPLS pseudowires and L3VPN, additional defaults are in
place to propagate QOS as follows:
–
For MPLS based L3 VPN and for the EoMPLS (both VPWS and VPLS), upon imposition of the
first (bottom of stack) MPLS label, MPLS EXP values are equal to the value is specified in the
internal qos-group setting (internal priority).
–
When adding additional MPLS label to an existing stack, the default MPLS EXP values are set
to match qos-group value.
This section contains the following topics:
• Modular QoS CLI, page 24-4
• Input and Output Policies, page 24-5
• Classification, page 24-7
• Table Maps, page 24-13
• Policing, page 24-14