Default Marking Behavior; Congestion Management - Cisco ASR 9000 Series Configuration Manual

Aggregation services router modular quality of service
Hide thumbs Also See for ASR 9000 Series:
Table of Contents

Advertisement

Modular QoS Overview
• Unconditionally—As part of the class-action.
• Conditionally—As part of a policer-action.
• Combination of conditionally and unconditionally.
For detailed conceptual and configuration information about packet marking, see the "Configuring Modular
Quality of Service Packet Classification on Cisco ASR 9000 Series Routers" module in this guide for
unconditional marking, and the "Configuring Modular Quality of Service Congestion Management on
Cisco ASR 9000 Series Routers" module in this guide for conditional marking.

Default Marking Behavior

When an ingress or egress interface adds VLAN tags or MPLS labels, it requires a default value for the CoS
and EXP values that go into those tags and labels. The default value can be then overridden based on the
policy map. The default value for CoS and EXP is based on a trusted field in the packet upon ingress to the
system. The router implements an implicit trust of certain fields based on the packet type and ingress interface
forwarding type (Layer 2 or Layer 3).
By default, the router does not modify the IP precedence or DSCP without a policy-map being configured.
The default behavior is described below.
On an ingress or egress Layer 2 interface, such as xconnect or bridge-domain, the outermost CoS value is
used for any field that gets added in the ingress interface. If there is a VLAN tag that gets added due to a
Layer 2 rewrite, the incoming outermost CoS value is used for the new VLAN tag. If an MPLS label is added,
the CoS value would be used for the EXP bits in the MPLS tag.
On an ingress or egress Layer 3 interface (routed or label weighted for IPv4 or IPv6 packets), the three DSCP
and precedence bits are identified in the incoming packet. For MPLS packets, the outermost label's EXP bit
is identified, and this value is used for any new field that gets added at the ingress interface. If an MPLS label
is added, then the identified precedence, DSCP, or MPLS EXP value is used for the EXP bits in the newly
added MPLS tag.
Provider Backbone Bridge (PBB) Configuration
In a PBB configuration, when a packet goes from a customer network to a service provider network using
PBB encapsulation, the class of service (CoS) and discard eligibility indicator (DEI) used in the backbone
VLAN tag (B-tag) and service instance tag (I-tag) of the PBB header is by default the CoS and DEI in the
topmost tag of the incoming packet.
When a packet goes from a service provider to a customer network, the PBB header is removed and the I-tag
CoS and DEI is used by default on any tags that are imposed on the customer interface. The default marking
occurs only on imposed tags, and not on existing or translated tags.

Congestion Management

Congestion management techniques control congestion after it has occurred. One way that network elements
handle an overflow of arriving traffic is to use a queuing algorithm to sort the traffic, then determine some
servicing method of prioritizing it onto an output link.
Cisco IOS XR software implements the low-latency Queuing (LLQ) feature, which brings strict priority
queuing (PQ) to the Modified Deficit Round Robin (MDRR) scheduling mechanism. LLQ with strict PQ
allows delay-sensitive data such as voice, to be dequeued and sent before packets in other queues are dequeued.
Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Configuration Guide, Release 6.1.x
QoS Techniques
5

Advertisement

Table of Contents
loading

Table of Contents