Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) - Manual (Page 551)

Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide 527
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How MPLS VLL works
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• On Brocade NetIron CES and Brocade NetIron CER devices, only one of the trunk ports is used
for a given VLL instance, depending on the VC label used by the instance.
QoS for VLL traffic
By default, packets travelling through an MPLS domain are treated equally from a QoS standpoint,
in a best effort manner. However, when a Layer 2 packet has an internal priority in its 802.1q tag,
or the LSP or VLL to which the packet is assigned has a configured Class of Service (COS) value,
QoS can be applied to the packet in the MPLS domain. The internal priority or COS value is mapped
to a value in the EXP field of the packet’s MPLS header. The value in the EXP field is then mapped
to an internal forwarding priority, and the packet is sent to the hardware forwarding queue that
corresponds to the internal forwarding priority.
QoS for VLL traffic at the ingress LER
The following methods can be used to provide QoS to packets entering a VLL:
• Use the COS value assigned to the tunnel LSP used to reach the VLL peer-
When a tunnel LSP has a user-configured COS value, all packets in all VLLs travelling through
the tunnel LSP receive the same QoS.
• Use the COS value assigned to the VLL-
When a COS value is set for the VLL, the device selects a tunnel LSP that also has this COS
value, when one is available. When no tunnel LSP with this COS value is available, the device
selects a tunnel LSP with the highest configured COS value (although never higher than the
COS setting for the VLL).
When the selected tunnel LSP does not have a COS value, the VLL’s configured COS value is
used to provide QoS. The VLL’s COS value is mapped to a value in the EXP field. This allows
traffic multiple VLLs using a single tunnel LSP, traffic from each VLL can receive different QoS
treatment.
• Use the priority in the packet’s 802.1q tag-
When neither the tunnel LSP nor the VLL has a configured COS value, the device examines the
priority in the Layer 2 packet's 802.1q tag, when the packet has one. Consequently, Layer 2
packets with the same 802.1q priority receive the same QoS in the VLL.
• Use the configured priority of the port-
When neither the tunnel LSP nor the VLL has a configured COS value, and the Layer 2 packet
does not have an 802.1q priority, QoS can be provided based on the priority of the incoming
port. A port can be assigned a priority from 0 (lowest priority) to 7 (highest priority). The default
port priority is 0.
By assigning different priorities to the ports where customer edge (CE) devices are connected
(that is, the VLL endpoints), the user can provide QoS to untagged Layer 2 traffic received from
different customer locations.
When a packet enters a VLL, the PE router that serves as both the VLL endpoint and the ingress of
a tunnel LSP pushes two labels onto the packet the inner VC label and the outer tunnel label. The
packet’s priority resides in the EXP field of the MPLS label header. The VC label and the tunnel label
carry the same value in the EXP field.
The following table lists how a Layer 2 packet’s priority is mapped to a value in the EXP field and
how the EXP value is mapped to a priority queue.