1. Setting the MPLS EXP bits
Spirent TestCenter Testing MPLS QoS

2. MPLS Traffic endpoints
There are mainly 3 types of MPLS traffic endpoints in a test
IP Router running MPLS or IP Host /Network located behind an MPLS Router
Layer-3 CE Device/Network in an L3VPN
Layer-2 CE Device/Host in an L2VPN
The MPLS traffic endpoint can be an Emulated device (Host or Router) or a Simulated device (Route/Network Block or an LSP Tunnel Block)
For simulated device endpoints, by default MPLS EXP = 0b000 and MPLS TTL =64
The MPLS traffic endpoints can be L3 endpoints (e.g. IP addresses) or L2 endpoints (e.g. Ethernet MAC addresses)
For emulated device endpoints, MPLS EXP is set to the device interface QoS

3. MPLS Traffic endpoint is a Layer3 device - Setting device interface QoS
Set the IPv4 ToS or IPv6 Traffic Class on the device interface by clicking “Edit Interface” in the Device Grid (next page)
Control Plane protocols (like BGP, OSPF) use the IPv4 ToS or IPv6 Traffic Class specified on the interface to transmit control plane packets (OSPF Hellos, BGP Updates).
Modifying the interface QoS settings should not be taken lightly.
Lowering the QoS values on routers running routing protocols can cause critical control plane packets to be dropped.
MPLS header if present in the interface stack cannot be edited.
MPLS EXP bits and TTL are automatically set by the control plane protocols
Control plane protocols running over MPLS tunnels (BGP or LDP Target) will copy the interface IP ToS and IP TTL to all the MPLS headers.

4. MPLS Traffic endpoint is a Layer3 device

5. MPLS Traffic Endpoint is a Layer3 device - Importing QoS into the Traffic Wizard
The Traffic Wizard will import the IPv4 ToS or IPv6 Traffic Class and the IP TTL specified on the device interface into the Frame tab when the stream-block is created (next page)
Traffic Wizard automatically sets MPLS EXP bits to use the IPv4 ToS or IPv6 Traffic Class in the following IP header
For example: If an IPv4 header immediately follows the MPLS headers then IP ToS will be copied to EXP in all MPLS headers
MPLS TTL is set to 64 by default. It is not copied from the following IP header in the Frame tab unlike the EXP bits.
If there are multiple source endpoint devices in the same stream-block the IP QoS on the interface of the first endpoint will be imported into the Frame tab of the Traffic Wizard.
If the QoS on the device interface is updated the stream-block will not be automatically updated. Stream-block will need to be re-created.

6. MPLS Traffic endpoint is a Layer3 device (page 3)

7. MPLS Traffic endpoint is a Layer2 device - Setting device interface QoS
L2 device is usually a CE device in a L2VPN test and is stacked behind an emulated PE device.
If a VLAN interface is present then set the VLAN Priority on the device interface by clicking “Edit Interface” in the Device Grid (next page)
If a control plane protocol (like STP, OSPF) is Active on the device(IP header may be added), then it would use the VLAN Priority specified on the interface to transmit control plane packets.
Currently, for control plane protocols, the MPLS EXP is being set to 0b000 and MPLS TTL is being set to 255 by default.
VLAN Priority and IP4 ToS or IPv6 Traffic Class are not being used to set the MPLS EXP bits for control plane protocols running on L2VPN CE devices.

8. MPLS Traffic endpoint is a Layer2 device

9. MPLS Traffic Endpoint is a Layer2 device - Importing QoS into the Traffic Wizard
The Traffic Wizard will import the VLAN Priority specified on the device interface into the Frame tab when the stream-block is created (next page)
Traffic Wizard automatically sets MPLS EXP bits to use the VLAN Priority in the following Ethernet+VLAN header when present
For example: If an Ethernet+VLAN header immediately follows the MPLS headers then VLAN Priority bits will be copied to EXP bits in all MPLS headers
If the VLAN header is not present on the L2 device endpoint, MPLS EXP is set to a default value of 0b000
MPLS TTL is set to 64 by default.

10. Setting MPLS QoS in the Traffic Wizard - L3 endpoints with IP QoS
A Layer3 MPLS endpoint (Router/Host or Route/LSP Block) will have an IP header following the MPLS headers
The IP QoS (ToS, DSCP or Traffic Class) of the IP header can be edited in the Frame tab
MPLS EXP will be automatically mapped to the 3 higher order IP ToS bits of the IP QoS in the following IP header
If a modifier is added to the IP QoS (ToS, DSCP or Traffic Class), then the modifier will automatically be copied to the EXP bits in all MPLS headers, provided that the Step only modifies the 3 higher order IP ToS bits. If the Step modifies other bits as well then the modifier is not copied to MPLS EXP and MPLS EXP bits are set to the 3 higher order ToS bits of the initial IP QoS.
Add an IP DiffServ/DSCP modifier: Start=AF23 (0x58), Step=40, Count=2,
1st packet: IP DSCP=0x58, MPLS EXP=0b010
2nd packet: IP DSCP=0x78, MPLS EXP=0b100
Only 6 Modifiers are allowed per stream

11. Setting MPLS QOS in the Traffic Wizard

12. Setting MPLS QoS in the Traffic Wizard - Setting the MPLS EXP bits
To set the MPLS EXP bits directly, select “Show All Headers” at the “Endpoints” selection tab, and go the “Frame” tab.

13. Setting MPLS QoS in the Traffic Wizard - Setting the MPLS EXP bits
MPLS header is now viewable and fields are editable
MPLS EXP bits for each header can be modified to override the defaults.
MPLS EXP bits can be set to values different from the IP QoS or Vlan Priority
A modifier can be added to MPLS EXP bits in one or more MPLS headers as needed
MPLS TTL and Bottom of stack bit are also editable

14. Setting MPLS QoS in the Traffic Wizard - Setting the MPLS EXP bits