Setting the MPLS EXP bits
Spirent TestCenter Testing MPLS QoS
PROPRIETARY AND CONFIDENTIAL
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
PROPRIETARY AND CONFIDENTIAL
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.
PROPRIETARY AND CONFIDENTIAL
MPLS Traffic endpoint is a Layer3 device
‹#›
PROPRIETARY AND CONFIDENTIAL
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.
PROPRIETARY AND CONFIDENTIAL
MPLS Traffic endpoint is a Layer3 device (page 3)
‹#›
PROPRIETARY AND CONFIDENTIAL
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.
PROPRIETARY AND CONFIDENTIAL
MPLS Traffic endpoint is a Layer2 device
‹#›
PROPRIETARY AND CONFIDENTIAL
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.
‹#›
PROPRIETARY AND CONFIDENTIAL
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
‹#›
PROPRIETARY AND CONFIDENTIAL
Setting MPLS QOS in the Traffic Wizard
‹#›
PROPRIETARY AND CONFIDENTIAL
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.
PROPRIETARY AND CONFIDENTIAL
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
PROPRIETARY AND CONFIDENTIAL
Setting MPLS QoS in the Traffic Wizard
- Setting the MPLS EXP bits
‹#›
PROPRIETARY AND CONFIDENTIAL