MPLS QoS EXP bits, Why two EXP Bits at the egress interface at the PE?

[Meddane]

From the figure below, when a PE router receives a packet with DSCP AF31, it copies the first three bits from the AF31 to the EXP field 3, why the figure from the WAN QoS design guide is showing two EXP fiels instead of one at the egress interface at the PE?

[Harold Ritter]

Hi @Meddane ,

It is a MPLS VPN scenario, therefore there is two labels. The outer label is the IGP label and the inner label is the service label. Each label includes an EXP field, hence two EXP field in this scenario.

Regards, 

[Meddane]

Hi @Harold Ritter 

So which field does the Service Provider use for its own QoS, IGP Label or Inner Label?

[Giuseppe Larosa]

Hello @Meddane ,

the more external label is checked for QoS in the SP network,  that is the IGP label ( the LSP to the remote PE loopback address = MP BGP next hop for recursion)

The inner label is in this case representing a VRF prefix locally on the remote PE and it  allows to find out the correct exit interface.

To be noted the original IP packet is not modified unless there is a marking down action like showed in the figure.

Hope to help

Giuseppe

[pigallo]

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@Meddane wrote:

Hi @Harold Ritter 

So which field does the Service Provider use for its own QoS, IGP Label or Inner Label?

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Both fields are used during QoS operations.

If inner label wouldn't be marked, imagine at PHP what would happen when topmost label is popped out and inner label is exposed without marking. QoS treatment simply cannot be assured anymore on the FEC.

Kind regards,