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Figure 59 on page 256 illustrates how the initial value of the EXP bits is set for the
first label pushed. Figure 60 on page 257 illustrates how the EXP bits can be changed
for all labels, including the first label, by attached policies or per-VR EXP rules. The
following section describes in detail how the EXP bits value is set for outgoing traffic.
Setting the EXP Bits for Outgoing Traffic
Different types of packets distributed into LSPs by the router have different default
settings for the EXP bits. For IP packets, the EXP bits value is set to match the IP
precedence value from the TOS field of the packet header. For non-IP packets, such
as Martini or VPLS packets, the EXP bits value is set to 000. You can use the
mpls
copy-upc-to-exp command to free the EXP bits value in IP packets from being tied
to the IP precedence value. Instead, this command sets the EXP bits value to match
the user packet class (UPC) value.
The IP precedence value can be copied back into the IP precedence field of the IP
packet header at the LSP endpoint on the ingress line module. This action takes place
only if the IP header is exposed after popping the MPLS labels and if the uniform
tunnel model is employed. The remaining bits of the TOS field are not touched.
In contrast, when you issue mpls copy-upc-to-exp command, the EXP bits value is
not copied to the UPC field at the LSP endpoint, because the UPC value might have
been set by a lower layer policy for a different purpose.
NOTE: For control traffic originated from this router, if an attached per-LSP policy
has rules to modify the EXP bits, or if per-VR EXP rules are configured, the EXP bits
value copied from the IP precedence value might be overwritten incorrectly because
the default traffic class/color combination for control traffic is best-effort/green. You
can avoid this situation by establishing an outgoing IP policy that sets the traffic
class/color combination for control traffic so that the policy or rules have the correct
traffic class/color to work with.
If per-LSP policies are used or per-VR rules are configured, by default all labels pushed
by the router for the same packet have the same EXP bits value. That value is
determined by the policies or rules.
You can use the mpls preserve-vpn-exp command to specify that the EXP bits value
for the VPN or Martini or VPLS label pushed by the router cannot be modified by
either policy for outer labels or by per-VR rules. This capability is useful if you want
the inner labels to have a different value for the EXP bits than do the outer labels.
For example, in a VPN you might want the inner label’s EXP bits value to be the
copied IP precedence value. You might want the base label’s EXP bits value set
according to the mapping of EXP bits to traffic class/color combination that is defined
in your network.
EXP Bits for Differentiated Services Overview
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Chapter 2: MPLS Overview