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MPLS Fundamentals: Forwarding Labeled Packets

Chapter Description

Forwarding labeled packets is quite different from forwarding IP packets in that not only is the IP lookup replaced with a lookup of the label in the label forwarding information base (LFIB), but different label operations are also possible. This chapter specifically focuses on how labeled packets are forwarded in an MPLS network.

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Unreserved Labels

Except for the reserved labels of 0 through 15, you can use all the label values for normal packet forwarding. Because the label value has 20 bits, the labels from 16 through 1,048,575 (220 – 1) are used for normal packet forwarding. In Cisco IOS, the default range is 16 through 100,000. This is more than enough for labeling all the IGP prefixes you have, but if you want to label the BGP prefixes, this number might be insufficient. You can change the label range with the mpls label range min max command. Example 3-9 shows how to change the default mpls label range.

Example 3-9. Changing the MPLS Label Range

event#show mpls label range
Downstream Generic label region: Min/Max label: 16/100000

event#conf t
Enter configuration commands, one per line. End with CNTL/Z.
event(config)#mpls label range ?
<16-1048575>  Minimum label value
event(config)#mpls label range 16 ?
  <16-1048575>   Maximum label value
event(config)#mpls label range 16 1048575

event#show mpls label range
Downstream Generic label region: Min/Max label: 16/1048575

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