Cisco Cisco IOS Software Release 12.0(13)S7

See the 

 for 

information on how to configure an allocation algorithm.

MPLS Safe Label Reuse After Router Restart supports the following features for Cisco IOS 
Release 12.0(32)SY and later releases:

All MPLS platforms 

Single and dual RP configurations

Redundancy modes: route processor redundancy (RPR), RPR+, MPLS SSO coexistence, and SSO

Any-version-to-any-version restarts, including non-In-Service Software Upgrade (ISSU)- 
capable-to-ISSU-capable restarts and vice versa

MPLS SSO and ISSU are not supported in Cisco IOS Release 12.0(23)SY.

Dense

The MPLS label allocator allocates the smallest free label even if the label 
allocator must wait for pblocks to transition from the timing-out state to the 
free state.

This algorithm results in a densely allocated label space at the cost of 
consuming all pblocks. This increases the likelihood of label allocation 
delays following restart.

Fractional

This algorithm confines label allocation to the lower half of the configured 
label space until the smallest available free label is outside the lower half of 
the label space. When this occurs the label allocator requests more pblocks 
and resumes confining label allocation to the lower half of the label space 
until the smallest available free label is outside the lower half, at which point 
it requests more pblocks. The label allocator continues in this fashion until 
all of the pblocks are in use at which time it always allocates the smallest 
free label.

This algorithm is a compromise between the Exhaust Available and Dense 
algorithms. The Fractional algorithm:

Provides a label space less densely populated than the Dense algorithm 
does, but more densely populated than the Exhaust Available algorithm 
does. 

Tends to use fewer pblocks than the Dense algorithm does, but more 
pblocks than the Exhaust Available algorithm does. 

Tends to leave more pblocks available for immediate use following 
restart than the Dense algorithm does and fewer pblocks available for 
immediate use following restart than the Exhaust Available algorithm 
does.