Behaviors and Limitations

• The default SRGB in IOS XR has a size of 8000 starting from label value 16000. The default range is 16000 to 23,999. With this size, and assuming one loopback prefix per router, an operator can assign prefix SIDs to a network with 8000 routers.

• There are instances when you might need to define a different SRGB range. For example:

  • Non-IOS XR nodes with a SRGB range that is different than the default IOS XR SRGB range.

  • The default SRGB range is not large enough to accommodate all required prefix SIDs.

• A non-default SRGB can be configured following these guidelines:

  • The SRGB starting value can be configured anywhere in the dynamic label range space (16,000 to 1,048,575).

  • In Cisco IOS XR release earlier than 6.6.3, the SRGB can have a maximum configurable size of 262,143.

  • In Cisco IOS XR release 6.6.3 and later, the SRGB can be configured to any size value that fits within the dynamic label range space.

• Allocating an SRGB label range does not mean that all the labels in this range are programmed in the forwarding table. The label range is just reserved for SR and not available for other purposes. Furthermore, a platform may limit the number of local labels that can be programmed.

• We recommend that the non-default SRGB be configured under the segment-routing global configuration mode. By default, all IGP instances and BGP use this SRGB.

• You can also configure a non-default SRGB under the IGP, but it is not recommended.

SRGB Label Conflicts

When you define a non-default SRGB range, there might be a label conflict (for example, if labels are already allocated, statically or dynamically, in the new SRGB range). The following system log message indicates a label conflict:

    %ROUTING-ISIS-4-SRGB_ALLOC_FAIL : SRGB allocation failed: 'SRGB reservation not 
    successful for [16000,80000], SRGB (16000 80000, SRGB_ALLOC_CONFIG_PENDING, 0x2) 
    (So far 16 attempts). Make sure label range is free'

To remove this conflict, you must reload the router to release the currently allocated labels and to allocate the new SRGB.

After the system reloads, LSD does not accept any dynamic label allocation before IS-IS/OSPF/BGP have registered with LSD. Upon IS-IS/OSPF/BGP registration, LSD allocates the requested SRGB (either the default range or the customized range).

After IS-IS/OSPF/BGP have registered and their SRGB is allocated, LSD starts serving dynamic label requests from other clients.

Note	To avoid a potential router reload due to label conflicts, and assuming that the default SRGB size is large enough, we recommend that you use the default IOS XR SRGB range.

Note	Allocating a non-default SRGB in the upper part of the MPLS label space increases the chance that the labels are available and a reload can be avoided.

Caution

Modifying a SRGB configuration is disruptive for traffic and may require a reboot if the new SRGB is not available entirely.

Behaviors and Limitations

• The default SRLB has a size of 1000 starting from label value 15000; therefore, the default SRLB range goes from 15000 to 15,999.

• A non-default SRLB can be configured following these guidelines:

  • The SRLB starting value can be configured anywhere in the dynamic label range space (16,000 to 1,048,575).

  • In Cisco IOS XR release earlier than 6.6.3, the SRLB can have a maximum configurable size of 262,143.

  • In Cisco IOS XR release 6.6.3 and later, the SRLB can be configured to any size value that fits within the dynamic label range space.

SRLB Label Conflicts

When you define a non-default SRLB range, there might be a label conflict (for example, if labels are already allocated, statically or dynamically, in the new SRLB range). In this case, the new SRLB range will be accepted, but not applied (pending state). The previous SRLB range (active) will continue to be in use.

To remove this conflict, you must reload the router to release the currently allocated labels and to allocate the new SRLB.

Caution

You can use the clear segment-routing local-block discrepancy all command to clear label conflicts. However, using this command is disruptive for traffic since it forces all other MPLS applications with conflicting labels to allocate new labels.

Note	To avoid a potential router reload due to label conflicts, and assuming that the default SRGB size is large enough, we recommend that you use the default IOS XR SRLB range.

Note	Allocating a non-default SRLB in the upper part of the MPLS label space increases the chance that the labels are available and a reload can be avoided.