Introduction

Shared Risk Link Groups are considered when establishing backup tunnels in MPLS FRR. It is a mechanism which provides information on the diversity of circuits between the primary tunnel and the backup tunnel. This provides a better way of selecting the backup tunnel's path, avoiding common links, which could lead to primary and backup failing.

MPLS TE SRLG

• SRLG feature comes into play when backup tunnels for link or node protection are created automatically. It provides for a way to disqualify links during the CSPF calculations of the backup tunnel by specifying which links share the same common risk. A common risk can be same physical circuit, same terminating POP..etc.
• SRLG is also referred to as Fate-sharing group.
• SRLG information is distributed using the IGP protocol (IS-IS and OSPF).
• This feature only works for backup tunnels that are automatically created using the Autotunnel Backup feature. See MPLS-TE-Auto-Tunnel-Backup for more information.
• SRLG is configured statically per each interface using mpls traffic-eng srlg (group-id). An interface can be part of multiple groups.
• Example could be circuits that terminated in the same POP or enter the facility via the same entrance..all of these could be part of the same group and can be configured to create autotunnel using different POP or facility entrance.
• There are two options with backup tunnel calculation:
  • Force all backup paths to avoid common SRLG (mpls traffic-eng auto-tunnel backup srlg exclude force).
  • Try to avoid common SRLG, but if that's not possible still use them (mplst traffic-eng auto-tunnel backup srlg exclude preferred).

Relevant Commands

mpls traffic-eng auto-tunnel backup srlg exclude force

R1(config)#        mpls traffic-eng auto-tunnel backup srlg exclude force

mpls traffic-eng auto-tunnel backup srlg exclude preferred

R1(config)#        mpls traffic-eng auto-tunnel backup srlg exclude preferred

mpls traffic-eng srlg (group-id)

R1(config-if)# mpls traffic-eng srlg (group-id)

R5#sh mpls  traffic-eng topology srlg 
(ospf 1  area 0)
  SRLG:      Intf Addr:      MPLS TE Id:
  45         20.4.5.5        5.5.5.5        

  54         20.4.5.5        5.5.5.5        

  56         20.5.6.5        5.5.5.5

Additional Resources

MPLS TE Shared Risk Link Group - Cisco

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