This article describes the meaning of an Unknown SRLG group name and how to fix the issue.
In MPLS Traffic Engineering (TE), Shared Risk Link Groups (SRLGs) identify links sharing a common physical risk (e.g. sharing the same fiber or conduit) to prevent backup paths from failing simultaneously with primary paths, to enhance resilience; in other words, SRLGs are used to ensure that primary and backup paths remain physically diverse.
A standard configuration example is as below to set a SRLG Group Name, Value, and Cost:
set routing-options srlg srlg-a srlg-value 101set routing-options srlg srlg-a srlg-cost 10
user@PE1# show routing-optionsrouting-options { srlg { srlg-a { srlg-value 101; srlg-cost 10; } }}
"srlg-cost" is a configurable numerical value like a risk factor that tells the Constraint-based Shortest Path First (CSPF) algorithm how much to penalize using links in that group, with higher costs making backup paths avoid them more strictly, ensuring a better diversity. This is not the link bandwidth or IGP cost. CSPF Calculation: The CSPF algorithm uses this cost to calculate diverse paths; a higher cost makes CSPF prune (avoid) links in that SRLG more aggressively when finding a backup.
However, why is sometimes "Unknown" is seen in SRLG Group Name and cost as 0 in below output, when doing 'show mpls srlg'?
netuser@ptx5k> show mpls srlg
SRLG Value Cost
Unknown-0x100 256 0
Unknown-0x200 512 0
Unknown-0x300 768 0
Unknown-0x400 1024 0
Unknown-0x1000 4096 0
The srlg srlg-name statement has the following options:
set routing-options srlg srlg-a srlg-value 101srlg-cost — Include a cost for the SRLG ranging from 1 through 65535. The cost of the SRLG determines the level of impact this SRLG has on the CSPF algorithm for path computations. The higher the cost, the less likely it is for a secondary path to share the same SRLG as the primary path. By default, the srlg-cost is 1.
srlg-value — Include a group ID for the SRLG ranging from 1 through 4294967295.
"Unknown SRLG" means the router sees links or resources that are part of a SRLG, but doesn't have specific, identified information about what shared risk they have locally. It's essentially a placeholder for risk information that the Interior Gateway Protocol (IGP) OSPF-TE or ISIS is carrying, but can't fully categorize or map to a known configuration, then preventing CSPF from accurately avoiding shared-risk backups. Hence, impacting reliability.
Unidentified Shared Risk links are grouped with a generic "Unknown" SRLG ID instead.
In other words, "Unknown SRLGs" are SRLGs for which there is no corresponding local configuration. Their existence is learnt via link-state protocols. Since the "SRLG-cost" is not advertised in the link-state protocols, the cost for "Unknown SRLGs" is zero. "Zero cost" means that there is no bias applied during the protection path computation (in scenarios where "exclude-srlg" is not configured). The default cost for "configured/known SRLGs" is 1.
It means unidentified shared risk: Links are grouped with a generic "Unknown" SRLG ID instead of a specific number (srlg-value). Also, it's a potential for overlap: The local router can't guarantee that a primary path and its backup LSP will not use the same underlying physical resource (like the same cable bundle) if the backup path includes an "Unknown" SRLG link, unexpectedly leading to potential simultaneous failures.
To fix it:1. Check IGP/BGP-LS configuration: Ensure SRLG TLVs are being correctly advertised and received in OSPF/IS-IS/BGP-LS.2. Manually assign/refine: Use SRLG management tools to add or modify SRLGs for specific links.