MPLS Commands
Page 154 7210 SAS M, T, X, R6, Mxp MPLS Configuration
Guide
srlg
Syntax [no] srlg
Context config>router>mpls>lsp>secondary
Description This command enables the use of the SRLG constraint in the computation of a secondary path for an
LSP at the head-end LER.
When this feature is enabled, CSPF includes the SRLG constraint in the computation of the
secondary LSP path. This requires that the primary LSP already be established and is up since the
head-end LER needs the most current ERO computed by CSPF for the primary path. CSPF would
return the list of SRLG groups along with the ERO during primary path CSPF computation. At a
subsequent establishment of a secondary path with the SRLG constraint, the MPLS/RSVP task will
query again CSPF providing the list of SLRG group numbers to be avoided. CSPF prunes all links
with interfaces which belong to the same SRLGs as the interfaces included in the ERO of the primary
path. If CSPF finds a path, the secondary is setup. If not, MPLS/RSVP will keep retrying the requests
to CSPF.
If CSPF is not enabled on the LSP name, then a secondary path of that LSP which has the SRLG
constraint included will be shut down and a specific failure code will indicate the exact reason for the
failure in show>router>mpls>lsp>path>detail output.
At initial primary LSP path establishment, if primary does not come up or primary is not configured,
SRLG secondary will not be signaled and will put to down state. A specific failure code will indicate
the exact reason for the failure in show>router>mpls>lsp>path>detail output. However, if a non-
SRLG secondary path was configured, such as a secondary path with the SRLG option disabled,
MPLS/RSVP task will signal it and the LSP use it.
As soon as the primary path is configured and successfully established, MPLS/RSVP moves the LSP
to the primary and signals all SRLG secondary paths.
Any time the primary path is re-optimized, has undergone MBB, or has come back up after being
down, MPLS/RSVP task checks with CSPF if the SRLG secondary should be re-signaled. If MPLS/
RSVP finds that current secondary path is no longer SRLG disjoint, for example, it became
ineligible, it puts it on a delayed MBB immediately after the expiry of the retry timer. If MBB fails at
the first try, the secondary path is torn down and the path is put on retry.
At the next opportunity the primary goes down, the LSP will use the path of an eligible SRLG
secondary if it is UP. If all secondary eligible SLRG paths are Down, MPLS/RSVP will use a non
SRLG secondary if configured and UP. If while the LSP is using a non SRLG secondary, an eligible
SRLG secondary came back up, MPLS/RSVP will not switch the path of the LSP to it. As soon as
primary is re-signaled and comes up with a new SLRG list, MPLS/RSVP will re-signal the secondary
using the new SRLG list.
A secondary path which becomes ineligible as a result of an update to the SRLG membership list of
the primary path will have the ineligibility status removed on any of the following events:
1. A successful MBB of the standby SRLG path which makes it eligible again.
2. The standby path goes down. MPLS/RSVP puts the standby on retry at the expiry of the retry
timer. If successful, it becomes eligible. If not successful after the retry-timer expired or the
number of retries reached the number configured under the retry-limit parameter, it is left
down.
3. The primary path goes down. In this case, the ineligible secondary path is immediately torn
down and will only be re-signaled when the primary comes back up with a new SRLG list.
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