When a router has fewer than the full set of SFMs functioning, the forwarding capacity can be reduced. Some scenarios include:
fewer than the maximum number of SFMs installed in the system
one or more SFMs have failed
the system is in the ISSU process and the SFM is co-located on the CPM
An overload condition can be set for IS-IS and OSPF to enable the router to still participate in exchanging routing information, but route all traffic away from it when insufficient SFMs are active. This is achieved using the following CLI commands:
config>router>single-sfm-overload [holdoff-time hold-off-time]
config>service>vprn>single-sfm-overload [holdoff-time hold-off-time]
tools>perform>redundancy>forced-single-sfm-overload
These cause an overload state in the IGP to trigger the traffic reroute by setting the overload bit in IS-IS or setting the metric to maximum in OSPF. When PIM uses IS-IS or OSPF to find out the upstream router, a next-hop change in the IS-IS or OSPF will cause PIM to join the new path and prune the old path, which effectively also reroutes the multicast traffic downstream as well as the unicast traffic.
When the problem is resolved, and the required compliment of SFMs become active in the router, the overload condition is cleared, which will cause the traffic to be routed back to the router.
The conditions to set overload are:
7950 XRS-20, 7750 SR-12/SR-7 and 7450 ESS-12/ESS-7 platforms: if an SF/CPM fails, the protocol will set the overload
7950-40 XRS and 7750 SR-12e platforms: if two SFMs fail (a connected pair on the XRS-40) the protocol will set the overload