This attribute indicates the substitute RID in use for the MID. This
RID will be substituted for the RID in DPN packets destined to the
local node's RID and the MID in question when the MID is
unreachable (when, for example, all links to an AM from the
Passport subnet are down). It will also be used in the case where the
path through the substitute RID is better than that available through
the subnet's connection to the cluster containing the AM MID.
Substitute RIDs are learned from AM clusters that are supported by
two RIDs (the Passport subnet's RID and another one). The
operator can check this value to determine if a possible back up
route to an AM exists. When there are no delayNextHopLinkGroups
or tputNextHopLinkGroups shown to reach the MID, the substitute
RID will be used.
A value of 128 indicates that no substitute RID is known.