Spatial reuse protocol (SRP) is a media access control (MAC)-layer protocol that operates over a double counter-rotating ring network topology. SRP is designed to enhance the SONET network so that it can handle data traffic more efficiently. We study the all-terminal reliability of an SRP ring implementing intelligent protection switch (IPS) and enhanced intelligent protection switch (E-IPS). We calculate reliability for all scenarios under which all-terminal connectivity is maintained. By summing up the results arising from individual mutually exclusive events, all-terminal reliabilities for IPS and E-IPS are obtained. By further assuming constant and time-dependent component failure rates, mean time to failure (MTTF) and mean time to failure improvement factor (MTIF) measures are calculated. Results show that MTTF/MTIF vary as functions of network size, component reliability and coverage factor. The advantage of E-IPS over IPS is more obvious when network size is big and concentrator failure rate is small. Our analysis could be helpful to network designers who wish to determine whether the additional cost/complexity added by the E-IPS scheme is justified.