600-212 Sample Questions & Answers

In Cisco StarOS, Diameter endpoints can be configured with multiple peers, each assigned a priority. The PGW will always attempt to send requests to the peer with the highest priority (e.g., priority 1). If that peer becomes unreachable (detected via watchdog messages or transaction timeouts), the PGW automatically fails over to the next highest priority peer (e.g., priority 2). This provides a simple and effective active/standby redundancy model for PCRF connectivity.

The error 'IMSI unknown in HSS' indicates the HSS does not have a record for the subscriber the MME is querying. When this happens for multiple, otherwise valid subscribers, it strongly points to an internal HSS problem. In a distributed HSS architecture, a database synchronization failure or a partitioning event could make a subset of subscriber records unavailable to the HSS front-end handling the MME's requests, leading to this specific error.

This accurately describes the division of labor. The MME, as the primary control plane node, generates IRI for events like attaches, detaches, and handovers. The Call Content (the actual user data) is tapped from the user plane, which can be done at either the SGW or the PGW, depending on the network architecture and the location of the intercept point. Both streams of information are then sent to the LI Mediation Function.

SGW selection is often geographically based to keep the user plane path optimal. The MME uses the TAI of the UE's current location to construct a DNS query (for S-NAPTR and A records) to find topologically appropriate SGWs.

The PGW is the gateway to a specific packet data network (like the internet or a corporate network), and this is identified by the APN. The MME uses the APN provided by the UE (or the default from the HSS) to construct a DNS query to resolve the IP address of the appropriate PGW.

When a subscriber's quota is exhausted, the OCS sends a Credit-Control-Answer with a Final-Unit-Indication. The PGW's action upon receiving this is determined by the Final-Unit-Action (FUA), which should be 'TERMINATE'. If there's a delay, it could be due to misconfigured credit-reauthorization thresholds (e.g., the PGW waits too long to ask for a new quota) or improper handling of the FUA, allowing a small amount of data to pass before the session is torn down. This is the most direct cause of revenue leakage in this scenario.

Cisco StarOS provides extremely detailed statistics via SNMP, but they are organized into schemas. To get granular data like specific drop reasons or failure causes, one must poll the schema-level MIBs (e.g., pgw-schema) and bulk statistics MIBs. This approach allows an NMS to retrieve thousands of specific counters related to session management, bearer handling, and protocol errors, which is far more detailed than high-level system MIBs or generic traps.

The S10 interface is a control plane interface that connects two MMEs. Its primary purpose is to facilitate the transfer of UE context (such as security information and bearer details) from a source MME to a target MME when a UE moves between tracking areas that are served by different MMEs. This ensures a seamless handover process.

3GPP specifies QoS Class Identifier (QCI) 1 for conversational voice. It is a Guaranteed Bit Rate (GBR) bearer with high priority and low latency requirements (100ms packet delay budget) to ensure high-quality, real-time voice communication. QCI 5 is used for IMS signaling, and QCI 9 is typically for the default internet bearer.

When an SGW relocation occurs, the user plane anchor point for the eNodeB shifts to the new SGW. The MME signals this change to the PGW, which then updates its user plane context. It establishes a new GTP-U tunnel (over the S5/S8 interface) to the new target SGW and tears down the tunnel to the old source SGW. This ensures that downlink data is immediately routed to the new SGW and onward to the target eNodeB.

The S9 interface is specifically defined for inter-PCRF communication in roaming scenarios. When a subscriber is roaming, the Visited PCRF (V-PCRF) in the VPLMN communicates with the Home PCRF (H-PCRF) in the HPLMN over the S9 interface. This allows the home network to enforce its own policies and charging rules for its subscribers, even when they are connected to a visited network. This is crucial for consistent service experience and accurate billing for VoLTE services while roaming.