SNMP
Managers, agents, MIBs and OIDs; polling versus traps and informs; and what the SNMP versions add for security.
SNMP lets a manager (NMS) read and write management data on devices (agents). Data lives in a tree called the MIB; each leaf is an OID — a dotted identifier like 1.3.6.1.2.1.1.5.0 (sysName). The agent listens on UDP 161 for requests; the manager listens on UDP 162 for notifications.
SNMP message types
| Message | Direction | Purpose |
|---|---|---|
| GetRequest / GetNext / GetBulk | NMS → agent (161) | Read OID(s); GetBulk efficiently walks tables |
| SetRequest | NMS → agent (161) | Write a writable OID |
| Response | agent → NMS | Answer to a request |
| Trap | agent → NMS (162) | Unsolicited alert, no acknowledgment |
| InformRequest | agent → NMS (162) | Alert the manager must acknowledge — reliable but heavier |
SNMP versions
| Version | Security | Notes |
|---|---|---|
| v1 | Community string (cleartext) | Legacy, 32-bit counters only |
| v2c | Community string (cleartext) | Adds GetBulk, Inform, 64-bit counters |
| v3 noAuthNoPriv | Username only | No crypto |
| v3 authNoPriv | Auth (MD5/SHA) | Integrity, no encryption |
| v3 authPriv | Auth + encryption | The secure enterprise choice |
Practical OIDs for network study: sysDescr, sysUpTime, sysName identify the device; ifOperStatus/ifAdminStatus report interface state; ifHCInOctets/ifHCOutOctets are the 64-bit counters used for utilization math. A 32-bit byte counter wraps in ~3.4 seconds on a 10 Gb link — always poll HC counters on fast interfaces.
Related resources
Sources
- Cisco CCNA 200-301 v1.1 Exam Topics (official PDF) — reviewed 2026-11-02
- RFC 3418 — Management Information Base (MIB) for SNMP — reviewed 2026-11-02
- RFC 3416 — Protocol Operations for SNMPv2 — reviewed 2026-11-02
Targets CCNA 200-301 v1.1 objectives. Supplemental blocks are labeled beyond exam scope.