Cisco switches use different types of memory, each optimized for a specific task:
- Fast vs persistent
- Volatile vs non‑volatile
- Boot vs runtime vs configuration
If you know what lives where, you know:
✅ what survives a reboot
✅ what gets erased
✅ how the switch boots
✅ where problems usually occur
Memory Types Overview
| Memory Type | Volatile? | Purpose |
|---|---|---|
| RAM (DRAM) | Yes | Running config, processes, packet buffers |
| ROM | No | Bootloader, POST, recovery |
| NVRAM | No | Startup configuration |
| Flash | No | IOS image(s), system files |
1️⃣ RAM (Random Access Memory)
✅ What RAM is used for:
RAM is the working memory of the switch.
Stored in RAM:
- Running configuration
- Routing tables (if Layer 3)
- ARP table
- MAC address table (CAM)
- Packet buffers
- Active IOS processes
✅ Key characteristics:
- Volatile → Cleared on reload or power loss
- Fast access
- Changes constantly during operation
✅ Cisco commands related to RAM:
Plain Text
show running-config
show processes memory
show memory statistics
show mac address-table
✅ Important exam / real‑world fact:
If you reboot a switch without saving, all changes in RAM are lost.
2️⃣ ROM (Read‑Only Memory)
✅ What ROM is used for:
ROM contains low‑level startup code and diagnostics.
Stored in ROM:
- POST (Power‑On Self Test)
- Bootloader / ROMMON
- Mini‑IOS or recovery image (on some platforms)
✅ Key characteristics:
- Non‑volatile
- Rarely changes
- Minimal environment
✅ When do you actually use ROM?
- IOS image is corrupt or missing
- Password recovery
- Manual boot troubleshooting
✅ ROMMON prompt example:
Plain Text
switch:
“
✅ Cisco commands interacting with ROM (indirectly):
Shell
show version
boot
“
3️⃣ NVRAM (Non‑Volatile RAM)
✅ What NVRAM is used for:
NVRAM stores the startup configuration.
Stored in NVRAM:
startup-config- Configuration that loads at boot
✅ Lifecycle:
- You configure the switch → stored in RAM (
running-config) - You save the config: Shellcopy running-config startup-config
Show more lines - The config is written to NVRAM
- On reboot → copied back into RAM
✅ Key characteristics:
- Non‑volatile
- Small in size
- Critical for config persistence
✅ Cisco commands:
Shell
show startup-config
write memory
copy run start
Show more lines
✅ Common mistake:
If NVRAM is erased or corrupted, the switch boots with default settings.
4️⃣ Flash Memory
✅ What Flash is used for:
Flash stores IOS software and system files.
Stored in Flash:
- Cisco IOS image (
.bin) - VLAN database (
vlan.daton some models) - Backup IOS images
- Crash info
✅ Key characteristics:
- Non‑volatile
- Rewritable
- Similar concept to a USB drive or SSD
✅ Cisco commands:
Shell
dir flash:
show flash
copy tftp: flash:
delete flash:old_ios.bin
“
✅ Example Flash contents:
Plain Text
2960-lanbasek9-mz.152-7.E8.bin
vlan.dat
“
Show more lines
🧠 Memory Interaction During Boot (VERY IMPORTANT)
Here’s the Cisco boot sequence — memorize this:
1️⃣ ROM
- POST runs
- Bootloader starts
2️⃣ Flash
- IOS image located and loaded into RAM
3️⃣ RAM
- IOS runs from RAM
4️⃣ NVRAM
startup-configcopied into RAM asrunning-config
Boot Flow Diagram:
ROM → Flash → RAM → NVRAM → RAM
Summary Table (Exam Gold)
| Memory | Contains | Loses Data on Reboot? |
|---|---|---|
| RAM | Running-config, tables, buffers | ✅ YES |
| ROM | Bootloader, POST | ❌ NO |
| NVRAM | Startup-config | ❌ NO |
| Flash | IOS images | ❌ NO |
Real‑World Troubleshooting Tips
🔴 Switch boots but no config?
➡️ Check NVRAM
Plain Text
show startup-config
“
🔴 Switch stuck at switch: prompt?
➡️ Likely Flash or IOS issue
🔴 Changes lost after reload?
➡️ Config was never saved from RAM to NVRAM
Cisco Interview / CCNA Tip
If asked:
“Where is the running configuration stored?”
✅ RAM
If asked:
“Where is IOS stored?”
✅ Flash
If asked:
“What memory survives a power failure?”
✅ ROM, NVRAM, Flash
Leave a Reply