When replacing a faulty Cisco router, one of the most important steps before connecting any production cables is ensuring the replacement unit is running the exact same IOS image as the original. A software mismatch at cutover can cause unexpected behavior, feature gaps, or compatibility issues with the rest of the network.
This guide covers how to copy a Cisco IOS image directly from one router to another using the router's built-in TFTP server capability — no external TFTP server required. The same procedure applies whether you are working on a bench with console-only access or in a live environment where both routers can already reach each other over the network.
This procedure works on all Cisco IOS and IOS XE routers. Commands are identical regardless of model. The only difference between platforms is the storage location name — some routers use flash: and others use bootflash:. Both are covered throughout this guide.
Before running any commands, identify which storage name your router uses. This affects every copy, verify, dir, and boot system command in this guide.
Run the following command to confirm which storage your router uses:
Router# show file systems
Look for the entry marked * (default) with type disk. That is the storage name you will use throughout this procedure. Example output from an ISR 4000 series router:
File Systems:
Size(b) Free(b) Type Flags Prefixes
- - opaque rw archive:
- - opaque rw system:
- - opaque rw tmpsys:
* 3,697,541,120 2,900,000,000 disk rw bootflash:
- - opaque rw null:
If your router uses bootflash:, replace every occurrence of flash: in the commands below with bootflash:. Do not mix them — the commands will fail or point to the wrong location.
This procedure covers two common situations. The steps are nearly identical — the only difference is how connectivity is established between the two routers.
Follow Steps 1 through 5 for Scenario A. For Scenario B, complete Step 1 to record the source image MD5, then skip to Step 4 to enable the TFTP server and proceed from there.
Connect to the source router via console or SSH. Confirm the image is present in storage and calculate its MD5 checksum. You do not need to compare the MD5 here — you just need to record the value. The actual integrity check is done on the destination router after the transfer, where it matters most.
List the contents of storage:
SourceRouter# dir bootflash:
You should see the IOS image listed with its file size. Note the exact filename — you will need it precisely as shown, including capitalisation and file extension.
Calculate the MD5 checksum. This may take 1–3 minutes depending on the image size:
SourceRouter# verify /md5 bootflash:<image-filename.bin>
Example:
SourceRouter# verify /md5 bootflash:isr4300-universalk9.17.12.07b.SPA.bin verify /md5 (bootflash:isr4300-universalk9.17.12.07b.SPA.bin) = 9bc35e12bae261c8668903bac90edaba
Note down the MD5 value printed in the output — for example, 9bc35e12bae261c8668903bac90edaba. You will use this value in Step 6 to confirm the copied image on the destination router is identical. If you already have the MD5 from the Cisco software download page, you can use that instead.
Connect to the destination (replacement) router and check available storage space:
NewRouter# dir bootflash:
The last line of the output shows total and free space. You need free space at least equal to the size of the image you are copying. If the router shipped with a factory default IOS image, both images can coexist in storage as long as there is enough room. If space is tight, you can delete the factory image after the transfer is verified.
Example output:
Directory of bootflash:/ 1 498073600 Jan 01 2000 00:00:00 +00:00 isr4300-universalk9.16.09.06.SPA.bin 2 10537 Jan 01 2000 00:00:00 +00:00 packages.conf 3,697,541,120 bytes total (3,180,000,256 bytes free)
In this example, approximately 2.96 GB is free — sufficient for most IOS XE images which typically range from 400 MB to 700 MB.
If both routers are already reachable over the network (Scenario B), skip this step and go to Step 4.
Plug an Ethernet cable between any available GigabitEthernet port on the source router and any available GigabitEthernet port on the destination router. Modern routers support auto-MDI/MDIX, so either a straight-through or crossover cable will work.
Assign a temporary IP address on each router using the console. Use any private range — these IPs exist only for the duration of the transfer and are removed afterward.
SourceRouter# configure terminal SourceRouter(config)# interface GigabitEthernet0/0/0 SourceRouter(config-if)# ip address 10.0.0.1 255.255.255.252 SourceRouter(config-if)# no shutdown SourceRouter(config-if)# end
NewRouter# configure terminal NewRouter(config)# interface GigabitEthernet0/0/0 NewRouter(config-if)# ip address 10.0.0.2 255.255.255.252 NewRouter(config-if)# no shutdown NewRouter(config-if)# end
Verify the link is up on both sides:
SourceRouter# show interfaces GigabitEthernet0/0/0 | include line protocol GigabitEthernet0/0/0 is up, line protocol is up
Confirm basic reachability from the destination router:
NewRouter# ping 10.0.0.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.0.0.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/2 ms
If the ping fails, confirm both interfaces show up/up using show interfaces GigabitEthernet0/0/0. Check that the cable is firmly seated on both ends. Do not proceed until pings are successful.
Cisco IOS has a built-in TFTP server. Enable it on the source router and point it at the image file in storage. This allows the destination router to pull the file directly without any external TFTP server.
SourceRouter# configure terminal SourceRouter(config)# tftp-server bootflash:<image-filename.bin> SourceRouter(config)# end
Example:
SourceRouter(config)# tftp-server bootflash:isr4300-universalk9.17.12.07b.SPA.bin
Verify the TFTP server entry is active:
SourceRouter# show run | include tftp-server tftp-server bootflash:isr4300-universalk9.17.12.07b.SPA.bin
For Scenario B (production routers): Use the source router's existing production IP address as the TFTP server address in the next step. No interface or IP changes are needed.
Before starting the transfer, increase the TFTP block size on the destination router. The default TFTP block size is 512 bytes, which results in a large number of round-trips for a file several hundred megabytes in size. Setting it to 8192 bytes significantly reduces transfer time.
NewRouter# configure terminal NewRouter(config)# ip tftp blocksize 8192 NewRouter(config)# end
The block size setting takes effect immediately for the next TFTP transfer. It does not require a reload. You can verify it is set with show running-config | include tftp blocksize.
Now initiate the TFTP copy. When prompted, enter the source router's IP address and the exact image filename:
NewRouter# copy tftp: bootflash: Address or name of remote host []? 10.0.0.1 Source filename []? isr4300-universalk9.17.12.07b.SPA.bin Destination filename [isr4300-universalk9.17.12.07b.SPA.bin]? [press Enter]
The transfer begins immediately. Exclamation marks (!) indicate data blocks being received. With a block size of 8192, IOS XE images of 400–700 MB typically transfer in 5 to 15 minutes over a direct GigabitEthernet link — considerably faster than the default block size.
Loading isr4300-universalk9.17.12.07b.SPA.bin from 10.0.0.1 (via GigabitEthernet0/0/0): !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! [OK - 598298624 bytes] 598298624 bytes copied in 312.540 secs (1914210 bytes/sec)
Do not reboot, power cycle, or disconnect either router during the transfer. A partial copy will leave a corrupted or incomplete file in storage, and you will need to delete it and start over.
Once the transfer completes, run the MD5 verification on the destination router. Pass the expected MD5 value directly in the command so IOS gives you an immediate pass/fail result:
NewRouter# verify /md5 bootflash:<image-filename.bin> <expected-md5-hash>
Example:
NewRouter# verify /md5 bootflash:isr4300-universalk9.17.12.07b.SPA.bin 9bc35e12bae261c8668903bac90edaba verify /md5 (bootflash:isr4300-universalk9.17.12.07b.SPA.bin) = 9bc35e12bae261c8668903bac90edaba Verified (bootflash:isr4300-universalk9.17.12.07b.SPA.bin) = 9bc35e12bae261c8668903bac90edaba MD5 Hash MATCHES
MD5 Hash MATCHES confirms the copied image is bit-for-bit identical to the source. It is safe to set this as the boot image. If you see MD5 Hash DOES NOT MATCH, delete the file, confirm the source image is still healthy, and repeat the transfer.
Configure the destination router to boot from the newly copied image. Clear any existing boot statements first to avoid conflicts:
NewRouter# configure terminal NewRouter(config)# no boot system NewRouter(config)# boot system bootflash:<image-filename.bin> NewRouter(config)# end NewRouter# write memory
Example:
NewRouter(config)# boot system bootflash:isr4300-universalk9.17.12.07b.SPA.bin
Confirm the boot statement saved correctly:
NewRouter# show run | include boot system boot system bootflash:isr4300-universalk9.17.12.07b.SPA.bin
Also verify the boot path in NVRAM reflects the new image:
NewRouter# show boot BOOT path-list : bootflash:isr4300-universalk9.17.12.07b.SPA.bin Config file : bootflash:/startup-config
Reload the destination router and allow 3–5 minutes for it to fully boot. Reconnect via console and verify the running version:
NewRouter# reload Proceed with reload? [confirm]
After boot:
NewRouter# show version Cisco IOS XE Software, Version 17.12.07b ...
The router is now running the correct IOS version — identical to the source router. For a bench replacement scenario, you can now proceed with configuration and production cable installation.
Once the destination router is confirmed healthy, remove the temporary TFTP and interface configuration from the source router. This is especially important if the source router is in production or will be reused:
SourceRouter# configure terminal SourceRouter(config)# no tftp-server bootflash:<image-filename.bin> SourceRouter(config)# end SourceRouter# write memory
If you added temporary IPs to the source router's interface (Scenario A — bench setup only), remove those as well:
SourceRouter# configure terminal SourceRouter(config)# interface GigabitEthernet0/0/0 SourceRouter(config-if)# no ip address SourceRouter(config-if)# shutdown SourceRouter(config-if)# end SourceRouter# write memory
On the destination router, if the factory default image is still present and storage space is needed, you can safely delete it now that the new image is verified and booted:
NewRouter# delete bootflash:<old-factory-image.bin> Delete filename [old-factory-image.bin]? [confirm] Delete bootflash:old-factory-image.bin? [confirm]
On IOS XE platforms, deleted files are immediately freed — there is no need to run squeeze. On older IOS platforms using flash:, you may need to run squeeze flash: after deletion to reclaim the space.
show interfaces GigabitEthernet0/0/0 on both routers and confirm the status shows up/up. Check that the Ethernet cable is firmly seated. Also confirm you assigned IPs on the same subnet (e.g., /30 means only .1 and .2 are valid hosts).
tftp-server entry is still in the running config of the source router with show run | include tftp. Ensure you are using the correct IP address and that the filename is typed exactly as it appears in storage, including case.
dir bootflash: (or dir flash:) and identify older images or unnecessary files. Delete them with delete bootflash:filename. On older IOS platforms, follow with squeeze flash: to reclaim space.
show boot, re-enter the boot system command, run write memory, and reload again.
show file systems and ensure you are using the correct prefix (flash: or bootflash:) consistently.
Copying a Cisco IOS image directly between two routers without an external TFTP server is straightforward. The source router acts as its own TFTP server using the built-in tftp-server command, and the destination router pulls the file over any available Ethernet path. The procedure is identical across router models — only the storage prefix (flash: vs bootflash:) differs by platform.
verify /md5 bootflash:<image> and note the valuetftp-server on the source router pointing at the imageip tftp blocksize 8192 on the destination router for faster transfercopy tftp: bootflash: from the destination routerwrite memoryshow version reflects the correct IOS versionStandardising IOS versions before a router goes into production eliminates one variable at cutover and makes troubleshooting significantly easier if issues arise after the swap.
