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60.1 Unexplained Phenomenon
»One app didn't work over Frame Relay; worked over ISDN
Further, sometime upgrading IOS image version solves weird problems. If you are looking for certain feature to work, jumping from 12.x to 15.x IOS version sometimes is the only choice due to certain bugs in earlier IOS version.
* Routers that are originally designed to work with 12.3 or 12.4 should stay on the same version. Upgrading to 15.x are usually unnecessary unless certain features are needed that are only available in 15.x
* Switches that are originally shipped with 12.2.x might be working fine with 15.x assuming the switches were made when 15.x was already available to download from Cisco website
* Reach out to TAC support for possible bugs in certain IOS images and whether upgrade to newer 15.x is needed to fulfill your objective
Check out following thread for illustration.
Downgrade from 12.4.x to 12.3.x
»Cisco 2621XM, Poor Performance with WIC-1ADSL, IOS 12.4
»[HELP] Cisco 877: connection drops when launching Emule
Upgrade from older 12.3.x to newer 12.3.x
»[Config] Cisco 2801 + WIC-1T problem
»[HELP] Cisco 7200 VXR uses up all of the I/O memory
Upgrade from 12.4.x to 15.x
»[HELP] configure GLBP with IPv6
Unidentified interface with factory-shipped IOS image
»[Config] Cisco router 867 what is the DSL interface?
Upgrade IOS image version for possible solutions
»Cisco 2821 ISR w/ Bad Ping, Error Inputs, CRC Errors & FiOS
»[Config] Adding 7920 knocks other phones off CME
Check out this FAQ for possible bug within the IOS image that might explain the phenomenon.
»Cisco Forum FAQ »Where can I find Field notices to confirm to during troubleshooting?
10G Ethernet - The aggregation problem
Here's a fun one....