IOS upgraded on Cisco 4331 Router.

[Zee-Far-Man]

Good Day.

I recently upgraded Cisco 4331 Router to 17.12.5a.  The upgrade went fine no issue. All interfaces showing up/up. But the Serial 0/1/0 was showing as down on the other end (WAN Side) and no communication. I rolled back to previous IOS IOS is isr4300-universalk9.17.03.06.SPA.bin and Serial interface came back online.

Question: Is it necessary to have all routers same upgraded IOS on both side?

Thanks in advanced.

[Enes Simnica · Accepted solution]

hello. No, routers don’t need to run the exact same IOS version to communicate over serial links, but differences between IOS versions can impact how serial interfaces behave. In ur case, the upgrade to 17.12.5a likely introduced changes to how the router handles encapsulation, clocking, or interface defaults, which could have caused incompatibility with the remote device (possibly ur ISP).

and man, serial interfaces are particularly sensitive to these low-level changes, especially if PPP or HDLC settings are altered or reset during the upgrade. Rolling back to 17.03.06 restored the original behavior that the other end expected, which is why the link came back up. While newer IOS versions can offer features or fixes, stability and compatibility on serial links are more important. Unless you specifically need something in 17.12.5a, it's safer to stick with the version that works or test the upgrade in coordination with your provider.

https://www.cisco.com/c/en/us/support/ios-nx-os-software/ios-xe-17/products-release-notes-list.html 

hope it helps..

-Enes

[Zee-Far-Man]

Thanks for the quick response. Yes that makes sense.  I will keep the pervious IOS to let it work.

Thanks gain, Much appreciated.

[David Ruess]

Hello,

No they do not have to be the same version in order to communicate or pass traffic.

Does the interface show admin down? If that's the case maybe the upgrade reverted the interface to be admin down for some reason.

Coordinate the upgrade with your ISP so when you complete it you can verify with them if they see traffic.

-David

[Enes Simnica]

u welcome man!

[Joseph W. Doherty]

Question: Is it necessary to have all routers same upgraded IOS on both side?

As the others have noted, no it shouldn't be necessary.  If fact, unless your router's serious interface is providing both CSU/DSU functions, your router isn't, directly, in physical communication with the far side router.

Whatever serial interface is being used, there are standards for communication.  I.e. excluding introduced bugs, or changes in some of the operational parameters, a software upgrade shouldn't have broken the connection.  Unfortunately, there's sometimes a big difference between how things ought to work and how they do work.

So, why the jump from 17.3.6 to 17.12.5a?   Was this just to jump to the latest star release?  Did you carefully review the release notes?

As these routers are going EoL, if you have a support contract, it might be worthwhile obtaining last MD train in 17.3.x and later.  Then carefully review release notes, and consider the need for a software update.  For instance, some new feature you desire to use?  Some bug you want fixed, etc.?

Plan on upgrade validation and rollback.  For example, what if there was a more subtle issue?

BTW, the fact this release has a letter suffix, and there are two more suffixed letter releases, doesn't inspire my confidence, but I haven't read their release notes.

[Zee-Far-Man]

So, why the jump from 17.3.6 to 17.12.5a?   Was this just to jump to the latest star release?  Did you carefully review the release notes?

The ACAS Scan was showing (vulnerability CVE 2025-20197 thru 20201 for priv escalation). A Cisco TAC Engineer suggested upgrade to 17.12.5a, we upgraded to one 1 out of 2. 1 does not having any issue, but the 2nd having serial interface problem.

Thanks.