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The output of show vlan brief includes VLANs 1002 - 1005:

1002 fddi-default                     active    
1003 trcrf-default                    active    
1004 fddinet-default                  active    
1005 trbrf-default                    active

What was the intended use case for these VLANs? What are fddi trcrf fddinet trbrf and why did they need a dedicated VLAN reserved?

1 Answer 1

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Those are "translation" VLANs for bridging ethernet and FDDI / Token-ring. You'll only find this hardcoded nonsense on older IOS switches. (It took Cisco eons after they stopped making FDDI and TR to finally remove this mess.)

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    Additionally, those are just "default" names. There's nothing stopping OP from using vlan ID 1003 for a guest network or CCTV cameras.
    – Criggie
    Commented Aug 5, 2021 at 5:42
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    @Criggie, except that e.g. the one Catalyst I have access to says "%Default VLAN 1002 may not have its name changed.". And trying to set them as the access VLAN of a port gives "% Warning: port will be inactive in non-ethernet VLAN". Not that I checked if there's a way to disable that and make them into normal VLANs, but at least it's extra steps.
    – ilkkachu
    Commented Aug 5, 2021 at 8:02
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    @Criggie Negative. Those are very special purpose VLANs -- there's "mountains" of code behind them. They cannot be used for any other purpose. They cannot be deleted, renamed, disabled, or otherwise altered in any way. It's been an annoyance in CatOS and IOS(switch) for as long as I can remember. (20+ years.)
    – Ricky
    Commented Aug 5, 2021 at 10:53
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    OK you're correct. On a much newer switch these VIDs can be used, but that may lead to interop problems if an older switch gets added to the network, like how some brands of switch make use of vlan 100 (vague memory that SMC did weird stuff with vid100 )
    – Criggie
    Commented Aug 5, 2021 at 11:18
  • There have been many a "weird thing" done with VLANs that make for interop nightmares. I could fill a (small) library with those cases. NX-OS was developed by "more enlightened" people, so it doesn't have that mess. As I recall, very later (15.0/152?) IOS finally removed it. (don't have any of those switches on at the moment.)
    – Ricky
    Commented Aug 5, 2021 at 12:06

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