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Input errors on the Layer-3 interface of Catalyst 6500 switch running Cisco IOS when connected to the Layer-2 switchport of CatOS switch

Core Issue

By default, all CatOS Layer-2 ports are trunking in the Auto mode.  This means they send out Dynamic Trunking Protocol (DTP) frames periodically.

When you connect this type of port to a native Cisco IOS® Layer-3 port, they do not understand any Layer 2 trunking frames, and hence, you receive Input errors, the Input queue drops and you receive WrongEncap errors respectively.

Resolution

Complete one of these steps in order to resolve this issue:

  • Turn off the default trunking on the CATOS switch with the set trunk mod/port off command.

  • Set the native Cisco IOS switch port to a Layer 2 port with the switchport command and move the Layer 3 information to a Switch Virtual Interface (SVI), for example, a layer 3 VLAN.

Note: When Cisco IOS port is made into Layer 2 switchport, the default trunking mode is dynamic desirable mode, which means it sends out the DTP frames and also understands the DTP frames sent by the CATOS switch. Since auto-desriable results in a trunk, the link comes up as trunk and works fine with no errors.

For more information on default trunking modes refer,

Dynamic Trunking Protocol part of Best Practices for Catalyst 4500/4000, 5500/5000, and 6500/6000 Series Switches Running CatOS Configuration and Management

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