when you use older Cisco routers together with more update routers in the same ethernet network and you are using CDP (Cisco Discovery Protocol) you maybe familiar with the following message.

000042: Jan 13 21:22:23.001: %CDP-4-DUPLEX_MISMATCH: duplex
mismatch discovered on FastEthernet0 (not half duplex), with
r2511a.arbzim.bogus Ethernet0 (half duplex).

I found this message on a Cisco 1712 router with a FastEthernet interface that saw a Cisco 2511 router with a Ethernet interface on the same Ethernet network. As not using CDP wasn’t an option and I couldn’t set the Ethernet interface on the Cisco 2511 router to full duplex and I don’t want to set the full duplex interface on the Cisco 1712 router to half duplex I searched for another solution.

Here are two commands that can’t solve the duplex mismatch problem but at least can prevent the duplex mismatch messages.

  • no cdp advertise-v2 English
  • This disables the duplex mismatch message. But it also disables the VTP Management Domain Name and Native VLAN messages. The advantage is that you only need to configure it on the router where you can’t fix the duplex setting on the interface (in my case the Cisco 2511 router).

  • no cdp log mismatch duplex English
  • This command only disables the duplex mismatch message. The advantage of this command is that you can configure it global or only on the (Ethernet) interface where you need it. The disadvantage is that you have to configure it on both routers (in my case on the Cisco 2511 router and on the Cisco 1712 router where I found the message).

Bye, Tore

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