Cisco IOS – Negating Terminal Commands

I always found it odd that I couldn’t negate terminal commands in IOS like other commands.

Eg;  no Terminal Monitor

I just assumed it wasn’t there, until I saw another engineer execute to disable term mon.

Router# terminal no monitor

It seems in IOS the terminal commands are special in that they require the terminal key word before the no keyword to negate sub-commands.

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Cisco 3750 – 3rd Party SFP

It is possible to use 3rd party SFP’s in a Cisco 3750 with the following commands:

Switch(config)#service unsupported-transceiver

and

Switch(config)#no errdisable detect cause gbic-invalid

The first command will generate the following warning from cisco :

” Warning: When Cisco determines that a fault or defect can be traced to
the use of third-party transceivers installed by a customer or reseller,
then, at Cisco’s discretion, Cisco may withhold support under warranty or
a Cisco support program. In the course of providing support for a Cisco
networking product Cisco may require that the end user install Cisco
transceivers if Cisco determines that removing third-party parts will
assist Cisco in diagnosing the cause of a support issue.”

I wouldn’t recommend using non-Cisco SFP’s in production environments, but for a lab save the bucks and go for it.

IOS – Ping Sweep

I discovered a really cool feature of IOS that is probably common knowledge but I was never aware of.

You can perform a ping sweep of a directly connected network by pinging the broadcast or Network address.

Example:

Router#ping 192.168.1.255

The output is as follows :
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.255, timeout is 2 sec

Reply to request 0 from 192.168.1.19, 4 ms
Reply to request 0 from 192.168.1.59, 40 ms
Reply to request 0 from 192.168.1.57, 40 ms
Reply to request 0 from 192.168.1.56, 40 ms

This is incredibly useful for doing discovery and populating the routers ARP table after a reboot.