Sunday, 5 February 2012

A few bits I forgot.. ICND2

The Boson exam environment software had a few trick questions for me today.

One such question was about being able to ping a computer on a different Vlan.  The question asked if the ICMP echo would traverse one particular switch in the diagram.  I answered NO.  The correct answer was YES.
When you ping from one device to the other and they are on different Vlans, the gateway will be contacted, so follow the path, too and from.  In my wisdom I thought the switch would check its mac table, notice the client in a different Vlan and route it, but switches aren't routers are they?

Another gotcha was to do with OSPF.  Routers can't have mismatched subnets or hello timers.  The network commands can be different (which enables OSPF on an interface) because that is merely ruling out which interfaces should run OSPF via a wildcard mask.

Finally, ISL is Cisco's own proprietry trunking protocol that encapsulates a normal ethernet frame before forwarding over a trunk.  I though it was open standard - stupid me.

Friday, 3 February 2012

Cold or Warm Reboot/Reloads on Cisco routers

Unbenown to me, you can warm reload Cisco routers and cut down the boot time.  I read this on Jermey Gaddis' blog from the states.  The guy knows his stuff, but on with the learning...

Have a go:

Conf t

Yes thats it.  You can configure other variables such as allowing the router to warm reboot a few times and when at a certain limit perform a cold restart etc, but the basic command is above.  I'm sure you know how to use the tab and question marks on your keyboard by now...

To then reload the router/switch from the prompt type 'reload warm'.

Obviously there are pro's and con's to this command.  The boot time is reduced, but you need a bit of room in RAM in order to buffer the command. 
The router simply saves the state in RAM and boots from that, rather than all the copying/decompressing the IOS image from flash each time.  I suppose if your IOS is corrupt (er... not sure how many times/if ever I've heard of this) in your running-config, then a cold restart will load a fresh IOS image back on.  But.. lets be realistic, we want quicker booting routers, so this should be a staple command.