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.

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