Some questions about portfast and CAM table

Lendros

Hi! I'm trying to get the CCNP Switch 300-115. Making some exams questions, i've found some topics where i have doubt. I hope you guys can help me.

- When a Portfast interface receives an unexpected BPDU, ¿which is it behaviour exactly? (from operational and STP views).

- One question declares: "By default, a Cisco Switch includes a MAC address table for every VLAN that exist on the switch." I thought that only exist a CAM instance per SW(with mac-port-vlan info), not a CAM instance per VLAN. ¿Is it correct or the book question is wrong?


Thanks!

EdTheLad

If a portfast interface receives a BPDU it will continue forwarding, which could lead to a loop. If you enable port-fast with bpdu-guard the port will go err-disabled.
The switch contains one CAM instance that's divided up per vlan. Which means a vlan will only see the mac addresses that are in that vlan.

tomtom1

EdTheLad wrote: »

If a portfast interface receives a BPDU it will continue forwarding, which could lead to a loop. If you enable port-fast with bpdu-guard the port will go err-disabled.

The better choice would be to use a RSTP edge port here, which also goes to forwarding immediately but will transition to a 'normal' STP port once it receives a BPDU.

Lendros

Thanks,

In CCNP Routing and Switching Exam Practice Pack (Cisco Press book), there is the next question with this answer and it confused me (I remembered that you said)

EdTheLad

When portfast is enabled on an interface, it means the interface will bypass the listening and learning stages and go directly to forwarding. If a bpdu is received on an interface which has portfast enabled, portfast will be disabled on that interface until it is shut/no shut. A portfast enabled interface will behave exactly like a regular interface, when it comes to stp roles and states. If a switch has two ports enabled with portfast and bpdu's are received on both, one will become the root and the other will either be designated forwarding or alternative blocked.

The question in the book doesn't make sense!