[c-nsp] wisdom of switchport block ...

András Tóth diosbejgli at gmail.com
Mon Feb 10 15:25:24 EST 2014


Let's not forget STP topology change notifications (TCNs) because they'll
cause the MAC address entries to age out in forward-delay (15 sec) or even
immediately with Rapid-STP. A STP topology change is observed (and TCN
generated) when a non-edge (non-portfast) port goes either from Forwarding
to Blocking or from Blocking to Forwarding. With RSTP non-edge port moving
to Forwarding will generate TCNs.

This can lead to hosts becoming unreachable with unicast blocking even with
a carefully chosen ARP aging timer.


On Mon, Feb 10, 2014 at 7:30 PM, Tarko Tikan <tarko at lanparty.ee> wrote:

> hey,
>       I am looking at tightening up my subscriber access network and, if
>> I understand the documentation correctly, 'switchport block unicast'
>> will prevent a cisco switch (3560g in this case) from flooding unicast
>> frames out any port so configured, unless the destination mac address
>> was learned from that port.
> Blocking unknown unicast is very typical for access networks using
> service-vlans (or N:1, whatever you like to call it).
> MAC aging and DHCP lease timers will have to be tuned accordingly, make
> sure DHCP < aging. This way DHCP renewals will keep active addresses in the
> MAC table.
> --
> tarko
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