When one uses “switchport block unicast”, the third type on the list, the so called “unknown unicast” traffic is not forwarded to the specific port, so the only traffic exiting the port will be the broadcast (ff…) or the packets that have as destination mac, one of the mac-addresses learned on the port.

A problem arises when a host connected somehow to that port does not send any traffic for more than $mac_address_aging_time (usually 5 min): the mac will be erased from the mac address table, and the traffic will not be forwarded to the port (due to the blocking of unknown unicast) until a packet is received again and the mac is relearned. This setting should be mostly used on port with hosts connected, not a “core” port. On most of the cases, there will no problems, since all operating systems (programs, daemons, servers) send packets, and one of these packets is enough to make the mac address of the host be addded to the table on the switch, and the probability of a “normal” host to not send packets for 5 minutes (especially if it is Windows :) ) is very very small.

PoE (Cisco that's not using 802.3af by default)

 interface fa x/y
 power inline delay shutdown 5 initial 100

for each interface

Make Catalyst switch silent (The Invisible Catalyst Switch)

If you've ever looked at a packet capture from a Cisco Catalyst switch with a default configuration, you know they produce a good amount of ambient noise out of the box. CDP, STP, DTP, keepalives - these are all turned on by default and, combined, generate a significant amount of traffic.

Original notes: http://packetlife.net/blog/2010/apr/15/invisible-catalyst-switch/ (10x Jeremy Stretch)

Turning off CDP

 S1(config)# no cdp run

Turning off DTP

 S1(config)# interface range f0/1 -24
 S1(config-if-range)# switchport mode access
 S1(config-if-range)# switchport nonegotiate

Turning off STP

 S1(config)# no spanning-tree vlan 1

Turning off Ethernet Keepalives

 S1(config)# interface range f0/1 -24
 S1(config-if-range)# no keepalive

Security on L2 based equipment

see Security Features on Switches on Access Lists

VLAN ACL (VACL)

VACL on a Bridged Port
VACL on a Routed Port

Configuring VACL

Define the standard or extended access list to be used in VACL.
Define a VLAN access map.
Configure a match clause in a VLAN access map sequence.
Configure an action clause in a VLAN access map sequence.
Apply the VLAN access map to the specified VLANs.
Display VLAN access map information.

Switch(config)#access-list 1 permit 192.168.1.0 0.0.0.255
Switch(config)#access-list 2 permit any
Switch(config)#vlan access-map mymap 10
Switch(config-access-map)#match ip address 1
Switch(config-access-map)#action drop
Switch(config-access-map)#exit
Switch(config)#vlan access-map mymap 20
Switch(config-access-map)#match ip address 2
Switch(config-access-map)#action forward
Switch(config-access-map)#exit
Switch(config)# vlan filter mymap vlan-list 5-10
Switch(config-access-map)#end

Switch# show vlan access-map
Vlan access-map "mymap"  10                              
  Match clauses:                                         
    ip address: 1                                        
  Action:                                                
    drop                                                 
Vlan access-map "mymap"  20                              
  Match clauses:                                         
    ip address: 2                                        
  Action:                                                
    Forward                                              

Switch# show vlan filter
VLAN Map mymap is filtering VLANs:                       
  5-10

MAC ACL

MAC ACL, also known as Ethernet ACL, can filter non-IP traffic on a VLAN and on a physical Layer 2 interface by using MAC addresses in a named MAC extended ACL. The steps to configure a MAC ACL are similar to those of extended named ACLs. MAC ACL supports only inbound traffic filtering.

Switch(config)# mac access-list extended my-mac-acl
Switch(config-ext-macl)# deny any any aarp
Switch(config-ext-macl)# permit any any
Switch(config-ext-macl)# exit
Switch(config)# interface Fastethernet0/10
Switch(config-if)# mac access-group my-mac-acl in
Switch(config-if)# end
Switch#