Learning IINS (Part 8) – L2 Security

Using SPAN feature

Switch port analyzer (SPAN) feature can be configured to allow a copy of traffic destined for another port to be sent out the SPAN port, thus allowing an attacted IDS sensor to capture a copy of traffic, as illustrated in figure below:

Configuration example:

monitor session 1 source interface gigabitethernet0/2
monitor session 1 destination interface gigabitethernet0/3

Cisco Catalyst Switches also support Remote SPAN (RSPAN) feature, which allows a SPAN port to be configured on a different switch.

VLAN Access Control List (VACL)

We can filter inter-vlan traffic by a VLAN access control list (VACL). Here is configuration example:

access-list 100 permit tcp any host eq 80 (Permit HTTP to be sent to host & deny all others)

vlan access-map ALLOWHTTP 10
(Fow sequence number 10, the specific action is to forward traffic matching ACL above)
match ip address 100
action forward

Vlan filter ALLOWHTTP vlan-list 5-10 (VACL is applied to VLANs in range of 5 to 10)

Isolating traffic within a VLAN using Private VLAN

Another way to provide traffic control withing a VLAN is the use of private VLANs (PVLAN). A PVLAN domain has a single primary VLAN. Additionally, the PVLAN domain also contains secondary VLANs that provide isolate between ports in a PVLAN domain. PVLAN ports fall into one of three categories:

  • Promiscuous ports: can communicate with all other PVLAN ports.
  • Isolated VLANs ports: can comunicate with only promiscuous ports.
  • Community ports: can communicate with other ports in their community and also with promiscuous ports.

Configuration example:

vtp mode transparent  (required, be careful !)
vlan 50
private vlan community


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