1. – Chapter 5 – Secure LAN Switching
Layer 2 security
Port security
IP permit lists
Protocol filtering
Controlling LAN floods (using port filtering, protocol filtering, etc.)
Private VLANs
Using IEEE 802.1x for port authentication and access control
Network Security

2. Switch and Layer 2 security
Security of lower layer devices is important, because some threats are initiated on Layer 2 rather than Layer 3 and above.
Example: A firewall or a router cannot block a compromised server on a DMZ LAN from connecting to another server on the same segment.  because the connection occurs at Layer 2
More example attacks:
Focus of the chapter: Cisco Catalyst 5000 series switches
Network Security

3. Switch and Layer 2 security (cont.)
Rules of thumb:
Use VLANs to create logical groupings of devices  Each of the groups may have different security levels.
Disable unused ports, and place them in a VLAN with no Layer 3 access.
Besides VLANs, other mechanisms must be used.
Separate devices should be used for zones at different security levels.
Disable Layer 3 connection (e.g., Telnet, HTTP) to the switch.
Disable trunking on ports that do not require it.
A trunk is an interface on a switch that can carry packets for any VLAN. When packets get sent between switches, each packet gets tagged, based on the IEEE standard for passing VLAN packets between bridges, 802.1Q. The receiving switch removes the tag and forwards the packet to the correct port or VLAN in the case of a broadcast packet.
 “VLAN Insecurity” (
Network Security

4. Switch and Layer 2 security (cont.)
How about attacks launched from hosts sitting on a LAN?
In general, those hosts are considered as trusted entities.
So it is difficult to stop a host when it becomes an attacker.
Solution: Make sure access to the LAN is secured.
 MAC address filtering (e.g., Cisco’s port security, DHCP)
Network Security

5. Port security
A mechanism to restrict the MAC addresses that can connect via a particular port of the switch
Allows a range of MAC addresses to be specified for a particular port
Only frames with a right MAC address can go through the switch.
Useful for preventing MAC address flooding attacks
CAM overflow: Content-Addressable Memory (aka. associated memory)
CAM table stores information such as MAC addresses available on physical ports, with their associated VLAN params.
CAM table has fixed size.
When a CAM table is full, the switch is unable to create a new entry.
 It forwards a received frame to all ports, resulting in increased traffic and allowing the attacker to examine all frames.
So, CAM overflow attacks may lead to subsequent DoS and traffic analysis attacks (next slide)
Network Security

6. MAC Address Flooding
Network Security

7. MAC Address Flooding (cont.)
Counter-measures:
Hard-coding the MAC addresses that are allowed to connect on a port, or
Limiting the number of hosts that are allowed to connect on a port
Example 5-1: approach 1 + timed suspension
Example 5-2: approach 2
Network Security

8. IP permit lists
Purpose: To restrict higher layer traffic, such asTelnet, SSH, HTTP, and SNMP, from entering a switch
Allows IP addresses to be specified that are allowed to send these kinds of traffic through the switch
Example 5-3
Network Security

9. Protocol Filtering
Purpose: To limit broadcast/multicasts for certain protocols
With Cisco Catalyst 5000 series of switches, packets are classified into protocol groups:
IP 2. IPX
AppleTalk, DECnet, Banyan VINES 4. Other protocols
A port is configured to belong to one or more of these groups.
For each of the groups a port belong to, the port is in one of the following states (for that group):
On  Receive all broadcast/multicast traffic for that protocol
Off  no broadcast/multicast traffic for that protocol
Auto  auto-configured port
The port becomes a member of the protocol group only after the device connected to the port transmits packets of that specific protocol group.
Once the attached device stops transmitting packets for that protocol for 60 minutes, the port is removed form that protocol group.
Example 5-4
Network Security

10. Controlling LAN floods
Attackers may cause frame flood (e.g., CAM flooding), or send broadcast/multicast messages to flood the LAN.
Counter-measures:
Protocol filtering
Setting up threshold limits for broadcast/multicast traffic on ports
Catalyst switches allow thresholds for broadcast traffic to be set up on a per-port basis.
The thresholds can depend on either the bandwidth consumed by broadcasts or the number of broadcast packets being sent across a port.
‘Bandwidth consumed’ is a preferred measure. (Why?)
Example: Console> (enable) set port broadcast 2/1-6 75%
Other broadcast/multicast traffic is dropped when the bandwidth consumed by broadcast/multicast traffic reaches 75%.
Network Security

11. Private VLANs An enhancement to Catalyst 6000 switches
Traditional VLAN: no layer 2 segregation of devices of the same VLAN  So when one of the devices in a VLAN is compromised, other devices on the same VLAN may be compromised as well.
Purpose of private VLANs: To allow restrictions to be placed on the Layer 2 traffic of a VLAN.
Three types of private VLAN ports:
Promiscuous ports: communicate with all other private VLAN ports
Isolated ports: have complete Layer 2 isolation from other ports within the same private VLAN
Community ports: communicate among themselves and with their promiscuous ports
Network Security

12. Using IEEE 802.1x Purpose: (a) port authentication; (b) access control
Other usage: used in i for WLAN security
Network Security

13. Summary
Next: NAT and security
Network Security