Layer 2 Switch Security Requirements Guide
| Version 1 Release |
| U_Layer_2_Switch_SRG_V1R |
| This Security Requirements Guide is published as a tool to improve the security of Department of Defense (DoD) information systems. The requirements are derived from the National Institute of Standards and Technology (NIST) 800-53 and related documents. Comments or proposed revisions to this document should be sent via e-mail to the following address: disa.stig_spt@mail.mil. |
- All
- Updated 5
- Added 2
- Removed 3
Vulnerabilities (30)
V-62157
The layer 2 switch must authenticate all endpoint devices before establishing a network connection using bidirectional authentication that is cryptographically based.
Discussion
Controlling LAN access via 802.1x authentication can assist in preventing a malicious user from connecting an unauthorized PC to a switch port to inject or receive data from the network without detection.
Fix Text
Configure the switch to implement 802.1.x using EAP-TLS or PEAP-MSCHAPv2. Both implementations will encapsulate the EAP packets within a TLS tunnel and provide bidirectional authentication between supplicant and a RADIUS server.
Check Content
Review the switch configuration and verify that the 802.1x implementation is using bidirectional authentication between the supplicant and the authentication server that is cryptographically based such as EAP-TLS or PEAP-MSCHAPv2. If the switch is not using bidirectional authentication between the supplicant and the authentication server that is cryptographically based, this is a finding.
V-62199
The layer 2 switch must only allow a maximum of one registered MAC address per each user-facing or untrusted access port.
Discussion
Limiting the number of registered MAC addresses on a switch access port can help prevent a Content Addressable Memory (CAM) table overflow attack. This type of attack lets an attacker exploit the hardware and memory limitations of a switch. If there are enough entries stored in a CAM table before the expiration of other entries, no new entries can be accepted into the CAM table. An attacker will be able to flood the switch with mostly invalid MAC addresses until the CAM table’s resources have been depleted. When there are no more resources, the switch has no choice but to flood all ports within the VLAN with all incoming traffic. This happens because the switch cannot find the switch port number for a corresponding MAC address within the CAM table, allowing the switch to become a hub and traffic to be monitored.
Fix Text
Configure the switch to limit the maximum number of registered MAC addresses on each user-facing or untrusted access switch port to one.
Check Content
Review the switch configuration to verify each access port is configured for a single registered MAC address. Configuring port-security on the Cisco switch access port interface will automatically set the maximum number of registered MAC addresses to one. If any user-facing or untrusted switch port has more than one MAC address assigned to it, this is a finding. Exemptions: Some deployments are exempt from requiring a single MAC address per access switch port. VoIP or VTC endpoints may provide a PC port thereby enabling a PC to be connected using the same switch port. The MAC address of each device will need to be registered to the appropriate access switch port. Another exempt case scenario is “hot-desking”, where a single connection is shared among several devices and several people are assigned to work at the same desk at different times, each user with their own PC. In this case, a different MAC address needs to be permitted for each PC that is connecting to the LAN drop in the workspace.
V-100097
The layer 2 switch must be configured in accordance with the security configuration settings based on DoD security configuration or implementation guidance, including STIGs, NSA configuration guides, CTOs, and DTMs.
Discussion
Configuring the network device to implement organization-wide security implementation guides and security checklists ensures compliance with federal standards and establishes a common security baseline across DoD that reflects the most restrictive security posture consistent with operational requirements. Configuration settings are the set of parameters that can be changed that affect the security posture and/or functionality of the network device. Security-related parameters are those parameters impacting the security state of the network device, including the parameters required to satisfy other security control requirements.
Fix Text
Configure the switch to be configured in accordance with the security configuration settings based on DoD security configuration or implementation guidance, including STIGs, NSA configuration guides, CTOs, and DTMs.
Check Content
Determine if the switch is configured in accordance with the security configuration settings based on DoD security configuration or implementation guidance, including STIGs, NSA configuration guides, CTOs, and DTMs. If it is not configured in accordance with the designated security configuration settings, this is a finding.
V-95879
The layer 2 switch must have Storm Control configured on all host-facing switch ports.
Discussion
A traffic storm occurs when packets flood a LAN, creating excessive traffic and degrading network performance. Traffic storm control prevents network disruption by suppressing ingress traffic when the number of packets reaches a configured threshold levels. Traffic storm control monitors ingress traffic levels on a port and drops traffic when the number of packets reaches the configured threshold level during any one-second interval.
Fix Text
Configure storm control on each host-facing switch ports.
Check Content
Review the switch configuration to verify that storm control is enabled on host-facing interfaces. If storm control is not enabled on all host-facing switch ports, this is a finding.
V-95881
The layer 2 switch must have IGMP or MLD Snooping configured on all VLANs
Discussion
IGMP and MLD snooping provides a way to constrain multicast traffic at Layer 2. By monitoring the IGMP or MLD membership reports sent by hosts within a VLAN, the snooping application can set up Layer 2 multicast forwarding tables to deliver specific multicast traffic only to interfaces connected to hosts interested in receiving the traffic, thereby significantly reducing the volume of multicast traffic that would otherwise flood the VLAN.
Fix Text
Configure IGMP or MLD snooping for IPv4 and IPv6 multicast traffic respectively for each VLAN.
Check Content
Review the switch configuration to verify that IGMP or MLD snooping has been configured for IPv4 and IPv6 multicast traffic respectively. If the switch is not configured to implement IGMP or MLD snooping for each VLAN, this is a finding.
The layer 2 switch must uniquely identify all network-connected endpoint devices before establishing any connection.
Discussion
Controlling LAN access via 802.1x authentication can assist in preventing a malicious user from connecting an unauthorized PC to a switch port to inject or receive data from the network without detection.
Fix Text
Configure 802.1 x authentications on all host-facing access switch ports. To authenticate those devices that do not support 802.1x, MAC Authentication Bypass must be configured.
Check Content
Verify if the switch configuration has 802.1x authentication implemented for all access switch ports connecting to LAN outlets (i.e., RJ-45 wall plates) or devices not located in the telecom room, wiring closets, or equipment rooms. MAC Authentication Bypass (MAB) must be configured on those switch ports connected to devices that do not support an 802.1x supplicant. If 802.1x authentication or MAB is not configured on all access switch ports connecting to LAN outlets or devices not located in the telecom room, wiring closets, or equipment rooms, this is a finding.
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Discussion
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An example is a firewall that blocks all traffic rather than allowing all traffic when a firewall component fails (e.g., fail closed and do not forward traffic). This prevents an attacker from forcing a failure of the system in order to obtain access. Abort refers to stopping a program or function before it has finished naturally. The term abort refers to both requested and unexpected terminations.
Fix Text
Configure the layer 2 switch to fail to a secure state upon failure of initialization, shutdown, or abort actions.
Check Content
Review the vendor documentation to determine if the layer 2 switch will fail to a secure state in the event that the system initialization fails, shutdown fails, or abort fails. If the layer 2 switch does not fail to a secure state in the event that the system initialization fails, shutdown fails, or abort fails, this is a finding.
The layer 2 switch must have Root Guard enabled on all switch ports connectingwhere tothe accessroot layerbridge switchesshould andnot hostsappear.
Discussion
Spanning Tree Protocol (STP) does not provide any means for the network administrator to securely enforce the topology of the switched network. Any switch can be the root bridge in a network. However, a more optimal forwarding topology places the root bridge at a specific predetermined location. With the standard STP, any bridge in the network with a lower bridge ID takes the role of the root bridge. The administrator cannot enforce the position of the root bridge but can set the root bridge priority to 0 in an effort to secure the root bridge position. The root guard feature provides a way to enforce the root bridge placement in the network. If the bridge receives superior STP Bridge Protocol Data Units (BPDUs) on a root guard-enabled port, root guard moves this port to a root-inconsistent STP state and no traffic can be forwarded across this port while it is in this state. To enforce the position of the root bridge it is imperative that root guard is enabled on all ports where the root bridge should never appear.
Fix Text
Configure the switch to have Root Guard enabled on all switch ports connectingwhere tothe accessroot layerbridge switchesshould andnot hostsappear.
Check Content
Review the switch topology as well as the switch configuration to verify that Rootroot Guardguard is enabled on all switch ports connectingfacing tousers accessor layer switches andthat hostsare downstream from the root bridge.
If the switch has not enabled Root Guard on all switch ports connectingwhere tothe accessroot layerbridge switchesshould andnot hostsappear, this is a finding.
The layer 2 switch must have DHCP snooping for all user VLANs to validate DHCP messages from untrusted sources as well as rate-limit DHCP traffic.
Discussion
In an enterprise network, devices under administrative control are trusted sources. These devices include the switches, routers, and servers in the network. Host ports and unknown DHCP servers are considered untrusted sources. An unknown DHCP server on the network on an untrusted port is called a spurious DHCP server, any device (PC, Wireless Access Point) that is loaded with DHCP server enabled. The DHCP snooping feature determines whether traffic sources are trusted or untrusted. The potential exists for a spurious DHCP server to respond to DHCPDISCOVER messages before the real server has time to respond. DHCP snooping allows switches on the network to trust the port a DHCP server is connected to and not trust the other ports. The DHCP snooping feature validates DHCP messages received from untrusted sources and filters out invalid messages as well as rate-limits DHCP traffic from trusted and untrusted sources. DHCP snooping feature builds and maintains a binding database, which contains information about untrusted hosts with leased IP addresses, and it utilizes the database to validate subsequent requests from untrusted hosts. Other security features, such as IP Source Guard and Dynamic Address Resolution Protocol (ARP) Inspection (DAI), also use information stored in the DHCP snooping binding database. Hence, it is imperative that the DHCP snooping feature is enabled on all VLANs.
Fix Text
Configure the switch to have DHCP snooping for all user VLANs to validate DHCP messages from untrusted sources as well as rate-limit DHCP traffic.
Check Content
Review the switch configuration and verify that DHCP snooping is enabled on alla userper-VLAN VLANsbasis.
If the switch does not have DHCP snooping enabled for all user VLANs to validate DHCP messages from untrusted sources as well as rate-limit DHCP traffic, this is a finding.
Note: Enabling DHCP snooping on a range of VLANs is permissible.
The layer 2 switch must not have any access to switch ports assigned to the native VLAN.
Discussion
Double encapsulation can be initiated by an attacker who has access to a switch port belonging to the native VLAN of the trunk port. Knowing the victim’s MAC address and with the victim attached to a different switch belonging to the same trunk group, thereby requiring the trunk link and frame tagging, the malicious user can begin the attack by sending frames with two sets of tags. The outer tag that will have the attacker’s VLAN ID (probably the well-known and omnipresent default VLAN) is stripped off by the switch, and the inner tag that will have the victim’s VLAN ID is used by the switch as the next hop and sent out the trunk port.
Fix Text
Configure all access switch ports to a VLAN other than the native VLAN.
Check Content
Review the switch configurations and examine all access switch ports. Verify that they do not belong to the native VLAN. If any access switch ports have been assigned to the same VLAN ID as the native VLAN, this is a finding.
The layer 2 switch must be configured to disable non-essential capabilities.
Discussion
A compromised switch introduces risk to the entire network infrastructure as well as data resources that are accessible via the network. The perimeter defense has no oversight or control of attacks by malicious users within the network. Preventing network breaches from within is dependent on implementing a comprehensive defense-in-depth strategy, including securing each device connected to the network. This is accomplished by following and implementing all security guidance applicable for each node type. A fundamental step in securing each switch is to enable only the capabilities required for operation.
Fix Text
Remove unneeded services and functions from the switch. Removal is recommended since the service or function may be inadvertently enabled otherwise. However, if removal is not possible, disable the service or function.
Check Content
Review the switch configuration to determine if services or functions not required for operation, or not related to switch functionality, are enabled. If unnecessary services and functions are enabled on the switch, this is a finding.
The layer 2 switch must authenticate all VLAN Trunk Protocol (VTP) messages with a hash function using the most secured cryptographic algorithm available.
Discussion
VLAN Trunk Protocol (VTP) provides central management of VLAN domains, thus reducing administration in a switched network. When configuring a new VLAN on a VTP server, the VLAN is distributed through all switches in the domain. This reduces the need to configure the same VLAN everywhere. VTP pruning preserves bandwidth by preventing VLAN traffic (unknown MAC, broadcast, multicast) from being sent down trunk links when not needed, that is, there are no access switch ports in neighboring switches belonging to such VLANs. An attack can force a digest change for the VTP domain enabling a rogue device to become the VTP server, which could allow unauthorized access to previously blocked VLANs or allow the addition of unauthorized switches into the domain. Authenticating VTP messages with a cryptographic hash function can reduce the risk of the VTP domain's being compromised.
Fix Text
Configure the switch to authenticate all VLAN Trunk Protocol (VTP) messages with a hash function using the most secured cryptographic algorithm available.
Check Content
Review the switch configuration to verify if VTP is enabled. If VTP is enabled, verify that authentication has been configured. If VTP has been configured on the switch and is not authenticating VTP messages with a hash function using the most secured cryptographic algorithm available, this is a finding.
The layer 2 switch must manage excess bandwidth to limit the effects of packet flooding types of denial of service (DoS) attacks.
Discussion
Denial of service is a condition when a resource is not available for legitimate users. Packet flooding DDoS attacks are referred to as volumetric attacks and have the objective of overloading a network or circuit to deny or seriously degrade performance, which denies access to the services that normally traverse the network or circuit. Volumetric attacks have become relatively easy to launch by using readily available tools such as Low Orbit Ion Cannon or by using botnets. Measures to mitigate the effects of a successful volumetric attack must be taken to ensure that sufficient capacity is available for mission-critical traffic. Managing capacity may include, for example, establishing selected network usage priorities or quotas and enforcing them using rate limiting, Quality of Service (QoS), or other resource reservation control methods. These measures may also mitigate the effects of sudden decreases in network capacity that are the result of accidental or intentional physical damage to telecommunications facilities (such as cable cuts or weather-related outages).
Fix Text
Implement a QoS policy for traffic prioritization and bandwidth reservation. This policy must enforce the traffic priorities specified by the Combatant Commanders/Services/Agencies.
Check Content
Review the switch configuration to verify that QoS has been enabled to ensure that sufficient capacity is available for mission-critical traffic such as voice and enforce the traffic priorities specified by the Combatant Commanders/Services/Agencies. If the switch is not configured to implement a QoS policy, this is a finding.
The layer 2 switch must provide the capability for authorized users to select a user session to capture.
Discussion
Without the capability to select a user session to capture/record or view/hear, investigations into suspicious or harmful events would be hampered by the volume of information captured. The volume of information captured may also adversely impact the operation for the network. Session audits may include port mirroring, tracking websites visited, and recording information and/or file transfers.
Fix Text
Enable the feature or configure the switch so that it is capable of capturing ingress and egress packets from any designated switch port for the purpose of monitoring a specific user session.
Check Content
Verify that the switch is capable of capturing ingress and egress packets from any designated switch port for the purpose of monitoring a specific user session. If the switch is not capable of capturing ingress and egress packets from a designated switch port, this is a finding.
The layer 2 switch must provide the capability for authorized users to remotely view, in real time, all content related to an established user session from a component separate from the layer 2 switch.
Discussion
Without the capability to remotely view/hear all content related to a user session, investigations into suspicious user activity would be hampered. Real-time monitoring allows authorized personnel to take action before additional damage is done. The ability to observe user sessions as they are happening allows for interceding in ongoing events that after-the-fact review of captured content would not allow.
Fix Text
Enable the feature or configure the switch so that it is capable of capturing ingress and egress packets from any designated switch port for the purpose of monitoring a specific user session.
Check Content
Verify that the switch is capable of capturing ingress and egress packets from any designated switch port for the purpose of remotely monitoring a specific user session. If the switch is not capable of capturing ingress and egress packets from a designated switch port for the purpose of remotely monitoring a specific user session, this is a finding.
The layer 2 switch must authenticate all network-connected endpoint devices before establishing any connection.
Discussion
Without authenticating devices, unidentified or unknown devices may be introduced, thereby facilitating malicious activity. For distributed architectures (e.g., service-oriented architectures), the decisions regarding the validation of authentication claims may be made by services separate from the services acting on those decisions. In such situations, it is necessary to provide authentication decisions (as opposed to the actual authenticators) to the services that need to act on those decisions. This requirement applies to applications that connect either locally, remotely, or through a network to an endpoint device (including, but not limited to, workstations, printers, servers (outside a datacenter), VoIP Phones, and VTC CODECs). Gateways and SOA applications are examples of where this requirement would apply. Device authentication is a solution enabling an organization to manage devices. It is an additional layer of authentication ensuring only specific pre-authorized devices can access the system.
Fix Text
Configure 802.1 x authentications on all host-facing access switch ports. To authenticate those devices that do not support 802.1x, MAC Authentication Bypass must be configured.
Check Content
Verify if the switch configuration has 802.1x authentication implemented for all access switch ports connecting to LAN outlets (i.e., RJ-45 wall plates) or devices not located in the telecom room, wiring closets, or equipment rooms. MAC Authentication Bypass (MAB) must be configured on those switch ports connected to devices that do not provide an 802.1x supplicant. If 802.1x authentication or MAB is not on configured on all access switch ports connecting to LAN outlets or devices not located in the telecom room, wiring closets, or equipment rooms, this is a finding.
The layer 2 switch must have BPDU Guard enabled on all user-facing or untrusted access switch ports.
Discussion
If a rogue switch is introduced into the topology and transmits a Bridge Protocol Data Unit (BPDU) with a lower bridge priority than the existing root bridge, it will become the new root bridge and cause a topology change, rendering the network in a suboptimal state. The STP PortFast BPDU guard enhancement allows network designers to enforce the STP domain borders and keep the active topology predictable. The devices behind the ports that have STP PortFast enabled are not able to influence the STP topology. At the reception of BPDUs, the BPDU guard operation disables the port that has PortFast configured. The BPDU guard transitions the port into errdisable state and sends a log message.
Fix Text
Configure the switch to have BPDU Guard enabled on all user-facing or untrusted access switch ports.
Check Content
Review the switch configuration to verify that BPDU Guard is enabled on all user-facing or untrusted access switch ports. If the switch has not enabled BPDU Guard, this is a finding.
The layer 2 switch must have STP Loop Guard enabled on all non-designated STP switch ports.
Discussion
The Spanning Tree Protocol (STP) loop guard feature provides additional protection against STP loops. An STP loop is created when an STP blocking port in a redundant topology erroneously transitions to the forwarding state. In its operation, STP relies on continuous reception and transmission of BPDUs based on the port role. The designated port transmits BPDUs, and the non-designated port receives BPDUs. When one of the ports in a physically redundant topology no longer receives BPDUs, the STP conceives that the topology is loop free. Eventually, the blocking port from the alternate or backup port becomes a designated port and moves to a forwarding state. This situation creates a loop. The loop guard feature makes additional checks. If BPDUs are not received on a non-designated port and loop guard is enabled, that port is moved into the STP loop-inconsistent blocking state.
Fix Text
Configure the switch to have STP Loop Guard enabled globally or at a minimum on all non-designated STP switch ports.
Check Content
Review the switch configuration to verify that STP Loop Guard is enabled. If STP Loop Guard is not configured globally or on non-designated STP ports, this is a finding.
The layer 2 switch must have Unknown Unicast Flood Blocking (UUFB) enabled.
Discussion
Access layer switches use the Content Addressable Memory (CAM) table to direct traffic to specific ports based on the VLAN number and the destination MAC address of the frame. When a router has an Address Resolution Protocol (ARP) entry for a destination host and forwards it to the access layer switch and there is no entry corresponding to the frame's destination MAC address in the incoming VLAN, the frame will be sent to all forwarding ports within the respective VLAN, which causes flooding. Large amounts of flooded traffic can saturate low-bandwidth links, causing network performance issues or complete connectivity outage to the connected devices. Unknown unicast flooding has been a nagging problem in networks that have asymmetric routing and default timers. To mitigate the risk of a connectivity outage, the Unknown Unicast Flood Blocking (UUFB) feature must be implemented on all access layer switches. The UUFB feature will block unknown unicast traffic flooding and only permit egress traffic with MAC addresses that are known to exit on the port.
Fix Text
Configure the switch to have Unknown Unicast Flood Blocking (UUFB) enabled.
Check Content
Review the switch configuration to verify that UUFB is enabled on all access switch ports. If any access switch ports do not have UUFB enabled, this is a finding.
The layer 2 switch must have IP Source Guard enabled on all user-facing or untrusted access switch ports.
Discussion
IP Source Guard provides source IP address filtering on a Layer 2 port to prevent a malicious host from impersonating a legitimate host by assuming the legitimate host's IP address. The feature uses dynamic DHCP snooping and static IP source binding to match IP addresses to hosts on untrusted Layer 2 access ports. Initially, all IP traffic on the protected port is blocked except for DHCP packets. After a client receives an IP address from the DHCP server, or after static IP source binding is configured by the administrator, all traffic with that IP source address is permitted from that client. Traffic from other hosts is denied. This filtering limits a host's ability to attack the network by claiming a neighbor host's IP address.
Fix Text
Configure the switch to have IP Source Guard enabled on all user-facing or untrusted access switch ports.
Check Content
Review the switch configuration to verify that IP Source Guard is enabled on all user-facing or untrusted access switch ports. If the switch does not have IP Source Guard enabled on all untrusted access switch ports, this is a finding.
The layer 2 switch must have Dynamic Address Resolution Protocol (ARP) Inspection (DAI) enabled on all user VLANs.
Discussion
DAI intercepts Address Resolution Protocol (ARP) requests and verifies that each of these packets has a valid IP-to-MAC address binding before updating the local ARP cache and before forwarding the packet to the appropriate destination. Invalid ARP packets are dropped and logged. DAI determines the validity of an ARP packet based on valid IP-to-MAC address bindings stored in the DHCP snooping binding database. If the ARP packet is received on a trusted interface, the switch forwards the packet without any checks. On untrusted interfaces, the switch forwards the packet only if it is valid.
Fix Text
Configure the switch to have Dynamic Address Resolution Protocol (ARP) Inspection (DAI) enabled on all user VLANs.
Check Content
Review the switch configuration to verify that Dynamic Address Resolution Protocol (ARP) Inspection (DAI) feature is enabled on all user VLANs. If DAI is not enabled on all user VLANs, this is a finding.
The layer 2 switch must implement Rapid STP where VLANs span multiple switches with redundant links.
Discussion
Spanning Tree Protocol (STP) is implemented on bridges and switches to prevent layer 2 loops when a broadcast domain spans multiple bridges and switches and when redundant links are provisioned to provide high availability in case of link failures. Convergence time can be significantly reduced using Rapid STP (802.1w) instead of STP (802.1d), resulting in improved availability. Rapid STP should be deployed by implementing either Rapid Per-VLAN-Spanning-Tree (Rapid-PVST) or Multiple Spanning-Tree Protocol (MSTP), the latter scales much better when there are many VLANs.
Fix Text
Configure Rapid STP to be implemented at the access and distribution layers where VLANs span multiple switches.
Check Content
In cases where VLANs do not span multiple switches, it is a best practice to not implement STP. Avoiding the use of STP will provide the most deterministic and highly available network topology. If STP is required, then review the switch configuration to verify that Rapid STP has been implemented. If Rapid STP has not been implemented where STP is required, this is a finding.
The layer 2 switch must enable Unidirectional Link Detection (UDLD) to protect against one-way connections.
Discussion
In topologies where fiber optic interconnections are used, physical misconnections can occur that allow a link to appear to be up when there is a mismatched set of transmit/receive pairs. When such a physical misconfiguration occurs, protocols such as STP can cause network instability. UDLD is a layer 2 protocol that can detect these physical misconfigurations by verifying that traffic is flowing bidirectionally between neighbors. Ports with UDLD enabled periodically transmit packets to neighbor devices. If the packets are not echoed back within a specific time frame, the link is flagged as unidirectional and the interface is shut down.
Fix Text
Configure the switch to enable Unidirectional Link Detection (UDLD) to protect against one-way connections. Note: UDLD is a Cisco-proprietary protocol. However, other switch vendors, such as 3Com, Extreme, and D-Link, have similar functionality in their products, respectively: Device Link Detection Protocol (DLDP), Extreme Link Status Monitoring (ELSM), and D-Link Unidirectional Link Detection (DULD).
Check Content
If any of the switch ports have fiber optic interconnections with neighbors, review the switch configuration to verify that UDLD is enabled globally or on a per interface basis. If the switch has fiber optic interconnections with neighbors and UDLD is not enabled, this is a finding.
The layer 2 switch must have all trunk links enabled statically.
Discussion
When trunk negotiation is enabled via Dynamic Trunk Protocol (DTP), considerable time can be spent negotiating trunk settings (802.1q or ISL) when a node or interface is restored. While this negotiation is happening, traffic is dropped because the link is up from a layer 2 perspective. Packet loss can be eliminated by setting the interface statically to trunk mode, thereby avoiding dynamic trunk protocol negotiation and significantly reducing any outage when restoring a failed link or switch.
Fix Text
Configure the switch to enable trunk links statically.
Check Content
Review the switch configuration to verify that trunk negotiation is disabled by statically configuring all trunk links. Configuring a command to manually disable negotiation may also be required for some switch platforms. If trunk negotiation is enabled on any interface, this is a finding.
The layer 2 switch must have all disabled switch ports assigned to an unused VLAN.
Discussion
It is possible that a disabled port that is assigned to a user or management VLAN becomes enabled by accident or by an attacker and as a result gains access to that VLAN as a member.
Fix Text
Assign all switch ports not in use to an inactive VLAN. Note: Switch ports configured for 802.1x are exempt from this requirement.
Check Content
Review the switch configurations and examine all access switch ports. Each access switch port not in use should have membership to an inactive VLAN that is not used for any purpose and is not allowed on any trunk links. If there are any access switch ports not in use and not in an inactive VLAN, this is a finding. Note: Switch ports configured for 802.1x are exempt from this requirement.
The layer 2 switch must not have the default VLAN assigned to any host-facing switch ports.
Discussion
In a VLAN-based network, switches use the default VLAN (i.e., VLAN 1) for in-band management and to communicate with other networking devices using Spanning-Tree Protocol (STP), Dynamic Trunking Protocol (DTP), VLAN Trunking Protocol (VTP), and Port Aggregation Protocol (PAgP)—all untagged traffic. As a consequence, the default VLAN may unwisely span the entire network if not appropriately pruned. If its scope is large enough, the risk of compromise can increase significantly.
Fix Text
Remove the assignment of the default VLAN from all access switch ports.
Check Content
Review the switch configurations and verify that no access switch ports have been assigned membership to the default VLAN (i.e., VLAN 1). A good method of ensuring there is not membership to the default VLAN is to have it disabled (i.e., shutdown) on the switch. This technique does not prevent switch control plane protocols such as CDP, DTP, VTP, and PAgP from using the default VLAN. If there are access switch ports assigned to the default VLAN, this is a finding.
The layer 2 switch must have the default VLAN pruned from all trunk ports that do not require it.
Discussion
The default VLAN (i.e., VLAN 1) is a special VLAN used for control plane traffic such as Spanning-Tree Protocol (STP), Dynamic Trunking Protocol (DTP), VLAN Trunking Protocol (VTP), and Port Aggregation Protocol (PAgP). VLAN 1 is enabled on all trunks and ports by default. With larger campus networks, care needs to be taken about the diameter of the STP domain for the default VLAN. Instability in one part of the network could affect the default VLAN, thereby influencing control-plane stability and therefore STP stability for all other VLANs.
Fix Text
Best practice for VLAN-based networks is to prune unnecessary trunk links from gaining access to the default VLAN and to ensure that frames belonging to the default VLAN do not traverse trunks not requiring frames from the VLAN.
Check Content
Review the switch configuration and verify that the default VLAN is pruned from trunk links that do not require it. If the default VLAN is not pruned from trunk links that should not be transporting frames for the VLAN, this is a finding.
The layer 2 switch must not use the default VLAN for management traffic.
Discussion
Switches use the default VLAN (i.e., VLAN 1) for in-band management and to communicate with directly connected switches using Spanning-Tree Protocol (STP), Dynamic Trunking Protocol (DTP), VLAN Trunking Protocol (VTP), and Port Aggregation Protocol (PAgP)—all untagged traffic. As a consequence, the default VLAN may unwisely span the entire network if not appropriately pruned. If its scope is large enough, the risk of compromise can increase significantly.
Fix Text
Configure the switch for management access to use a VLAN other than the default VLAN.
Check Content
Review the switch configuration and verify that the default VLAN is not used to access the switch for management. If the default VLAN is being used to access the switch, this is a finding.
The layer 2 switch must have all user-facing or untrusted ports configured as access switch ports.
Discussion
Double encapsulation can be initiated by an attacker who has access to a switch port belonging to the native VLAN of the trunk port. Knowing the victim's MAC address and with the victim attached to a different switch belonging to the same trunk group, thereby requiring the trunk link and frame tagging, the malicious user can begin the attack by sending frames with two sets of tags. The outer tag that will have the attacker's VLAN ID (probably the well-known and omnipresent default VLAN) is stripped off by the switch, and the inner tag that will have the victim's VLAN ID is used by the switch as the next hop and sent out the trunk port.
Fix Text
Disable trunking on all user-facing or untrusted switch ports.
Check Content
Review the switch configurations and examine all user-facing or untrusted switch ports. If any of the user-facing switch ports are configured as a trunk, this is a finding.
The layer 2 switch must have the native VLAN assigned to an ID other than the default VLAN for all 802.1q trunk links.
Discussion
VLAN hopping can be initiated by an attacker who has access to a switch port belonging to the same VLAN as the native VLAN of the trunk link connecting to another switch that the victim is connected to. If the attacker knows the victim’s MAC address, it can forge a frame with two 802.1q tags and a layer 2 header with the destination address of the victim. Since the frame will ingress the switch from a port belonging to its native VLAN, the trunk port connecting to the victim’s switch will simply remove the outer tag because native VLAN traffic is to be untagged. The switch will forward the frame on to the trunk link unaware of the inner tag with a VLAN ID of which the victim’s switch port is a member.
Fix Text
To ensure the integrity of the trunk link and prevent unauthorized access, the ID of the native VLAN of the trunk port must be changed from the default VLAN (i.e., VLAN 1) to its own unique VLAN ID. The native VLAN ID must be the same on both ends of the trunk link; otherwise, traffic could accidentally leak between broadcast domains. Note: An alternative to configuring a dedicated native VLAN is to ensure that all native VLAN traffic is tagged. This will mitigate the risk of VLAN hopping since there will always be an outer tag for native traffic as it traverses an 802.1q trunk link.
Check Content
Review the switch configurations and examine all trunk links. Verify the native VLAN has been configured to a VLAN ID other than the ID of the default VLAN (i.e. VLAN 1). If the native VLAN has the same VLAN ID as the default VLAN, this is a finding.