The Cisco BGP switch must be configured to enable the Generalized TTL Security Mechanism (GTSM).

Severity	Group ID	Group Title	Version	Rule ID	Date	STIG Version
low	V-221021	SRG-NET-000362-RTR-000124	CISC-RT-000470	SV-221021r856414_rule	2024-06-06	3

Description
As described in RFC 3682, GTSM is designed to protect a switch's IP-based control plane from DoS attacks. Many attacks focused on CPU load and line-card overload can be prevented by implementing GTSM on all Exterior Border Gateway Protocol-speaking switches. GTSM is based on the fact that the vast majority of control plane peering is established between adjacent switches; that is, the Exterior Border Gateway Protocol peers are either between connecting interfaces or between loopback interfaces. Since TTL spoofing is considered nearly impossible, a mechanism based on an expected TTL value provides a simple and reasonably robust defense from infrastructure attacks based on forged control plane traffic.

Description

As described in RFC 3682, GTSM is designed to protect a switch's IP-based control plane from DoS attacks. Many attacks focused on CPU load and line-card overload can be prevented by implementing GTSM on all Exterior Border Gateway Protocol-speaking switches. GTSM is based on the fact that the vast majority of control plane peering is established between adjacent switches; that is, the Exterior Border Gateway Protocol peers are either between connecting interfaces or between loopback interfaces. Since TTL spoofing is considered nearly impossible, a mechanism based on an expected TTL value provides a simple and reasonably robust defense from infrastructure attacks based on forged control plane traffic.

ℹ️ Check
Review the BGP configuration to verify that TTL security has been configured for each external neighbor as shown in the example below: router bgp xx no synchronization bgp log-neighbor-changes neighbor x.1.1.9 remote-as yy neighbor x.1.1.9 password xxxxxxxx neighbor x.1.1.9 ttl-security hops 1 neighbor x.2.1.7 remote-as zz neighbor x.2.1.7 password xxxxxxxx neighbor x.2.1.7 ttl-security hops 1 If the switch is not configured to use GTSM for all Exterior Border Gateway Protocol peering sessions, this is a finding.

ℹ️ Check

Review the BGP configuration to verify that TTL security has been configured for each external neighbor as shown in the example below: router bgp xx no synchronization bgp log-neighbor-changes neighbor x.1.1.9 remote-as yy neighbor x.1.1.9 password xxxxxxxx neighbor x.1.1.9 ttl-security hops 1 neighbor x.2.1.7 remote-as zz neighbor x.2.1.7 password xxxxxxxx neighbor x.2.1.7 ttl-security hops 1 If the switch is not configured to use GTSM for all Exterior Border Gateway Protocol peering sessions, this is a finding.

✔️ Fix
Configure TTL security on all external BGP neighbors as shown in the example below: SW1(config)#router bgp xx SW1(config-switch)#neighbor x.1.1.9 ttl-security hops 1 SW1(config-switch)#neighbor x.2.1.7 ttl-security hops 1