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


Finding ID Version Rule ID IA Controls Severity
V-221021 CISC-RT-000470 SV-221021r856414_rule Low
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.
Cisco IOS XE Switch RTR Security Technical Implementation Guide 2023-09-13


Check Text ( C-22736r408857_chk )
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 Text (F-22725r408858_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