From Cisco IOS XE Release 3 RTR Security Technical Implementation Guide
Part of SRG-NET-000191-RTR-000081
Associated with: CCI-002385
As described in RFC 3682, Generalized TTL Security Mechanism (GTSM) is designed to protect a router'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 routers. GTSM is based on the fact that the vast majority of control plane peering is established between adjacent routers; 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.
Review the Cisco ISR 4000 Series router configuration and verify that the neighbor command "ttl-security" is configured for all eBGP peering sessions. The configuration would look similar to the following: router bgp 100 neighbor 10.1.1.1 remote-as 222 neighbor 10.1.1.1 ttl-security hops 1 If the "ttl-security" command is not configured for all eBGP peering sessions, this is a finding.
Configure all eBGP neighbors with GTSM. The configuration would look similar to the following: router bgp 100 neighbor 10.1.1.1 remote-as 222 neighbor 10.1.1.1 ttl-security hops 1
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