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Routing & Switching · Advanced

Generalized TTL Security Mechanism (GTSM)

Protection that accepts BGP packets only if their TTL is at or near 255, blocking spoofed sessions from distant attackers.

Definition

Peers send with TTL 255; the receiver drops packets with TTL below 255 minus the allowed hop count. Remote attackers cannot forge packets that arrive with such a high TTL. Defined in RFC 5082 and configured with ttl-security hops on Cisco.

Example

neighbor 203.0.113.1 ttl-security hops 1

Exam tip

ttl-security and ebgp-multihop are mutually exclusive on the same neighbor.

Related terms