UCF STIG Viewer Logo

The Cisco MPLS switch must be configured to have TTL Propagation disabled.


Finding ID Version Rule ID IA Controls Severity
V-221036 CISC-RT-000620 SV-221036r622190_rule Medium
The head end of the label-switched path (LSP), the label edge switch (LER) will decrement the IP packet's time-to-live (TTL) value by one and then copy the value to the MPLS TTL field. At each label-switched switch (LSR) hop, the MPLS TTL value is decremented by one. The MPLS switch that pops the label (either the penultimate LSR or the egress LER) will copy the packet's MPLS TTL value to the IP TTL field and decrement it by one. This TTL propagation is the default behavior. Because the MPLS TTL is propagated from the IP TTL, a traceroute will list every hop in the path, be it routed or label switched, thereby exposing core nodes. With TTL propagation disabled, LER decrements the IP packet's TTL value by one and then places a value of 255 in the packet's MPLS TTL field, which is then decremented by one as the packet passes through each LSR in the MPLS core. Because the MPLS TTL never drops to zero, none of the LSP hops triggers an ICMP TTL exceeded message, and consequently, these hops are not recorded in a traceroute. Hence, nodes within the MPLS core cannot be discovered by an attacker.
Cisco IOS XE Switch RTR Security Technical Implementation Guide 2023-09-13


Check Text ( C-22751r408902_chk )
Review the switch configuration to verify that TTL propagation is disabled as shown in the example below:

no mpls ip propagate-ttl

If the MPLS switch is not configured to disable TTL propagation, this is a finding.
Fix Text (F-22740r408903_fix)
Configure the MPLS switch to disable TTL propagation as shown in the example below:

SW1(config)#no mpls ip propagate-ttl