When vRtrIfIcmpTtlExpired is set to 'true', the rate that
ttl-expired messages by this interface is throttled based on
the values of vRtrIfIcmpNumTtlExpired and vRtrIfIcmpTtlExpiredTime.
When vRtrIfIcmpTtlExpired is set to 'false', unlimited ttl-expired
messages are sent.