Specifications

Tracing a Route
RoamAbout Mobility System Software Configuration Guide 5-29
Tracing a Route
YoucantracetherouterhopsnecessarytoreachanIPhost.
ThetraceroutefacilityusestheTTL(TimetoLive)fieldintheIPheadertocauseroutersand
serverstogeneratespecificreturnmessages.TraceroutestartsbysendingaUDPdatagramtothe
destinationhostwiththeTTLfield
setto1.IfarouterfindsaTTLvalueof1or0,itdropsthe
datagramandsendsbackanICMPTimeExceededmessagetothesender.
Thetraceroutefacilitydeterminestheaddressofthefirsthopbyexaminingthesourceaddress
fieldoftheICMPtime‐exceededmessage.
Toidentifythenexthop,tracerouteagainsendsaUDPpacket,butthistimewithaTTLvalueof2.
ThefirstrouterdecrementstheTTLfieldby1andsendsthedatagramtothenextrouter.The
secondrouterseesaTTLvalueof1,discardsthedatagram,andreturns
theTimeExceededmessage
tothesource.ThisprocesscontinuesuntiltheTTLisincrementedtoavaluelargeenoughfor the
datagramtoreachthedestinationhost(oruntilthemaximumTTLisreached).
Todeterminewhenadatagramhasreacheditsdestination,traceroutesetstheUDPdestination
portin
thedatagramtoaverylarg evalue,onethatthedestinationhostisunlikelytobeusing.In
addition,whenahostreceivesadatagramwithanunrecognizedportnumber,itsendsanICMP
PortUnreachableerrortothesource.Thismessageindicatestothetraceroutefacilitythatithas
reachedthedestination.
Totracearoutetoadestinationsubnet,usethefollowingcommand:
traceroute host [dnf] [no-dns] [port port-num] [queries num] [size size]
[ttl hops] [wait ms]
Example
Totracetheroutetohostserver1,typethefollowingcommand:
RBT-8100# traceroute server1
traceroute to server1.example.com (192.168.22.7), 30 hops max, 38 byte packets
1 engineering-1.example.com (192.168.192.206) 2 ms 1 ms 1 ms
2 engineering-2.example.com (192.168.196.204) 2 ms 3 ms 2 ms
3 gateway_a.example.com (192.168.1.201) 6 ms 3 ms 3 ms
4 server1.example.com (192.168.22.7) 3 ms * 2 ms
Inthisexample,server1isfourhopsaway.Thehopsarelistedinorder,beginningwiththehop
thatisclosesttotheRoamAboutSwitchandendingwiththeroute’sdestination.Forinformation
aboutthecommandoptions,seetheRoamAboutMobilitySystemSoftwareCommandLineInterface
Reference.