Configuring Load Sharing Network Address Translation (LSNAT)
19-2 LSNAT Configuration
•WhendifferentvirtualserverIPs(VIPs)sharethesamerealserverindifferentserverfarms,
thepersistencelevelmustbesetthesame.
•Ingeneral,inordertoeditordeleteavirtualserverorrealserver(serverfarm)configuration,
thedevicesmustbefirstconfigured“outofservice”(noinservice)beforethechangeswillbe
allowed.

Session Persistence

LoadbalancingclientsconnecttoavirtualIPaddresswhich,inreality,isredirectedtooneof
severalphysicalserversinaloadbalancingserverfarmgroup.Inmanywebpagedisplay
applications,aclientmayhaveitsrequestsredirectedtoandservicedbydifferentserversinthe
group.Incertainsituations,however,itmaybecriticalthatalltrafficfortheclientbedirectedto
thesamephysicalserverforthedurationofthesession—thisistheconceptofsessionpersistence.
Whentherouterreceivesanewsessionrequestfromaclientforaspecificvirtualaddress,the
routercreatesabindingbetweentheclient(source)IPaddress/portsocketandthe(destination)IP
address/portsocketoftheloadbalancingserverselectedforthisclient.Subsequentpacketsfrom
clientsarecomparedtothelistofbindings.Ifthereisamatch,thepacketissenttothesameserver
previouslyselectedforthisclient.Ifthereisnotamatch,anewbindingiscreated.Howtherouter
determinesthebindingmatchforsessionpersistenceisconfiguredwiththepersistencelevel
commandwhenthevirtualserveriscreated.
Therearethreeconfigurablelevelsofsessionpersistence:
TCPpersistenceabindingisdeterminedbythematchingthesourceIP/portaddressas
wellasthevirtualdestinationIP/portaddress.Forexample,requestsfromtheclientaddress
of134.141.176.10:1024tothevirtualdestinationaddress207.135.89.16:80isconsideredone
sessionandwouldbedirectedtothesameloadbalancingserver(forexample,theserverwith
IPaddress10.1.1.1).Arequestfromadifferentsourcesocketfromthesameclientaddressto
thesamevirtualdestinationaddresswouldbeconsideredanothersessionandmaybe
directedtoadifferentloadbalancingserver(forexample,theserverwithIPaddress10.1.1.2).
Thisisthedefaultlevelofsessionpersistence.
SSLpersistenceabindingisdeterminedbymatchingthesourceIPaddressandthevirtual
destinationIP/portaddress.NotethatrequestsfromanysourcesocketwiththeclientIP
addressareconsideredpartofthesamesession.Forexample,requestsfromtheclientIP
addressof134.141.176.10:1024or134.141.176.10:1025tothevirtualdestinationaddress
207.135.89.16:80wouldbeconsideredonesessionandwouldbedirectedtothesameload
balancingserver(forexample,theserverwithIPaddress10.1.1.1).
StickypersistenceabindingisdeterminedbymatchingthesourceanddestinationIP
addressesonly.Thisallowsallrequestsfromaclienttothesamevirtualaddresstobedirected
tothesameloadbalancingserver.Forexample,bothHTTPandHTTPSrequestsfromthe
clientaddress134.141.176.10tothevirtualdestinationaddress207.135.89.16wouldbe
directedtothesameloadbalancingserver(forexample,theserverwithIPaddress10.1.1.1).

Sticky Persistence Configuration Considerations

Stickypersistencefunctionalityprovideslesssecuritybutthemostflexiblecapabilityforusersto
loadbalanceallservicesthroughavirtualIPaddress.Inaddition,thisfunctionalityprovides
betterresourceusagebytheLSNATrouter,aswellasbetterperformanceforthesameclients
tryingtoreachthesamerealserversacrossdifferentservicesthroughavirtualserver.
Forexample,withstickypersistence,HTTP,HTTPS,TELNETandSSHrequestsfromaclient
(200.1.1.1)tothevirtualserveraddress(192.168.1.2)wouldallbedirectedtothesamerealserver.
Theclientalwaysgoestothesamerealserverforalltheservicesprovidedbythatserver,andit