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Dual Router Mode
DRM istheoriginal MSFCconfiguration forredundant supervisorengine orMSFC configurations.In thismode, bothMSFCs
are active routers on the network. Having two active MSFCs ina single chassis does not mean having two separaterouters. In
fact, both MSFCs must have a nearly identical configuration, as described below in more detail. The main idea for DRM is
that each MSFC independently builds an accurate picture of the Layer 3 network.
DRM Operation
The failover mechanismbetween MSFCsinDRM isthe HotStandby RoutingProtocol (HSRP).HSRPallowsthe twoMSFCs
to maintain internal communication and react to an MSFC failover. HSRP needs to be configured on both MSFCs for each
VLAN where first hop default gateway redundancy is required. Internal HSRP between MSFCs works in the same manner as
HSRP between physically separate devices by sending hello messages between the routing engines. For more information
about configuring HSRP, see the Cisco IOS Software Configuration Guides at:
http://www/univercd/cc/td/doc/product/software/ios121/121cgcr/ip_c/ipcprt1/1cdip.htm - xtocid26
and
http://www/univercd/cc/td/doc/product/lan/cat6000/sw_7_3/confg_gd/redund.htm
Because both MSFCs have independent routing tables, there is little routing protocol convergence necessary in the event of
an MSFC failure. With DRM and based on HSRP timers, the MSFC failover can be configured to less than three seconds for
LAN interfaces, thus aligning the Layer 3 failover of the MSFC with the supervisor engine failover time.
Because each MSFC has the potential for taking over for the other one, they need to maintain identical configurations. This
is an extremely important point to understand in DRM. Configuration parameters such as interfaces, access lists, policy
routing, etc. must be configured exactly the same on both MSFCs. Parameters that cannot be duplicated on a network such as
IP addresses and HSRP settings are the only parameters that are configured differently on each MSFC.
The MSFC isresponsible for programmingcertain functions of theASIC hardware on the PFCx.The first MSFCto go online
is considered the designated router and the second MSFC is considered the nondesignated router. In a supervisor engine 1A
system, boththedesignated routerand thenondesignated routerare ableto programLayer 3entries intothe PFCNetflow table
for routing functions. In a supervisor engine 2 system, only the designated router programs the Layer 3 entries in the PFC2s
Cisco Express Forwarding table. For both a supervisor engines 1A and 2, all router ACLs and multicast shortcuts are
programmed from the designated router. As you can see, the requirement for each MSFC to have an identical configuration
is a necessity. If the MSFCs in DRM have different configurations, the forwarding ASICs will be programmed incorrectly
resulting in unexpected behavior.
MSFC Configuration Synchronization
Beginningwith theMSFC CiscoIOS SoftwareRelease 12.1(3a)E4,an MSFCredundancy featurecalled config-synchas been
available to streamline the redundant MSFC configuration process for both MSFC and MSFC2. This feature can be used to
simplify configuration of the two MSFCs and to ensure that the MSFC configurations match. Both the startup and running
configurations betweenthedesignated (primary)and nondesignated(secondary) MSFCsare synchronized.Specifically,when
a write memory orcopy <source> startup-config command isissued on thedesignated MSFC, thestartup configurations in
NVRAMof bothMSFCs areupdated. Thisallows theconfigurations onthedesignated andnondesignated MSFCstomaintain
the same configuration without having to manually type each command twice.
