Dual Router Mode
Dual Router Mode (DRM) represents the original MSFC
high-availability option for dual Supervisor engines with MSFCs. In dual router
mode, both MSFCs are active routers on the network. Although both MSFCs are
active and can be configured independently, they are not to be used as
independent routers. In reality, both MSFCs must have nearly identical
configurations to function properly. The importance of requirement in DRM cannot
be overstated. Configuration parameters such as interfaces, access lists, policy
routing, and so on 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 first MSFC to go online is considered the designated
router, and the second MSFC is considered the nondesignated router. The MSFC is
responsible for programming certain functions of the ASIC hardware on the PFC.
In a Supervisor Engine IA system, both the designated router and the
nondesignated router are able to program Layer 3 entries into the PFC Netflow
table for routing functions. With the Supervisor IIs, only the designated router
programs the Layer 3 entries in the PFC2 Cisco Express Forwarding (CEF) table.
For both Supervisor Engines IA and II, all router ACLs and multicast shortcuts
are programmed from the designated router. If the MSFCs in DRM have different
configurations, the forwarding ASICs will be programmed incorrectly, resulting
in an unsupported and unreliable configuration.
Failover in DRM relies upon HSRP, which allows the two MSFCs to
maintain internal communication and react to an MSFC failover. HSRP on the dual
MSFCs is configured in the same way as any two independent routers. Because both
MSFCs have independent routing tables, little routing protocol convergence is
necessary in the event of an MSFC failure. Using DRM and tuned HSRP timers, MSFC
failover can be configured to occur in less than 3 seconds for LAN interfaces,
matching the Layer 3 failover of the MSFC with Supervisor engine failover
time.
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