Dual Router Mode, DRM Operation | Cisco Systems 6503 specification

Dual Router Mode

DRM is the original MSFC conﬁguration for redundant supervisor engine or MSFC conﬁgurations. In this mode, both MSFCs are active routers on the network. Having two active MSFCs in a single chassis does not mean having two separate routers. 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 mechanism between MSFCs in DRM is the Hot Standby Routing Protocol (HSRP). HSRP allows the two MSFCs 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 is responsible for programming certain functions of the ASIC hardware on the PFCx. The ﬁrst MSFC to go online is considered the designated router and the second MSFC is considered the nondesignated router. In a supervisor engine 1A system, both the designated router and the nondesignated router are able to program Layer 3 entries into the PFC Netﬂow 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

Beginning with the MSFC Cisco IOS Software Release 12.1(3a)E4, an MSFC redundancy feature called conﬁg-sync has 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 between the designated (primary) and nondesignated (secondary) MSFCs are synchronized. Speciﬁcally, when a write memory or copy <source> startup-conﬁgcommand is issued on the designated MSFC, the startup conﬁgurations in NVRAM of both MSFCs are updated. This allows the conﬁgurations on the designated and nondesignated MSFCs to maintain the same conﬁguration without having to manually type each command twice.

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