Implementing Hardware and Software Redundancy on Modular Switches
What is Single Router Mode – SRM?

In SRM redundancy, only the designated router (MSFC) is visible to the network at any given time. Dual Router Mode (DRM) had both MFSCs active and used HSRP to maintain an active and secondary relationship. DRM had the problem of extra complexity and routing protocol peering, which is overcome by using SRM. The non-designated router is booted up completely and participates in configuration synchronization, which is automatically enabled when entering SRM. The configuration of the non-designated router is exactly the same as the designated router, but its interfaces are kept in a "line down" state and are not visible to the network. Processes, such as routing protocols, are created on the non-designated router and the designated router, but all non-designated router interfaces are in a "line down" state; they do not send or receive updates from the network.

When the designated router fails, the non-designated router changes its state to become the designated router and its interface state changes to "link up." It builds its routing table while the existing Supervisor engine switch processor entries are used to forward Layer 3 traffic. After the newly designated router builds its routing table, the entries in the switch processor are updated.

Because only one MSFC is visible to the network at a given time, multiple BGP peering sessions do not have to exist between two MSFCs. In the event of a failure of the designated MSFC, the non-designated MSFC re-establishes BGP peering. Therefore, it always appears as a single BGP peer to the network and simplifies the network design, but it gives the same level of redundancy in case an MSFC has a failure.