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.