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3Com Router 3000 Ethernet Family
Configuration Guide
Chapter 2 Dynamic Routing Standby
2.1 Introduction to Dynamic Routing Standby
Dynamic routing standby provides routing-based dial backup. It uses legacy DCC,
including both C-DCC and RS-DCC, to maintain dial links dynamically.
Taking advantage of convergence time and related features of dynamic routing
protocols, dynamic routing standby perfectly combines backup and routing. It breaks
through the limitation of interesting packet-triggered dial in legacy DCC and provides
reliable connections and standard dial-on-demand services.
Dynamic routing standby is an enhancement to legacy DCC backup. With it, the
primary and secondary interfaces can be of any type, because its backup is dynamic
routing based rather than interface-specific or link-specific. It is thus suitable for
situations where multiple interfaces and routers are involved.
Dynamic routing standby does not depend on a particular routing protocol. It can run
multiple dynamic routing protocols such as RIP1, RIP2, and SOPF.
It does not depend on interesting packets to trigger dial; thus the secondary link starts
automatically when the primary link disconnects without dialing delay (excluding route
convergence time).
By configuring a set of destination IP addresses carried on the primary link, dynamic
routing standby can monitor the changes of the routes on the primary link. The following
describes how it monitors routes and starts the secondary link:
Dynamic routing standby registers a network segment (IP address range)
requiring observation with the system;
The system listens in on the observed network segment for any updates. If the
route has been deleted, dynamic routing standby looks up the routing table for a
valid route to the observed network segment;
If a valid route is available and if this route sets out from an interface other than the
one configured with the standby routing-group command, the primary link is
regarded UP.
If no valid route is available, the primary link is regarded disabled and unavailable.
Dynamic routing standby immediately informs the routing protocol to trigger dial
right away to start the secondary link.
Once the secondary link (dialup link) is enabled, communication data is switched over
to it. When the primary link is available again, the system adds the route destined for
the monitored segment, which is carried by the primary link. After that, the system can
disconnect the secondary link right away or enable a "disable" timer for the route. In the
latter case, if the route is still available when the timer expires, the system disconnects
3Com Corporation
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Chapter 2 Dynamic Routing Standby
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