HP MSR2000/3000/4000 Router Series Layer 2 - WAN Configuration Guide

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Configuring dynamic route backup through DDR
The dynamic route backup function is available on the following dialup interfaces: dialer interfaces, PRI
interfaces, BRI interfaces, serial interfaces operating in asynchronous mode, AM interfaces, AUX
interfaces, and asynchronous interfaces.
Introduction to dynamic route backup
The dynamic route backup function employs DDR to dynamically maintain dialup links. It can provide
backup for dialup links based on routes.
The dynamic route backup function combines the backup function and the routing function. It provides
reliable connections and standard dial-on-demand services.
Dynamic route backup features
The dynamic route backup function is mainly used to back up dynamic routes. Moreover, it can also back
up static routes and directly connected routes.
The dynamic route backup function is suitable for scenarios with multiple interfaces and multiple routers.
It is not dedicated to a specific interface or link.
With dynamic route backup enabled, the backup link will be activated automatically when the primary
link fails. The primary-backup switchover does not incur dialup delay (the route convergence time is not
counted in).
The dynamic route backup function is routing protocol-independent. It can collaborate with the RIPv1,
RIPv2, OSPF, IS-IS, and BGP. However, some routing protocols (such as BGP) use the optimal routes by
default. Therefore, with BGP employed, when the backup link is activated due to a failure of the primary
link to the monitored network segment, the device will learn a route to the monitored network segment
through BGP. When the primary link recovers, the device will also learn a route to the monitored network
segment through BGP. However, the route learned for the primary link might be less optimal than that
learned for the backup link. As a result, the route learned for the backup link remains activated, dynamic
route monitoring fails, and the switchover from backup link to primary link fails.
To address this problem, do the following:
• Make sure the IP address assigned to the backup link is greater than that assigned to the primary
link.
• Make sure the same route can be learned for multiple links (which can be achieved through load
balancing configuration).
How dynamic route backup works
The dynamic route backup function is implemented by using dynamic route backup groups. In a dynamic
route backup group, the backup link is activated when the primary link leading to the monitored network
segment fails.
Dynamic route backup operates in the following workflow:
• The system monitors whether the routes to the monitored network segment are updated and checks
whether at least one route to the monitored network segment exists.
• If at least one route to the monitored network segment exists, and the route is originated from an
interface with dynamic route backup disabled, the primary link is considered to be operating
correctly.