| > 2) When one of the two serial ports is used for leased-line, the other
> port can support dial up for backup. When one of the two serial ports
> is used for X.25 or Frame Relay, how about the other ports? Can it be
> used as dial up for backup? If it can, what signal or parameter it will
> detect when the router shift its backup ports to active.
The problem is that a dialup line is a point to point link, and the
primary X.25/FR network is a pseudo-broadcast circuit (it's actually
modelled that way in the code). So you cannot directly use Wan
Restoral (link layer backup) or Wan Reroute (network layer backup) on
these types of links. In the special case of PPP over FR, you can use
WAN Reroute because the PPP over FR circuit is a point to point link.
The other option is static routes, which can work passably well in
these situations, especially when OSPF is used on the primary (link
failure detection times are quite speedy with link state protocols).
So you would put a high-cost static route on the dialup line, and it
would only come into use when the primary OSPF link failed. Obviously
the topology can be tricky (consider the case of the whole FR network
going down, vs. the loss of a single adjacency).
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