You can achieve the same with a Cisco router by using Policy Based Routing (PBR). PBR overrides routing decisions based on the IP routing table with policy routing. I assume you have a single LAN interface and IP address and two serial interfaces and IP addresses:

LAN Interface:

ip policy route-map POLICY

-> you apply PBR to the interface

route-map POLICY permit 10

match ip address HighPrio

set ip default next-hop 1stSerial 2nd Serial

-> you define your policy in a route map. You match your high priority traffic and define the IP next-hop as the 1st serial interface (or IP@). You define the 2nd serial interface as backup next hop.

ip access-list HighPrio

permit ip source IP@ any eq source port

-> you describe your high priority traffic using an access list whre you specify the source IP address of your high priority traffic and the port of your high priority traffic.

Please verify the mentioned commands in the configuration guides for PBR and access-list, but it should help to clarify the logic.

In my view, PBR is often not the best solution of these situations, but you houls think of implementing Quality of Service instead (see a previous conversation where the same issue was discussed: http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40.2cc229a4/5#selected_message

HTH, Thomas