Questions on OSPF

We have a WAN that consists of 4 locations. Site A has 2 T1 connections to the Internet and there is a 3825 router running BGP to handle the connections. Behind it are 2 PIX 515e's in failover mode. Site B has a connection to Site A using SBC's Gigaman line, which is a Gigabit fiber connection. The main route for Internet is through Site A. Site B has a

3825 as it's core router. Site B also has a DSL connection that is being used by consultants, which is connected by a Cisco 871 router. Site C has a T1 line to the Internet, connected through a 2600 router and a T3 line to Site B, connected through a 3825 router. Also, Site C is connected to Site A through a PIX-2-PIX VPN through the Internet. Site D has a T1 connection to the Internet, connected through a 2600 router, with a PIX-2-PIX VPN over the Internet to Site A. Hope that made sense.

We are planning on changing from static routing to dynamic routing to reroute traffic in case a connection were to go down. We are thinking about using OSPF to update the routes on the routers. There is also redundancy in Sites A, B, and C. If the Gigaman link between sites A and B were to go down, traffic can be rerouted through the DSL by establishing a VPN tunnel. If the T3 were to go down between sites B and C, traffic can be rerouted through the VPN tunnel across sites A and C and down the Gigaman. Site D can get to A by using dial-up Internet accounts if needed.

Time for questions:

How do we configure the route to the Internet and have Internet traffic reroute in case a link were to go down? For example, if the T1 line were to go down in Site B, how can we reroute Internet traffic through the T3 to Site B, through the Gigaman to Site A 's Internet link? I've tried looking at some sample configurations but they all had one gateway to the Internet and not multiple. We would prefer to have each site use it's own Internet link and use the VPN tunnels as backups in case the main WAN links were to go down.

Also how many areas would you recommend for this particular scenario?

Thanks for the help.

Reply to
Mario Lopez
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Hi,

I would advise to create just one OSPF area (0) for the whole setup. Your setup is not large enough to have to build a multi-area setup. As for the multiple internet connections. You'll have to play with Administrative Distance and OSPF costs of the default route on each location. Static routes will have preferance over dynamic routes unless their AD is raised. Local static routes redistributed into OSPF will have a lower cost than remote static routes redistributed in OSPF.

Regards, Erik

Reply to
Erik Tamminga

Now to add to the mystery. In Site A and C there are PIX firewalls sitting behind the Internet gateway routers. The internal routers use the PIX as the default route to the Internet. If the Internet link were to go down, how will the routes be rerouted to the alternative Internet routes since the internal routers "think" the default route is good since they are able to get to the PIX firewalls. Hope this makes sense.

Reply to
Mario Lopez

The PIX cannot determine if it's default gateway is still valid (unless the PIX gets its default from the internet gateway router). So you'll have to determine somewhere else if the default is still valid. This indeed complicates things a little more but it's still possible. You can have the internal routers check availability of the default tracking SAA probes. Have a look at Cisco.com for the rtr statement and tracking results on routes.

Erik

Reply to
Erik Tamminga

Thanks Erik. These seems to be the way to go.

Reply to
Mario Lopez

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