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ISDN Multisite dialup using OSPF cost issue

Hi All,

Need Assistance for setting up the

Pls refer attached topology, and Router configuration for Central site (Datacentre) routers, DR Site Router and Branches- 2 Location Config.

WAN for the attached topology is up and running on Channelized-E1 with IP Scheme as mentioned in the configuration file.

Need to implement PRI with any site to any site dialup option (Multiple level of backup from branch site to datacentre (Primary site and DR site).

IP Scheme for PRI is and on primary site router.

IP Scheme for PRI on DR site router is and

Need assistance in freezing the config as per the the proposed Topology.

OSPF is used with three areas defined

Area 0 - Backbone area for Datacentre router ethernet segment

Area 1 - For branches connecting to Primary datacentre site

Area 2 - For branches connecting to DR site router

Branch router is connected over lease line to primary datacentre channelized-E1, In case there is a Lease line failure at branch end or Channelized-E1 link failure at Central site , Branch router should dial in to PRI of Primary Datacentre site .

When primary datacentre site E1 as well as PRI link fails branch should dial the DR site PRI over ISDN.

Similarly branches having primary connectivity to DR site over lease line will dial the DR site PRI in case lease line fails and after PRI fails DR site branch should dial the Primary datacentre PRI over ISDN (Multiple backup scenario)

Just to summarize the requirement related to Dialup option...

Data flow for Central site branches should be in the listed order.

Data transfer from Branch (Lease Line) to Primary site (Channelized-E1) will be primary path for network operation/trafficlized-E1)

When the lease line at either branch or central site.

In case either branch lease link fails or primary site channelized-e1 link fails, branch router should automatically dial over isdn to Primary site PRI link.

In case of PRI link failure of Primary site, branch will dial to DR site PRI link as a seconday backup.

Also fourth condition ... very important.

When branch lease link and primary site channelized-e1 link comes up ISDN call initiated/connected from branch to either primary/DR site should be dropped and all the further data should take Lease line as the preferred path. As good as Fallback.Idea behind fallback is to minimize the ISDN call charges when the Lease line comec back up.

Same call flow is required for DR site branches.

Need your inputs whether the above scenario will be acheived with the OSPF cost parameter defined in the already freezed config.

Pls confirm.




Re: ISDN Multisite dialup using OSPF cost issue

What I think it is purely the routing issue. You have to sum up all cost in a path then compare them to prevent the looping or the traffic route through the remote back to hub.

I suggest to configure the OSPF cost between main & DR is high but smaller then hub to spoke + spoke to DR. Otherwise, traffic may route though hub to spoke then spoke to DR.

For the OSPF issue, you only need to ensure there is no traffic (e.g. broadcast, routing update) to maintain the connectivity and configure a suitable idle timer is fine. In this case, you may consider to use floating static for the backup link and disable the OSPF to prevent the routing update to keep the ISDN up. But you also need to configure those floating static for each site & backup connection to ensure the traffic can flow back to the remote.

Hope this helps.

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