Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

Pre-installed site has duplicate IP's on DUAL backup ISDN solution

A site I am reviewing for a customer has duplicate IP subnets designed into the cloud used on the WAN side of the ISDN links. The routing protocol is EIGRP. To me, this is bad design practice for troubleshooting and general IP design. Is there any reason one would use duplicate IP subnets on a parallel ISDN backup to backup for remote sites in a solution?

Here is a general idea

192.168.10.x-LAN--remote---ISDN 192.168.50.x-----core ISDN Backup----LAN 192.168.1.x

|--ISDN Backup 192.168.50.x----backup ISDN backup---LAN 192.168.2.x

thanks, I just want to double check before I recommend removal of duplicate IP subnetting



Re: Pre-installed site has duplicate IP's on DUAL backup ISDN so

From the "picture" you have posted, this looks like an old-time standard ISDN design, where the ISDN cloud is treated as a single physical point-to-multipoint network. This can create routing issues if two remotes, both on ISDN, want to communicate (the same problem you see on frame relay).

From the viewpoint of duplicate IP addresses, this configuration is probably correct and there are no duplicate addresses (unless a mistake is made and two remotes get the same value for "x"). However, I don't like to use it as you are limited to the number of remote sites that will fit in the subnet chosen before you have to reconfigure everyone to use a larger subnet.

My preference in a hub and spokes network is to use IP unnumbered for dial backup to eliminate scaling issues and simplify expanding the hub access server to multiple PRIs on multiple servers. See Chapters 4, 5 & 8 in my book for an exhaustive (or is that exhausting) exposition on the topic. Table of contents and all the example listings are on my web site.

Vincent C Jones