Switch – How to Connect Two Switches Together

Agree with both Ron and Mike Pennington here. The entire point of Spanning Tree is to prevent broadcast storms from blowing up your network and you have just recently observed a practical lesson in how that happens.

Knowing the version of your Netgear switch would be helpful. I'm looking at an older version of the ProSafe switch user manual for the GS748TS and the Spanning-Tree settings are quite granular, so you should be able to configure the switch to prevent such things from happening in the future.

On the older NetGear ProSafe switches I have used the default configuration is that STP is enabled in RSTP (rapid spanning tree) with BPDU flooded to all ports on the switch. This is a good configuration for everyday use. In order to help you more, we would need to know how many VLANs are on your network and so forth.

But at the end of the day this is a hard lesson in the need for company policies that govern what can and cannot be connected to a production network. At my employer (a large Fortune 500 manufacturing enterprise) our network policy is that all access ports on switches are configured with BPDUguard enabled such that when any unknown switching device is plugged into it the port becomes error disabled. If you have SNMP monitoring on your switches, then you will get an alert.

There is a small price to be paid in user frustration but this is more than made up for in the fact that you will automatically have visibility and hopefully control over what gets connected and you won't have to deal with the network outages in the future.

When companies merge, they often have this problem with overlapping addresses. Cisco has NAT Outside Source which helps to solve the problem. You can do an Internet search for "cisco ip nat outside source" to find information such as Sample Configuration Using the ip nat outside source list Command.

Introduction

This document provides a sample configuration with the ip nat outside source list command, and includes a brief description of what happens to the IP packet during the NAT process. You can use this command to translate the source address of the IP packets that travel from outside of the network to inside the network. This action translates the destination address of the IP packets that travel in the opposite direction—from inside to outside of the network. This command is useful in situations such as overlapping networks, where the inside network addresses overlap addresses that are outside the network. Let us consider the network diagram as an example.

Note: this should really only be a temporary situation, and you should strive to resolve the conflict. This is not something you want to maintain as a long term solution.

Another possible solution is to create VLANs on the switches just for the RANCID server connections, and you can set up different IP addresses for the switches on the server VLANs. This will not affect the switches and the existing LAN addressing, it just adds new addressing specifically for the server connections.