Before adding a new switch on your network pay attention to these 3 things
- VTP (Vlan Trunking Protocol): VTP is used to synchronize vlans and vlan information across switches. It allows you to create, modify and delete a vlan on one switch and these changes are sent to other switches in the same VTP domain. This means that If you add a new switch with a single vlan, this information might be propagated to other switches in the network thus causing your network to lose all the vlans. If you don’t use VTP, just disable it by using the command vtp mode off.
- STP (Spanning Tree Protocol) priority: STP is used to prevent loops in networks by logically blocking redundant paths. It operates by selecting a single root bridge and then determining the best path from each switch to the root. The STP priority value is a key parameter used in the process of electing the root bridge. The switch with the lowest combination of priority and MAC address becomes the root bridge. By default, all switches have a priority of 32768, so the MAC address becomes the deciding factor. This means that if the new switch has a lower priority it will be elected the root bridge thus altering the previous STP topology. Be aware of that!!
- Trunk: Trunking is essential when you want to extend VLANs across multiple switches. If trunking is enabled to allow all vlans, you’re creating an opportunity for a ton of problems in the future. By allowing all vlans on a trunk without proper restrictions, unauthorized people could have access to certain vlans/networks. It’s recommended to explicitly define which vlans should be allowed on the interface by using the command switchport trunk allowed vlan. And a very important tip, whenever you need to add an additional vlan, use the command switchport trunk allowed vlan add xyz.
These might be basic settings, but as a consultant I see this often on production environments and what most of network admins don’t realize is that some of the problems they experience in layer 3 or at application level, are actually related to something happening in layer 2.
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