Layer 2 Bridging
Device Requirement Device “A” needs to communicate with device “B” and those two devices are “required” to live on the same layer 2 broadcast domain. I haven’t come across any new devices or applications that fall into that spectrum, and it’s 2018. However, some enterprise organizations may still have legacy devices or poorly manufactured devices/applications with no foreseeable updates that may fall into this category.
Customer demand A customer you service in area “A” needs network services expanded to area “B.” They want their equipment to stay on the same subnet. Cough, cough, point of sales systems. I believe that modern POS systems can talk via IP across different subnets, but this can also be a possible use case that still comes up.
Data center disaster recovery or should I say “specific” DR models. I say “specific” because not all DC DR needs to be developed with an absolute layer 2 requirement extension model. Specific apps that are short-sighted will include layer 2 extension as a requirement. Someone insists that a VM pinned to a specific IP move from region “A” to region “B” and the IP needs to stay the same. What!?! Let’s think of better ways to do this, DNS, automate IP provisioning? However, this can still be a possible use case.
Ease of use Sometimes if you’re uncomfortable with routing protocols, it may seem easier to span a VLAN across the core of the network. Less IP provisioning, less ACL’s, potentially less firewall rules, and less management of those dreaded IP routing protocols. However, this is something we are in control of, so it’s OK to take time to research and learn what routing protocol would work best for your environment. Don’t let the lack of information drive your operation.
I can confirm that extending 100’s of VLAN’s through your core along with multiple instances of STP with a sprinkle of HSRP is NOT scalable. You will run into issues at some point. Others would say, “but my superior wants things done yesterday.” That’s another topic which may be worth blogging about in the future but hang in there.