What are the complexities of configuring a functional VoIP network using products from more than one company?

Natasha- "functional" may come across as a loaded question that non-IT folks such as lawyers, managers, doctors ... would use. There is nothing functional about networks in this matter of speaking; networks are living - breathing - and changing. More importantly, one industry vertical or even niche business would deem functional to them as being able to just make voice calls over the network but would never consider faxing - just a simple example.

Some will argue that because networks consists of equipment and software that are based on standards and that there is no problem. Then they will gladly cite the specs and jibber-jabber to support their claim.

The answer to your question is: it depends. But I want to focus on areas that could help you answer your own question or re-direct your thinking.

Here are the basic options:

-One (1) Vendor solution
-Multi-Vendor solution

With regards to hardware there are different approaches to take. The hardware could be singular in how it functions or it could be multi-purpose. Integrated Access Devices (IADs) do more functions, use less space and often cost less to own and maintain. The CONS could be that one specific feature or application may be weaker or not up to exactly what the customer is seeking. Some will argue that IADs are more difficult to setup because there's added complexity or that they are more disastrous in a single point of failure scenario.

The questions needing asked are costs, performance, need, manage-ability, TCO (total cost of ownership) and value.

Is it practical to use multi-vendor for a VoIP network? Yes but again it depends. Networks evolve over time and taking a plunge into VoIP means stepping back and looking at the network to see the existing network for what it is, how it is configured and then assessing what is needed to move beyond what we have today? Two assessments - one physical and rather simplistic while the other involves metrics and testing to see the actual network performance.

Another question you may ask yourself is who is going to maintain what you build? The vendor, vendors, internal staff or a combination? Then go a step further and explore how each party can sustain continued support and what is involved along with costs. Always then consider is what we are doing or trying to do practical or are we building in too much complexity to the solution? The warning is to not build a house of cards.

If you are looking for technical issues you may or may not find them immediately upon first glance but you need to consider a simple rule. Simplicity is often a good thing and when you add consistency to the fold then you are sure to be more successful in your venture.

I'll touch on "interoperability." None of this means a thing until you have "proverbial proof in the pudding." Of course there are issues and there are issues with using multi-vendors and even single vendors or less vendors. It's a bit more complex but again, not knowing are you speaking of a 5 user home office network that wants to use DSL to connect to the world for all your voice/data needs or if you are a large distributed global enterprise you still need to consider all the above but add in "What are your expectations."

You may find that by implementing with consistency will in fact change the dynamics of your solution. When you need to patch and change your switches, routers, firewalls for example- how easy or difficult is the task? How well do they work together? Are the costs lower for licensing and or support using one vendor's solutions vs many?

Another key concept to consider is quality and value. Would you consider using best-in-class products for each area of your network? Again, not knowing your topology/infrastructure - think in these terms. Would you use a home office router for an SMB connecting to clients in Europe? Some will argue YES passionately because it works. Does it support and provide sustainable use- meaning when things go awry do you have tools to expedite the troubleshooting and expedition of issues? Can you ID root causes of failures and issues?

It's a can of worms.

I hope this helps a bit.