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L3 Virtualization and VRFs

I got into an interesting discussion with Johannes Luther on the need for VRFs and he wrote:

If VRF = L3 virtualization technologies, then I saw that link. However, VRFs are again just a tiny piece of the whole story.

Of course he’s right, but it turns out that VRFs are the fundamental building block of most L3 virtualization technologies using a shared infrastructure.

The Basics

Each L3 virtualization technology must implement numerous separate (and optionally isolated) L3 domains (let’s call them tenants). Each tenant (= L3 domain) needs a forwarding table that could be different from any other L3 forwarding table in the system, just like you need a separate MAC table for each virtual L2 domain.

Whether the system uses a single L3 table and makes L3-domain-ID part of the lookup process is an implementation detail, just like some L2 switches use a single MAC lookup table and do lookups based on VLAN+MAC value.

You also need some way of populating that table, usually a routing protocol. Virtual Routing and Forwarding table (VRF) is thus a fundamental building block of every L3 virtualization technology regardless of how you call it.

Alternative Architectures

Obviously you could implement a distributed system where the routing (= collecting and distributing information) decisions are centralized, but forwarding (= sending received packets to correct output ports) decisions are distributed, so there would be a single copy of the tenant routing table and numerous copies of the tenant forwarding table.

You could also run isolated per-tenant control planes and push the results of these control planes to the forwarding tables. VMware NSX is a typical example of such architecture:

  • A dedicated routing VM is run for every tenant using a single routing table. Control-plane VM thus doesn’t use a VRF concept;
  • Routes collected by the control-plane VM are sent to the NSX controller and distributed to L3 forwarding elements in individual hypervisors where every tenant has its own virtual forwarding table.

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2 comments:

  1. So ... most people outside networking look at L3 as a solution for NOT needing anything 'virtual' in the network. A single L3 domain with distributed firewalls and IP policy driven by code.

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    Replies
    1. Unless you can force your users to agree on a common IP address plan, using a single L3 domain might be difficult. I would claim that for a hosted environment, multiple L3 domains is a requirement. One can of course build such domains using 464-XLAT or similar.

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