Mr. Anonymous (my most loyal reader and commentator) sent me this question as a comment to one of my blog posts:
Is there any use case of running EVPN (or PBB EVPN) in DC with MPLS Data Plane, most vendors seems to be only implementing NVO to my understanding.
Sure there is: you already have MPLS control plane and want to leverage the investment.
You might also want to persuade your customers that this is a must-have feature because your competitors don’t have it in their products.
- EVPN uses BGP Tunnel Encapsulation Attribute (RFC 5512, new version in draft-ietf-idr-tunnel-encaps) to indicate the encapsulation to use to reach the egress PE device (we shouldn’t call them routers anymore, right)?
- The values of this attribute can be found in IANA registry.
- Almost all of them rely on something-over-IP tunneling.
Maybe the real question should be “why is that the case?” In a word: decoupling.
Unrelated question: why do we need so many encapsulations? Because standards.
The Benefits of Decoupling
When using whatever-over-IP encapsulation technology with EVPN, the transport fabric remains simple and clean. It’s a pure IP fabric using a single encapsulation (IP) and running a single routing protocol.
MPLS encapsulation used with EVPN control plane requires end-to-end LSPs between PE devices. You’ll have to deal with two encapsulations (IP and MPLS), two sets of forwarding tables (FIB and LFIB), and additional control-plane protocols – LDP, MPLS-TE, BGP IPv4+labels or segment routing.
Each LSP has to exist in the forwarding table of every node it traverses. That introduces extra state in the transport fabric - not relevant if you have 10 switches, highly relevant if you have thousands of switches (example: WAN deployment) and the core switches use high-speed merchant silicon.
Admittedly, the MPLS encapsulation introduces lower overhead than whatever-over-IP encapsulation. That overhead becomes relevant when the bandwidth becomes expensive: in WAN networks, not in data centers.
To summarize: there’s no free lunch. You have to accept higher encapsulation overhead or more complex transport fabric. I know what I would do when designing data center infrastructure.
- We’ll do a deep dive into EVPN complexities with Dinesh Dutt on March 6th;
- We’ll discuss the implications of using EVPN in multi-site deployments in a live session with Lukas Krattiger (part of Leaf-and-Spine Fabric Architecture webinar) on March 29th;
Both live sessions are open to ipSpace.net subscribers.