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TRILL Switching for WISPS, Part 2

< Part 1… intro in case you missed it


Many WISPS start out with a bridged/switched network. It's cheap and easy to create (simply plug stuff in!) but doesn't scale well. Sooner or later, the only way to scale up is to scale out — using routers, not just switches and bridges.

I learned the practical limits of spanning tree the hard way, somewhere around the last century. Since then, I've been a proponent of Routing All The Bits.

So for me, moving back to 'switching,' even on a small test network segment, felt a bit grungy. We'll test a mesh of up to 10 TRILL switches from IgniteNet in a real deployment.

In Part 1, we saw that TRILL is an overlay network. It's actually a routed network underneath. In practice, it should be more efficient and resilient than a traditional bridged network.

Here in Part 2 we'll talk about how to configure TRILL (it's almost zero-config), and what effect it has on traffic paths. SPOILER: it picks the shortest path.

Design & Setup

Bridged Network: Foundation

We'll use a physical backhaul network of wireless and wired single links. We can use non-TRILL switches as well as TRILL switches; if a TRILL port sees Spanning Tree BPDU packets, it lets STP do its thing and steps out of the way. Consequently, we disable STP wherever we want to use TRILL to manage loop-free topology.

My original intent for proof-of-concept was to test gigabit-only links and TRILL-only switches. On the bench, this is easy to cable. In production, I used a mix of 1Gb copper ethernet, fiber, and a variety of wireless links:

  • Mimosa B24 (24 GHz bridge)
  • 1-gig Mikrotik Wireless Wire (60 GHz bridge, RSTP disabled)
  • 2.5Gb IgniteNet MetroLink PTP/PTMP radios (60 GHz) 

To augment testing with sub-gigabit connections, I added an SFP VDSL2+ link that tops out at about 110Mb, converted to a 1 Gig copper interface.

Each POP or hop contains a minimum of two backhauls or interconnections, a TRILL switch, and a router. We'll get to the routers in part 3.

Deployment Layer 2 topology


Routed IS-IS Network — Port Configuration

Screenshot 2018-09-01 11.28.18.png


Each switch needs a unique nickname/ID. (If you don't choose one, TRILL picks one for you.)

Each port needs to be designated either as an access port (accepting untagged and VLAN frames), or as a trunk port — encapsulating traffic between switches so they can be routed instead of bridged.

You can configure ports as access or trunk as needed. Your payload connects to an access port. You might connect a router (as I did), or even another Layer 2 segment.

TRILL switches run a link state routing protocol, IS-IS, on the trunk ports. IS-IS requires no additional IPv4 or IPv6 configuration. Runtime state and debugging is available via CLI.

Bridged TRILL Network — Status

Screenshot 2018-09-01 11.26.41.png


On trunk ports, a TRILL adjacency forms between neighbor peers.

If you look at the switch IDs on the deployment link map above, and compare it to the TOPOLOGY section of this status screenshot, you'll see that the TRILL switch with nickname/ID 10250 has an optimal path to every other peer — from its perspective in the mesh.

Metric has been manually overriden to route to peer 10080 via 10110 on fiber rather than taking the grey VDSL2 link. (Limiting port speed to 100 Mb would have the same effect.)

STP (spanning tree) needs to be disabled on all bridging devices and backhauls that connect a trunk port to another trunk port, or TRILL does not form an adjacency — instead, it falls back to Layer 2 switching.

Bridging Reconvergence

UDP Failover Test

TCP Failover Test

I simulated a link-down by pushing continuous UDP/TCP across the mesh, and disabling the RF side of test backhauls in the path — this causes packets to stop flowing, but does not physically signal an outage. (A switch would immediately detect a downed ethernet port.)

With the latest firmware, convergence occurs in 6-8 seconds on a link-down condition, and even faster on a link-up condition.

Load Balancing, kinda

Screenshot_2018-09-03_14_25_51_switching edited.png


The IgniteNet implementation of TRILL does not yet support multipath/ECMP. However, assuming port speed and metric are identical, TRILL prefers the first or lowest interface with the shortest path to the destination.

In this example, note that TX and RX are split across trunk ports #2 and #3. We can exploit this effect to balance traffic across multiple half-duplex radio links — without any additional router or switch configuration needed.


What's working

  • Self-configuring TRILL shortest-path-first mesh
  • Reasonably stable, fast-ish failover
  • Poor-man's full-duplexer/load balancer
  • Metric can be manipulated via CLI (persistently?)
  • Low cost for the feature set

What could use improvement

  • Some outstanding issues with accessing management IPs on trunks
  • No multipath/ECMP
  • GUI limits max MTU below what hardware supports
  • GUI doesn't allow setting interface metric
  • No SNMP/API monitoring of TRILL adjacency
These views are my own; IgniteNet did not sponsor this post, but did answer copious questions during the debugging and trial phases.

…coming in part 3…

In part 3, we'll talk about:

  • types of deployments where TRILL mesh is a good idea/bad idea
  • various routing topologies when using a Layer 2 mesh
  • additional types and methods of traffic steering

Testing Setup

  • IgniteNet Metrolinq 2.1.1 and 2.2.0
  • Mikrotik RouterOS 6.43rc47 and later, for Wireless Wire — earlier revisions have an MTU bug
  • Mikrotik RouterOS 6.42.1 and later, for routers
  • Mimosa B24 1.5.1
  • Managed and unmanaged fiber converters, various manufacturers and firmware revisions
Jeremy Austin1 Comment