Port numbering and naming in VLANd

This is a far more complex subject than it should be, and it may cause problems for end users depending on their usage models. This is particularly likely for those users with Cisco Catalyst 3750 switches configured with extra internal modules for extra copper/fibre ports.


For consistency, throughout this document and within VLANd two terms are used to identify switch ports:

  • a port name is the (typically) alphanumeric identifier for a switch port in software, i.e. when using the CLI to manage a switch. Common naming styles vary significantly here; examples from the VLANd test rack in Linaro’s lab include “Gi1/0/6”, “Gi1/1/4”, “Te1/1/2”, “fa24”, “gi3”, “1/0/23”
  • a port number is the (typically) numeric identifier for a switch port that is labelled visibly on the switch somewhere, typically next to the port or a group of ports. Not all switches limit themselves to numeric labels, though. The same examples from our test rack are described as “6”, “G4/Te2”, “24”, “G3”, “23G”, “23T”

These examples include some of the problems - how do we try to match these up!

The problems we’ve seen

On all of the switch models that we currently support, there is a disconnect between the port names and the associated port numbers. The port names are not directly shown to the user on any of our switches, which is not a good start!

To a human operator, it is likely not very difficult to perform the mapping from one scheme to the other in most cases. However, multiple cases exist where it is difficult to map reliably from one identifier to the other in software without needing extensive manual configuration. For our VLAN control to be useful, we need to be able to identify ports in a recognisable way for our users who have connected devices to their network.

In the examples above, I showed 6 identifiers in each list but they only correspond to 5 physical ports on the switches!

Problem 1: Disjoint port numbers

On our Cisco SF300, there are two sets of ports: 48 10/100M RJ45 ports and a further four 1G RJ45 ports. They’re labelled externally as ports 1-48 and G1-G4; internally the names are fa1-fa48 and gi1-gi4. Thankfully, this particular case isn’t too hard to deal with. The SG300 we have (which works with the same driver) simply has 52 ports labelled 1-52 and named gi1-gi52. The two switches look very similar to the user.

Problem 2: Single physical ports with multiple possible port names

On our Cisco Catalyst 3750-X 24P, most of this is easy. It has 24 1G ports, labelled 1 to 24 on the front panel. The port name “Gi/1/0/6” corresponds to “6”. However, we also have an extra SFP module fitted that exposes 4 extra 1G interfaces (labelled G1 to G4). However, there’s more complication yet: G2 and G4 are dual-personality ports and can also work as 10G ports when appropriate SFP+ modules are inserted. These 1G and 10G ports are named differently in software depending on which speed is in use:

Port number Port name
G1 Gi1/1/1
G2/Te1 Gi1/1/2 OR Te1/1/1
G3 Gi1/1/3
G4/Te2 Gi1/1/4 OR Te1/1/2

Our Catalyst 3750 also has an extra 10/100M port on the back as a management port; VLANd explicitly ignores this port. Various other models in the 3750 range can come in a whole range of different configurations with 10/100M, 1G and 10G ports and it’s very difficult to identify naming and numbering schemes for all of these without physical access to all of them.

Problem 3: Multiple physical ports which map to the same port name

Our Netgear M7300-24XF (aka XSM7224S) has 24 10G SFP+ ports and a further 4 10G RJ45 ports. Those 4 copper ports replace the last 4 of the SFP+ ports in a “combo” arrengement - if an SFP module is inserted, the RJ45 port is disabled. Thus, on this switch we have the following port numbers and port names:

Port number Port name
1 1/0/1
20 1/0/20
21F OR 21T 1/0/21
24F OR 24T 1/0/24

This combo approach is also used on the TP-Link TL-SG2216; it has 16 1G RJ45 ports and 2 1G SFP ports which over-ride them:

Port number Port name
1 Gi1/0/1
14 Gi1/0/14
15 OR 15F Gi1/0/15
16 OR 16F Gi1/0/16

These show the opposite problem to the Catalyst 3750 - if a user connects to port 16 on the TP-Link, that at least will show up as one consistent name in the CLI. However, should we be looking for port number 16 or 16F when talking to the user?

A (partial) solution in VLANd 0.4 onwards

Hopefully the above descriptions will make it clear that this is a hard problem to solve 100% reliably. However, it’s a problem that must at least partially be solved as end-users supplying port numbers cannot be expected to know the port names that are used internally by the switches but not exposed externally. Yet port names must be used internally for all the interactions with the switches directly - this is the only way that the software can work.

So, port numbers have been added as an extra column in the port database. New APIs and methods have been added to do lookups by port number as alternatives to lookups by port name, to make things easier for end users. When adding ports using the admin interface, a port number is now required too.

The backend of the auto_import_switch admin command (the recommended way to add switches) will attempt to automatically allocate sensible port numbers as it probes switches and finds new ports. The algorithm chosen here for assigning port numbers is the simplest one that might work for most of the switches and configurations that we have. Numeric port numbers (i.e. without any prefix or suffix letters) will be assigned to port names simply in the order that the port names are listed via a switch’s CLI, starting with “1”. For almost all our supported switches, this should make sense for a user when trying to supply port numbers. However, BE AWARE of the possibility that your port numbers may not always match the port numbers written on your switch in some cases (e.g. the Cisco Catalyst 3750 as described above).

If this port number to port name mapping algorithm proves not to be sufficient, future releases of VLANd may include options in the config file to provide exact mappings. We’d prefer not to have to do this due to the admin overhead it will impose (and the attendant possibility of errors).