Replaced 2009-9-23

Overview of creating VLANs

  • In principle, VLANs that never leave a switch don't need to be allocated and managed globally, but if one intends to use ONA to manage such switches, the VLANs need to be allocated.
  • For VLANS that span switches, there presumably will need to be a route-point for the associated network (discussed here-tbd).
  • Once a VLAN id has been assigned, it must be added to the list of manageable VLANs in ONA: ONA -> Maintenance -> Vlan Group Memberships.
  • Use the ONA "create VLAN" feature to allocate the VLAN within ONA. ONA will warn that unless the VLAN appears somewhere within 48 hours, it will be purged. The simple way to allocate the VLAN is to log into a switch and enter a context for the VLAN -- this will create it on the switch, satisfying ONA.

Rules for constructing VLAN ids for networks in 129.97/16

VLAN ids in the public IP space 129.97/16 are created according to the follow rules:

  • for /24 networks, 129.97. x.0/24, the VLAN id is x
  • for /25 networks of the form 129.97. x.0/25 and 129.97._x_.128/25:
    1. the network containing the ".0" address, ie 129.97. x.0/25 gets VLAN id x, and
    2. the other network's VLAN id is formed by appending a "1" to the string x: for example for 129.97. x.128/25 would be VLAN id x1
  • for /26 networks, the lower half of the network is appended "1" and the upper example is appended "2"

An example will help. Consider the /24 network 129.97.173, and suppose it is split into three networks: a /25 and a pair of /26s, as follows:

  •, ie IP addresses -> (128 addresses);
  •, ie IP addresses -> (64 addresses)
  •, ie IP addresses -> (64 addresses)

The VLAN id assignments would be:

Network VLAN id Comments 173 contains the .0 address 1731 append "1" to "173" 1732 append "2" to "173"

If the /25 is split into two /26s, or if one of the /26s is split into a pair of /27s, the suffixes "3" and "4" would be appended to "173" yielding "1733" and "1734". (To be pedantic, one could go back and renumber all of the original VLANs so that they occurred "in order" within the /24 block, but there's not technical advantage to doing that.

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Topic revision: r1 - 2009-09-23 - TrevorGrove
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