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Looking Glass


You can use Looking Glass to find various details about your network, e.g. what are your Unit's VLAN and network allocations?

At its simplest, just select the Unit from the dropdown list and click the "show unit" button. Other search criteria include your backbone VLAN ID as assigned by IT Services, the four letter oak code, an IP address, subnet, or hostname.

So in this example we see all the information we hold for Jesus College.

The above output will provide the following information:

1) Which networks are assigned to this unit?

2) On which core router the Unit's SVI (virtual router) is present and which IP addresses have been assigned on that interface (So basically what's the gateway per subnet)?

3) Which backbone VLAN IDs have been assigned to the Unit?

4) Which layer 2 switches carry those VLANs?


In short Jesus College has been assigned two subnets (1)

The upstream core router is CIND. Jesus has a routed configuration. So their SVI 617 is assigned the IP address and traffic for the second subnet goes over a static route that points to their firewall/router IP (2).

The VLAN numbers assigned to them are (3)

617 Main VLAN as described above

672 Annexe VLAN 682 Q-in-Q VLAN

The access switch (Frodo) for all VLANs located at Unit's main site (4) jesus.frodo and on which switch-port each VLAN terminates.


Annexe VLANs

To determine the upstream router of your Annex VLANs, you can lookup the Frodo's name. From the last two octets of the resulting IP address you can determine the Frodo number and finally use the table below to match it with the router it's connected to.


CRouter Router Number Frodo Number Range
COUCS1 0 1-99
CENG 1 101-199
CSUR 2 201-299
CMUS 3 301-399
CZOO 5 501-599
CIND 6 601-699
CASH 7 701-799
CIHS 8 801-899
COUCS2 9 901-999


e.g. VLAN 682 is present on Frodo

$ dig +short

Frodo number is 253. From the table you can determine that the upstream router is CSUR.

It's expected that a failure or maintenance task that takes place on any of the above network components will cause service disruption to at least one site for this specific VLAN (could be more if the main site goes down depending on how the services are distribute and delivered).

Rule of thumb: Further to the distribution routers are the core switches BMUS and BOUCS. These are configured to be the root bridges (RB) and secondary root bridges for Annexe and Q-in-Q VLANs. Even numbered VLANs' RB is on BMUS, odd numbered VLANs' RB on BOUCS. This means, that in the event of a reboot or outage of one of the backbone switches the Annexe VLANs will experience an outage equal to the time it will take for STP to re-converge.

Service area: 

Written by IT Services. Latest revision 13 January 2016