Fun With Optics

I recently had a deployment where we needed to connect Cisco 6500s to Juniper MX960s.

There was a lot of confusion surrounding what fibre and optic modules needed to be used, so I’m documenting the initial state and the successful state here.

Initially, the Junipers had the following model numbers used for the optics:

XFP-10G-LR (identified using a “show chassis hardware”)

The 6500s:

10Gbase-SR

Interestingly, one of the links came up between the devices, while one did not. I tried swapping the cable (Single Mode) for Multimode for the non-working link, and that did not fix the issue. We then swapped the Cisco optic for an LR optic and used Single Mode fibre which did the trick.

Confusingly for someone from a Cisco background like me, the Juniper optic was labelled  XFP-10G-L-OC192-SR1. I assumed the SR stood for short reach/range (it does) and tried to use Multimode fibre, as you would with Cisco SR optics. However, it turns out that the Juniper SR optic in this case used Single Mode fibre as its interface.

It is documented in this handy link here, which turned up with some Googling.

The confusion stems due to the reference to Short Reach differing between Cisco and Juniper. I need to figure out a simpler way to ID the type of fibre to be used. Perhaps using the core:cladding ratio is the best way? 9:125 micro meters seems to be what is used for SM fibre.

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