Fiber Light Level Good , but Layer 1 down

[ahmad82pkn]

Hi,

Can any one tell me possible reasons if Fiber light levels with in range but Layer 1 still shows down on both source and destination routers, what could be reason?

Considering there is a partner in between and a colocation as well.

Just want to understand possible reasons to troubleshoot an issue.

As i always thought if light levels are ok on both side with in range then layer 1 should come up. isnt it?

[Marvin Rhoads]

Any number of things could go wrong. Here are a few:

- TX/RX strand reversal

- incompatible signalling (for instance, one end with 100Base-FX and the other end with 1000Base-SX transceiver type)

- mismatch of fiber type along the path (for instance a multimide patch cable in an otherwise all single mode path).

You can get good light levels but not get a link to establish (Layer 2 technically) if any of the above are the case.

Link up / protocol up requires more than just good light levels. Depending on the link type and length, other more esoteric reasons can also come to bear (chromatic dispersion etc.).