A fire started by a homeless man knocked out service between Boston and New York on the experimental Internet2 network Tuesday night.

Chris Robb, an engineer at Indiana University's Global Network Operations Center who works on Internet2, says Level 3 Communications cables used by the network went up in flames. The cables were on the Longfellow Bridge, which connects Boston and Cambridge across the Charles River.

Robb, who co-authors the Internet2 Network Upgrade blog, writes that Level 3 engineers estimate it could take one to two days to restore the circuit. Engineers are looking at rerouting a Chicago-to-New York OC-192 circuit that normally goes through Boston to Washington until service is restored.

Robb writes: "Question: When can a cigarette take down your network? Answer: When you throw it at a bridge and light it on fire."

Authorities say the fire, which also disrupted service on the Red Line subway, started around 8:20 p.m. when a homeless man tossed a lit cigarette. The cigarette landed on a mattress, which ignited and led to a two-alarm fire.