BT restored its ADSL network to normal operation this afternoon after attempts to guard against a serious security problem overnight inadvertently disrupted the connections of a substantial minority of UK Net users this morning.

In response to a serious DoS risk affecting a wide range of Cisco routers and switches (which emerged yesterday), BT sensibly decided to upgrade the software on its core network routers to non-vulnerable versions of Cisco's IOS software. Unfortunately not everything went smoothly.

BT took a number of routers out of service overnight in order to apply patches. This left fewer routers to share the load, which shouldn't have been a problem.

However some of these routers stopped working normally, resulting in widespread packet loss which left a substantial number of ADSL users unable to get online.

Matthew Watkins of ISP IDNet explained: "We're a relatively small ISP, and we've got about 10 per cent of our customers affected with 90 per cent packet-loss on the broadband network."

"BT have given winks and nods to the recent Cisco bug. Looks like they're trying to apply filters and perform software updates to their Cisco kit."

According to Watkins, customers were having problems connecting to the Hitchin exchange. Letchworth, Cambridge and some parts of London were also affected, readers tell us, though the extent of the problem this morning remains unclear.

Mark Williamson, of telecoms reseller Gether, commented: "In trying to prevent a DoS attack they [BT] have managed to deny service to thousands of users."

This morning, BT told Williamson that all "the ADSL providers using the Central Pipe are unable to connect".

A BT spokeswoman confirmed that there were issues with BT's ADSL network this morning brought about as a side effect of "work to protect the network" against the Cisco router DoS vulnerability that emerged this week.

Although reluctant to go into too many details, she said that there was a problem with the operation of BT's ADSL network this morning which impacted a "number of service providers" until services were restored at 1:30pm this afternoon. ®

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