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either an ISP-side configuration or a network glitch is causing the problem
a routing loop is causing the problem
a software bug on one of the switches is causing the problem
a bridging loop is causing the problem
If an interface duplex mode is manually configured and does not match the duplex mode on the other end of the link, a bridging loop will occur even when STP is enabled on a switch. This happens because SW1's port to SW3 is hardcoded as full duplex, while SW3's port to SW1 is set to autonegotiate which sets the link to a half-duplex mode (CSMA/CD). This results in BPDUs being dropped which leads to the port on SW3 to be unblocked which creates bridging-loop. (http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10556-16.html#duplex)
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