Several enhancements were made to the layer 2 loop prevention mechanism, the original STP (IEEE 802.1d). Examples are multiple spanning tree, rapid spanning tree.
One of the major drawbacks of spanning tree is - it prohibits the use of multiple paths in layer 2. After BPDU exchange, STP will put redundant ports in blocking mode.
Earlier solutions to disable SPT and to take advantage of multiple layer 2 paths were the use of PortChannel - FEC or GEC (Proprietary), PagP (Proprietary) and LACP (802.3ad, later 802.3AX).
Biggest limitation of PortChannel is that it can only be used between two adjacent devices.
Cisco came up with vPC (virtual PortChannel) in its Nexus platform (NX-OS) which will let multiple active layer 2 paths exist. See here for detail explanation.
vPC is Cisco solution before a standard based technology is agreed upon. That technology is in work in progress for a few years (since 2007 I think) and it has a weird sounding acronym. It is IETF's TRILL - Transparent Interconnection of Lots of Links. TRILL uses IS-IS (yes that L3 routing protocol) to manage L2MP management traffic.
L2MP - Layer 2 MultiPath
Wednesday, March 4, 2009
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