Editing Spanning Tree Protocol (section)

==Shortest path bridging==
{{Main|IEEE 802.1aq}}

IEEE 802.1aq, also known as Shortest Path Bridging (SPB), allows redundant links between switches to be active through multiple equal cost paths, and provides much larger layer-2 topologies, faster convergence, and improves the use of the mesh topologies through increased bandwidth between all devices by allowing traffic to load share across all paths on a mesh network.<ref>{{cite web
 |title       = Shortest Path Bridging IEEE 802.1aq Overview
 |publisher   = Huawei
 |author      = Peter Ashwood-Smith
 |date        = 24 Feb 2011
 |url         = http://meetings.apnic.net/__data/assets/pdf_file/0012/32007/APRICOT_SPB_Overview.pdf
 |access-date  = 11 May 2012
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20130515115628/http://meetings.apnic.net/__data/assets/pdf_file/0012/32007/APRICOT_SPB_Overview.pdf
 |archive-date = 15 May 2013
}}</ref><ref>
{{cite web
 |title = Largest Illinois healthcare system uproots Cisco to build $40M private cloud
 |author = Jim Duffy
 |date = 11 May 2012
 |publisher = PC Advisor
 |quote = Shortest Path Bridging will replace Spanning Tree in the Ethernet fabric.
 |url = http://www.pcadvisor.co.uk/news/internet/3357242/largest-illinois-healthcare-system-uproots-cisco-build-40m-private-cloud/
 |access-date = 11 May 2012
 }}</ref> SPB consolidates multiple existing functionalities, including Spanning Tree Protocol (STP), Multiple Spanning Tree Protocol (MSTP), Rapid Spanning Tree Protocol (RSTP), Link aggregation, and [[Multiple MAC Registration Protocol]] (MMRP) into a one link state protocol.<ref name="IEEE">
{{cite news
 |title = IEEE Approves New IEEE 802.1aq Shortest Path Bridging Standard
 |publisher = Tech Power Up
 |date = 7 May 2012
 |url = http://www.techpowerup.com/165594/IEEE-Approves-New-IEEE-802.1aq-Shortest-Path-Bridging-Standard.html
 |access-date = 11 May 2012
 }}
</ref>

===System ID Extension===
The bridge ID (BID) is a field inside a BPDU packet. It is eight bytes in length. The first two bytes are the bridge priority, an unsigned integer of 0–65,535. The last six bytes are a [[MAC address]] supplied by the bridge. Prior to IEEE 802.1D-2004, the first two bytes gave a 16-bit bridge priority. Since IEEE 802.1D-2004, the first four bits are a configurable priority, and the last twelve bits carry the bridge system ID extension. In the case of MST, the bridge system ID extension carries the MSTP instance number. Some vendors set the bridge system ID extension to carry a VLAN ID allowing a different spanning tree per VLAN, such as Cisco's [[PVST]].