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 [[Multiple Spanning Tree Protocol|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]].