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Posts: 71
Registered: ‎03-04-2013
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Accepted Solution

CR - Change default STP priority from 0 to 32768

CHANGE REQUEST
==============

On the EdgeRouter Lite, when STP is enabled on a bridge interface, the default bridge priority is 0.

 

Please consider changing the default STP Bridge priority from 0 to 32768 to match the defaults of other major switch manufacturers.

 

JUSTIFICATION
=============

The STP bridge priority is used to determine which bridge (switch) will win an election to become the STP Root bridge. The switch running STP with the lowest priority value will become the Root Bridge.

 

Having the ERL use a default STP priority of 0 may cause unexpected behaviour for users accustomed to the defaults used by other manufacturers.  Changing the default priority from 0 to 32768 will address these concerns.

 

I can not immedately think of an environment where an ERL would be the preferred STP Root over another switch so having the default be 0 seems aggressive. Typically a "core" switch is manually configured to become the STP Root by lowering its configured priority but it is hard to go lower then 0 in order to beat the default of the ERL.

 

HOW TO REPRODUCE
================

 

( below behaviour confirmed on ERL versions "v1.5.0" and "v1.6.0rc2" )

 

Create a bridge, enable STP, and assign it to a physical interface. (do not specify a priority value)

ubnt@ubnt:~$ configure
ubnt@ubnt# set interfaces bridge br0 stp true
ubnt@ubnt# set interfaces ethernet eth0 bridge-group bridge br0
ubnt@ubnt# commit
ubnt@ubnt# save
ubnt@ubnt# exit

 

Running "show bridge" displays the priority value (in hex) as the first part of the "bridge id"

 

ubnt@ubnt:~$ show bridge 
bridge name     bridge id               STP enabled     interfaces
br0             0000.dc9fxxxxb9de       yes             eth0

 

A packet capture confirms that EdgeMax is using the above bridge-id in its BPDU frames.

 

ubnt@ubnt:~$ sudo tcpdump -vvv -p -n -i eth0 stp
tcpdump: WARNING: eth0: no IPv4 address assigned
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
10:49:49.177091 STP 802.1d, Config, Flags [none], bridge-id 0000.dc:9f:xx:xx:b9:de.8001, length 35
        message-age 0.00s, max-age 20.00s, hello-time 2.00s, forwarding-delay 15.00s
        root-id 0000.dc:9f:xx:xx:b9:de, root-pathcost 0

 

 

Info from the connected Cisco switch confirms the priority value of 0 of the ERL Root Bridge:

 

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    0
             Address     dc9f.xxxx.b9de
             Cost        4
             Port        35 (GigabitEthernet0/35)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    24577  (priority 24576 sys-id-ext 1)
             Address     0019.xxxx.9880
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/35              Root FWD 4         128.35   P2p Peer(STP) 

 


SUPPORTING DOCUMENTATION
=========================

 

Juniper default priority is 32,768
- http://www.juniper.net/techpubs/en_US/junos11.4/topics/reference/configuration-statement/bridge-prio...

 

Cisco default priority is 32,768
"...switch priority of all the switches is set to the default (32768)"
- http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960/software/release/12-2_53_se/configura...

 

HP ProCurve default is 32,768
- [PDF page 27] http://www.hp.com/rnd/pdfs/CLI_reference.pdf
"priority <0..65535> - Switch (or bridge) priority used along with the switch MAC address to determine which device is the root. The default is 32768. "

 

Mikrotik default is 32,768

- http://wiki.mikrotik.com/wiki/Manual:Interface/Bridge#Properties 

 

Wikipedia states default is 32,768
"...The bridge priority default is 32768"
- http://en.wikipedia.org/wiki/Spanning_Tree_Protocol


Accepted Solutions
Previous Employee
Posts: 13,551
Registered: ‎06-10-2011
Kudos: 5411
Solutions: 1657
Contributions: 2

Re: CR - Change default STP priority from 0 to 32768

Yeah that's likely a good idea and we should look into it. Thanks for the suggestion.

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Previous Employee
Posts: 13,551
Registered: ‎06-10-2011
Kudos: 5411
Solutions: 1657
Contributions: 2

Re: CR - Change default STP priority from 0 to 32768

Yeah that's likely a good idea and we should look into it. Thanks for the suggestion.

Emerging Member
Posts: 71
Registered: ‎03-04-2013
Kudos: 52
Solutions: 3

Re: CR - Change default STP priority from 0 to 32768

Cool!  Thank you for the quick response.

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