Bug 435757

Summary:	(kvm) bridge_forward_delay 15 seconds default
Product:	[Fedora] Fedora	Reporter:	Warren Togami <wtogami>
Component:	libvirt	Assignee:	Daniel Veillard <veillard>
Status:	CLOSED CURRENTRELEASE	QA Contact:	Fedora Extras Quality Assurance <extras-qa>
Severity:	low	Docs Contact:
Priority:	low
Version:	8	CC:	berrange, clalance
Target Milestone:	---
Target Release:	---
Hardware:	All
OS:	Linux
Whiteboard:
Fixed In Version:		Doc Type:	Bug Fix
Doc Text:		Story Points:	---
Clone Of:		Environment:
Last Closed:	2008-11-04 11:09:19 UTC	Type:	---
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:
Bug Depends On:
Bug Blocks:	188611

Description Warren Togami 2008-03-03 18:34:04 UTC

Sometime after F-8 GA the kernel changed the bridge forwarding delay to be 15
seconds.

kernel source:
net/bridge/br_if.c:     br->bridge_forward_delay = br->forward_delay = 15 * HZ;

This makes it really annoying for KVM PXE boot users, because DHCP fails for 15
seconds instead of working instantly like it did in the past.

Workaround:
brctl setfd virbr0 0.1

Two questions:
1) Why did the kernel change this?
2) Can we workaround this in the virbr0 creation?

Comment 1 Dave Jones 2008-03-03 18:51:19 UTC

The line you quoted was added in 2.3.47pre5
It hasn't changed since.   I think the problem is elsewhere.

Comment 2 Warren Togami 2008-03-03 20:40:54 UTC

OK, it was some other host kernel change that caused this problem.

Comment 3 Warren Togami 2008-07-22 15:00:14 UTC

Reassigning to libvirt.

> I think I understand what's going on here.
>
> - The libvirt 'default' network XML config (aka that for virbr0) does
>   not specify any forward delay or STP setting, so it defaults to
>   delay=0, and STP=on
>
> - The libvirt in F8 GA had a bug, where it called 'setfd' instead
>   'stp' when invoking  brctl.
>
> - Thus, the default config would result in a bridge with STP off
>   and a delay of 1 second.
>
> - A libvirt update fixed the bug in the way we call brctl. Fixing
>   the bug means we now by default create a bridge with STP on
>   and the default kernel delay setting of 15 seconds.
>
> - There is a further bug in that if you specify delay=0 in the XML
>   it'll never call 'brctl setfd 0', so you'll be stuck with the 15
>   second default still.
>
> So yes, we need to fix things such that it has STP=on and a tiny (or even 
> zer) forward forward delay again, as per F8 GA behaviour. Can you file
> a BZ about this problem against libvirt. It impacts F9 and rawhide too
>
> Daniel

Comment 4 Daniel Berrangé 2008-11-04 11:09:19 UTC

libvirt now explicitly sets FD to zero, from at least 0.4.5 release