[c-nsp] OSPF fast convergence

Geoffrey Pendery geoff at pendery.net
Thu May 14 09:44:58 EDT 2009

Well, if it's an SVI, then you're up a little higher than layer 1...
Presumably the VLAN still exists on other links (like a trunk over to
another 7600?) so the SVI itself doesn't go down, you just wait for
the neighbor relationship to timeout?  Or are you actually pulling a
cable and seeing "int Gig 1/1 up/up" for 3 seconds?  After the 3
seconds, do you actually see the VLAN interface go down/down?

Sorry if this is obvious stuff you already thought of, I'm just
baffled 'cause I've never seen slow link-down on 6500 or 7600
personally, though I admit I've mostly done my failure testing on
fiber not copper...

I suppose if you're in a bind, you could cobble together your own BFD
with EEM... have IPSLA ping the neighbor every 300 msec or so, and if
it fails three times you admin down the SVI...  Obviously it's a
kludge, but it might beat waiting on the proper BFD support...


On Thu, May 14, 2009 at 8:29 AM, Justin Shore <justin at justinshore.com> wrote:
> Phil Mayers wrote:
>> Justin Shore wrote:
>>> Phil Mayers wrote:
>>>> Common advice seems to be to make actual link-loss detection fast, in
>>>> preference to using BFD. That said, I know some people use BFD.
>>>> Assuming you're using LAN cards, you may want to see if you can make
>>>> router links as routed rather than SVI interfaces. Though routed interfaces
>>>> are implemented internally as VLANs, presentations I saw from Cisco claim
>>>> that this:
>>> I prefer to use BFD personally.  Link failure detection without BFD will
>>> be slow no matter what you do.  FRR doesn't gain you much if it takes you
>>> several seconds to realize that a link dropped.
>> Seconds? Wow. I'm curious - under what circumstances are you seeing such
>> length link-loss detection times?
> On our 7600s today.  We drop a link to one of them and the thing is
> oblivious to the drop for 2-3 seconds.  Dumber than a post...  L3 is waiting
> on L1 to wake up before it can start tearing down routing relationships and
> pulling routes.  I'm running SRB1 on my 7600s right now so that I can run
> BFD.  I'm trying to get my account team to carry my BFD on SVI request to
> the DE team for BFD or the product manager. Unfortunately I think I'm
> throwing pennies into a blackhole.
> Justin
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