[c-nsp] BFD feedback?

[Chris Woodfield]

I wasn't advocating that BFD *only* work at the link level, but for  
point-to-point links it's a feature I'd like to have as an option.  
There would obviously need to be some mechanism by which BFD packets  
would continue to be sent and received despite the interface protocol  
being shut down, but that's an implementation detail. My pain point  
is exactly what I described - ensuring proper and timely failover of  
point-to-point MAN access circuits that don't lose link state when a  
metro switch in the middle goes belly-up. If we could do this with  
BFD *instead* of, instead of an top of, a routing protocol between  
ourselves and our customers, that's a big win.

Having BFD trigger the withdrawal of a static route would be a nice  
interim step here. Did cisco ever put this on their feature timetable?

-C

On Oct 24, 2007, at 5:48 AM, Sam Stickland wrote:

> Chris Woodfield wrote:
>> BFD is a lifesaver where you have circuits such as metro ethernet   
>> links that don't lose link state when something in the middle  
>> blocks  connectivity. It's less useful across WAN links that  
>> depend on end-to- end connectivity to maintain line protocol.
>>
>> As Arie, said, the hardest part of implementation is the timer   
>> tuning, to get the best balance of response time vs. CPU  
>> utilization  as well as preventing false timeouts, another side  
>> effect of setting  the timers too low.
>>
>> That said, I do feel that tying BFD to routing protocol events  
>> only  is a bit shortsighted - why not have an option to just  
>> change line  protocol to down in a case of BFD timeout failure,  
>> and let the  routing protocols react the that naturally?
>>
> Surely this wouldn't work on multi-access networks (e.g.  
> ethernet ;) )? BFD forms adjacencies according to the routing  
> protocol neighbors, not according to the physcial links. Consider  
> the followinng topology:
>
> R1 ----- SW1 ---- R2
>                 |
>                R3
>
> R1, R2 and R3 share a broadcast domain via SW1 and each one has  
> formed an OSPF adjacency with each other (full mesh). This means  
> that there is a full mesh of BFD neighbors also. If the link from  
> SW1 to R3 goes down BFD will detect this and take down the  
> appropiate OSPF neighbors. If BFD shut the interface down instead  
> it would also sever the communication between R1 and R2, which  
> isn't want you want.
>
> Sam
>