Re: ping-pong phenomenon with p2p links & /127 prefixes

[Seiichi Kawamura <kawamucho@mesh.ad.jp>]

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Hi Ole-san

Ole Troan wrote:
>>>>>>> please ping my router, it's interface address is: fe80::20e:cff:fe5c:b001/64
>>>>>>>
>>>>>>> my monitoring system can't ping this to ensure liveness of the
>>>>>>> interface either :(
>>>>>> but they can ping whatever global /128 you put on that interface, so why doesn't that solve the problems?
>>>>> Because you are then using one set of addresses for protool peerings
>>>>> and another one for global ping - thus making life more complicated
>>>>> for the operator.
>>>> is that any more "complicated" (I don't quite understand that argument) than using IS-IS?
>>>>
>>> Yes. 
>> yes. I tried something close to it 5 years ago and it was hell.
> 
> how? Jared's "Yes" doesn't exactly help my understanding why this is operationally complex.

Jared's yes may be different from my yes, but

(quoting your exmaple)

> BGP peerings and what not could use link-local addresses. e.g:
>
> router A --------------  router B
> fe80::1                        fe80::2
> dead:beef::1/128     c001:cafe::2/128

if
   I get a BGP neighbor down message with fe80::2
then
   what address do I ping, trace? I can look at config of router A
   and my address is dead:beef::1. What's the other side's global address?
   If router B isn't mine, I may not have a clue.

The challenge here is that we don't always have the knowledge of
whats on the other side of the router. When you have tons of these
links on one router, this is just making trouble shooting harder.

Even if I did know the other side's global address, monitoring pings
cannot be sent to fe80::2. We'll have to ping c001:cafe::2 and
manually link that status with fe80::2 peering session on the NMS.
I would hate to do that with hundreds of sessions running inside my network.
That's always been a causes mistakes. We want to monitor what's
acutally running and not some alias address.

Regards,
Seiichi
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