[c-nsp] Label still appearing in traceroute after disabling ttl propagation

[John Neiberger]

For the record, someone had applied some LDP label filtering for testing,
which caused one of the P routers not to have labels for a set of prefixes
and allowed the original IP traceroute packet to be exposed within the core
before it reached the PE router. Once we removed the filtering, all was
well.


On Tue, Jul 30, 2013 at 12:28 PM, John Neiberger <jneiberger at gmail.com>wrote:

> After a little more investigation, I think the problem is that our P2
> router is not learning a set of prefixes via LDP that it should be, so it
> is sending them unlabeled to PE2. We assumed that both P routers had the
> right labels, but that doesn't appear to be the case.
>
>
> On Tue, Jul 30, 2013 at 12:20 PM, John Neiberger <jneiberger at gmail.com>wrote:
>
>> I guess I should rephrase. We have configured "mpls ip-ttl-propagate
>> disable" to try to hide the labeled part of the path. For whatever reason,
>> we always get something like the following:
>> CE1#trace 10.6.10.1 source lo0
>> Type escape sequence to abort.
>> Tracing the route to 10.6.10.1
>> VRF info: (vrf in name/id, vrf out name/id)
>>   1 192.168.105.50 0 msec 0 msec 0 msec
>>   2 192.168.62.2 [MPLS: Label 16018 Exp 0] 4 msec 0 msec 0 msec
>>   3 192.168.62.60 0 msec 0 msec 0 msec
>>   4 192.168.106.61 4 msec *  0 msec
>> CE1#
>> If I trace from CE1 to the loopback of PE2, which is the same path, it
>> works as expected and the labeled part of the path is hidden:
>>
>> CE5#trace 10.6.1.1 source lo0
>> Type escape sequence to abort.
>> Tracing the route to 10.6.1.1
>> VRF info: (vrf in name/id, vrf out name/id)
>>   1 192.168.105.50 4 msec 0 msec 0 msec
>>   2 192.168.62.60 0 msec *  0 msec
>>
>>
>> On Tue, Jul 30, 2013 at 11:43 AM, Jared Mauch <jared at puck.nether.net>wrote:
>>
>>> Disable TTL != don't copy label into ICMP TTL Expired message.
>>>
>>> - Jared
>>>
>>> On Jul 30, 2013, at 1:37 PM, John Neiberger <jneiberger at gmail.com>
>>> wrote:
>>>
>>> > I think either we're just doing something wrong or perhaps we're
>>> running
>>> > into a bug. I did find this one, which sounds similar:
>>> >
>>> > https://tools.cisco.com/bugsearch/bug/CSCtd17126
>>> >
>>> > I'm not sure if that is fixed in 4.1.0 or not.
>>> >
>>> >
>>> > On Tue, Jul 30, 2013 at 11:01 AM, John Neiberger <jneiberger at gmail.com
>>> >wrote:
>>> >
>>> >> W
>>> >> e're running into an interesting problem. We have a simple lab setup
>>> like
>>> >> this:
>>> >>
>>> >> CE1 -- PE1 --- P1 --- P2 --- PE2 --- CE2
>>> >>
>>> >> We have "mpls ip-ttl-propagate disable" on all PE and P routers, but
>>> if we
>>> >> trace from CE1 to CE2, we still see an MPLS label coming from the PE2
>>> >> router. If we trace CE2 to CE1, we see a label on the hop from P1 to
>>> PE1.
>>> >>
>>> >> If we have ttl propagation completely disabled, why would we still
>>> see the
>>> >> label and P-to-PE link in the path?
>>> >>
>>> >> All P and PE routers are IOS XR running 4.1.0.
>>> >>
>>> >> Thanks,
>>> >> John
>>> >>
>>> > _______________________________________________
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>>>
>>>
>>
>