Re: Router alert label : Use case

Gaurav,

One of the use cases for MPLS Router Alert Label is in VCCV type-2 pings.
VCCV uses a control channel to send/receive MPLS ping packets which aids in
Data path failure or fault isolation (OAM operations). A general case for
using RA Label is when Control plane is out of sync with Data plane and
hence User Traffic is Not delivered to end customer CE device.

If control word is enabled and successfully negotiated during PW capability
exchange (aka In-band or type-1), then Router-Alert Label is not used. If
the control word is absent, then Router-Alert Labels are used (aka
Out-of-band or type-2). To answer your Qs:

(1) What the control plane punt does on Egress PE is basically a FEC
validation against Label Stack and few sanity/error checks. If you are more
interested, check RFC 4379 section 4.4.

(2) The Reply mode specified in CLI says to Egress PE how the Echo Reply
packet should be sent back. Yes, LSPs are Uni-directional; however the
sender can indicate in the Echo request whether 'Not to send' (or) 'Send
IPv4/UDP reply with/without Router-Alert'. Normal case is to send without
Router-Alert. The CLI you specified controls this behavior. And yes, Label
for traffic on return path is Not tested.

In summary, LSP pings are diagnostic tools for basic connectivity checks.
If you need to isolate the fault location, use 'traceroute mpls ..' instead.

HTH,
Naveen.

PS: There is much more to this (in terms of ECMP considerations, Security
checks, Inter-AS cases, etc) that its almost impossible to hold in my
little wired-head :)

On Fri, Dec 26, 2014 at 1:39 AM, GAURAV MADAN <gauravmadan1177_at_gmail.com>
wrote:

> Hi All
>
> Is there a practical use case that you guys are running for Router Alert
> Label ( label =1) in your network ?
>
> 1) All the theories indicate that these pkts are punted to s/w for special
> investigation . But not sure what investigation and what are those special
> cases . Can someone please give an example .
>
> 2) For LSP ping option ; LSP pings are also unidirectional .. i.e LSP ping
> from A to B will check the labelled path from A to B only ( and not from B
> to A ) . Hence i dont understand the usage of option :
>
> ping mpls ipv4 44.44.44.44 255.255.255.255 reply mode router-alert
>
> Hence as per me ; for LSP ping from A to B ; if it goes A-C-D-E-B , and I
> am getting a reply as success ; this does not mean that
> a) return traffic would have taken the B-E-D-C-A path
> b) It also do not for sure mean that labelled path from B to A is
> intact .
>
> Can some one please clarify these for me .
>
> Regards
> Gaurav Madan
>
>
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Received on Sun Dec 28 2014 - 02:26:30 ART