[cisco-voip] Of Expressways and max-forwards...

[Gary Parker]

> On 15 Nov 2021, at 15:34, Gary Parker <g.j.parker at lboro.ac.uk> wrote:
> 
> ...
> 
> So…question: why is the max hops set so low (15) on expressway zones by default when it’s set to 70 on CUBEs, and is there anything this is likely to break/that I should look out for now I’ve made the change?

Thanks for all the feedback and taking the time to reply, folks. A few follow-ups below:

> My expressways were set up by professional services with almost little to no “learning” involved.

Same here. It’s one of those things that’s “always worked” since it was put in so I’ve never had the impetus to learn about it, sadly :-/

> Isn't Calmanager Service Parameter for max-forwards 12? Says if QSIG set to 15. Nothing about if SIP set to 70. 

Having just checked it, yes it is.

Cisco seems to use “maximum hops” and “maximum forwards” interchangeably on different systems, which is less than helpful.

In the CallManager Service Parameters we have “Forward Maximum Hop Count”, which controls the number of times a call can be *forwarded* within the cluster, ie. from one DN to another. I don’t believe this has an impact on SIP “max-forwards” when passing call from router to router when routing calls to PSTN. 

Damnit Cisco, pick a word and stick with it :-)

(I know, I know…there’s history…)

> is the value reset at CUBE to PSTN to 70 on outgoing? that is what logs seems to show. Makes sense if CUBE is IP-IP gateway.


Yes. I can see that the initial INVITE of a SIP call passed from CUCM to CUBE has Max-Forwards <70 (as it passes through my campus network), but the corresponding INVITE sent to my TSP has it reset to 70. 

> All the things Wes said

Thanks, that all makes sense wrt to causing internal loops. I think the problem here, as alluded to earlier, is that Cisco mixes use of maximum “hops" and “forwards" in different contexts (no doubt IETF SIP standards are also partly to blame), and that the defaults on Expressways weren’t set up with SIP PSTN access in mind.

I should also apologise at this point for an error in my previous post: it was not the “max hops” parameter that I had to change in the Zones on the core and edge expressways, but “Hop count”, which is somewhat unintuitive imho.

It’s interesting that CUBE seems to respect and preserve the max-forwards field that’s set on calls via the expressways, but not on those from directly registered CUCM clients. FWIW I’ve not looked at the behaviour wrt to SCCP devices; that may be different again.

Anyways, thanks again folks.

Gary