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New Member

IP Telephony over GRE

We have some customers that wish to start using IP telephony over GRE tunnels. We are currently baselining some plans to implement a network wide QoS. However, the use of GRE is going to mask the true nature of a packet as it passes over intermediate routers. This will present a problem in setting up the QoS.

Has anyone tackled a similar problem? What QoS have you used?

3 REPLIES
Cisco Employee

Re: IP Telephony over GRE

I assume that you are the provider of a transit network where customer GRE traffic including VoIP will traverse, and you want to insure proper QoS treatment of the RTP streams.

With LLQ -- frame relay example at http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121newft/121t/121t2/dtfrpqfq.htm -- you can configure certain packets to be given highest priority based on access-lists.

You can permit GRE packets in the access-lists but the problem is that you can't recursively look into the GRE packets to see if they are UDP voice streams, or telnet, or whatever.

However, if you have customers build 2 or more GRE tunnels, one of them specifically for voice traffic only, then you can permit GRE packets coming from and going to the network numbers they defined for that tunnel, and give it strict priority.

Note that if the links are relatively small (less than 768k) then you may also need to fragment the large data packets using FRF.12 if frame relay, or LFI if PPP on leased lines.

New Member

Re: IP Telephony over GRE

Your assumption is correct.

Thanks for the information. I've actually just found out that IP Precedence will automatically be re-created in a GRE header. If that is correct, then we should still be able to provide QoS mechanisms using that setting, thereby allowing us to use a single GRE tunnel.

Cisco Employee

Re: IP Telephony over GRE

If the IP packet's ToS is set in the GRE header ToS, then you would also have to beware of other data applications setting the ToS field. Many Windows-based applications now set the IP ToS field to nonzero values, some of them 5. This happens in both Microsoft applications and OS's such as IE 5.0 and higher, and Windows 2000. Evidence of this can be found by doing a Net search.

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