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New Member

rtp header-compression


i configured the following on my router:


interface Serial0/0

description Int S0/0 - Verbdg. zum BR-LinzLand

bandwidth 512

no ip address

max-reserved-bandwidth 95

encapsulation frame-relay IETF

load-interval 30

no snmp trap link-status

frame-relay traffic-shaping

frame-relay lmi-type q933a


interface Serial0/0.1 point-to-point

description Serial0/0.1: Verbindg. zum BR-LinzLand

ip address

no cdp enable

frame-relay class 512k_voice

frame-relay interface-dlci 16 CISCO

frame-relay ip rtp header-compression


fact is that i can not see any compressed rtp-packets - but i have voice-traffic!

is there anything wrong in the config (must i also turn on compression on the physical int ??)

r-traun1#sh ip rtp header-compression

RTP/UDP/IP header compression statistics:

DLCI 16 Link/Destination info: point-to-point dlci

Interface Serial0/0:

Rcvd: 0 total, 0 compressed, 0 errors, 0 status msgs

0 dropped, 0 buffer copies, 0 buffer failures

Sent: 0 total, 0 compressed, 0 status msgs

0 bytes saved, 0 bytes sent

Connect: 256 rx slots, 256 tx slots,

0 long searches, 0 misses 0 collisions, 0 negative cache hits


New Member

Re: rtp header-compression

CRTP feature used not to work under IETF encapsulation, not sure whether it does currently,

you might need to change it to cisco ecapsulation.

New Member

Re: rtp header-compression


thanks for answer - but:

i have enc cisco for dlci 16 which is the one where the branch is connected.

or do you mean that i have to change the enc to cisco on the maininterface ?

New Member

Re: rtp header-compression

That's true, I didn't notice that. So as long as both are set for cisco encapsulation, this should not be an issue. I suppose if there is no compression, there must be something else going on.

Hmm, do you know that the RTP stream conforms to the cRTP requirements ? Here's the link for what's required on those streams:

New Member

Re: rtp header-compression

hi thanks for the url.

it seems that in fact my rtp-stream is in a range which does cisco not compress (by default?).

i sniffered some phone-connection and saw that our

siemens-voip shelfs use udp port 4512 - 4600 for example.

do you know if there is a possibility to change the routers udp - ports for compressing ??


New Member

Re: rtp header-compression

As far as I know, no, there is no way to change the way packets are considered for compression with cRTP.

However, you might be able to just use a simple header compression. Your compression ratio will not be as great as instead of compressing 40 bytes like in cRTP you will be only compressing 28 bytes. But, at least you still have an option. You'd need to use

'ip rtp header-compression iphc-format' under the serial interface or modular QOS cli, here is an example:

class-map match-any identify-voip

mathc ip address 180

policy-map compress-voip

class identify-voip

compress header ip rtp

access-list 180 permit udp any any range 4512 4600

Unfortunately, this might not work under frame-relay, unless you use ppp over frame-relay. I remember it being supported under PPP and HDLC only. Also, the statistics for rtp compression will not show anything being compressed, as no RTP was identified. I know it doesn't look like this should work, but that's how it is. You are still able to compress the header, but it's not an RTP compression no more.

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