50 Chapter 2: End-to-End QoS: Quality of Service at Layer 3 and Layer 2
You can verify the configuration of LFI for Frame Relay with the show frame-relay
fragment command. These commands are, at present, only applicable to the FlexWAN
module for the Catalyst 6500 Family of switches.
Real Time Protocol Header Compression (cRTP)
Real Time Protocol (RTP) is the standard protocol for the transport of real-time data, and is
defined in RFC 1889. As defined, an RTP packet includes the payload plus an RTP/UDP/
IP header. The total header is 40 bytes and breaks down as follows: The RTP header is 12
bytes, the UDP header is 8 bytes, and the IP header is 20 bytes. Depending on the appli-
cation, the payload is typically between 20 bytes and 160 bytes. That means that it’s
possible that the header would be twice the size of the payload! Clearly, that is not efficient,
so the folks at the IETF came up with a way to take advantage of similarities between
successive packets and reduce the 40-byte header to somewhere between 2 and 5 bytes. For
VoIP, this header compression can yield an overall reduction in packet size on the order of
200 percent or more.
Although the idea of using cRTP everywhere sounds good at first glance, the trade-off, in
terms of performance, on link speeds higher than T1 are not desirable. Therefore, use of
cRTP should be limited to links with speeds of T1 or less. Speaking of performance, prior
to Cisco IOS 12.0(7)T, cRTP happened in the process switching path, which often slowed
the processing of packets to the point that it was not desirable to use cRTP. As of 12.0(7)T,
however, cRTP will use the Cisco Express Forwarding (CEF) switching path, or fast
switching path (if CEF is not enabled). Only if both CEF and fast switching are disabled
will cRTP packets be process switched.
The configuration of cRTP on a serial interface is simple, as shown in the following
example:
Router(config-if)#ip rtp header-compression [passive]
The passive keyword tells the routers to compress outgoing RTP packets only if the
incoming RTP packets on that interface are compressed.
For Frame Relay PVCs, the configuration of cRTP is as follows:
router(config-if)#frame-relay map ip
ip-address dlci
[broadcast] rtp header-
compression [active | passive]
To verify the configuration and operation of cRTP, use the following commands for Frame
Relay and non-Frame Relay interfaces, respectively:
Router#show frame-relay ip rtp header-compression [interface
type number
]
Router#show ip rtp header-compression [
type number
] [detail]