Cisco Field Manual[c] Router Configuration

• Link Fragmentation and Interleaving (LFI) fragments large packets, reducing the serialization times as packets are sent out a serial interface.

• LFI also allows other, smaller packets to be interleaved with fragmented packets. Time-critical protocols can be sent more predictably, because they don't have to wait for large packets to be forwarded.

• LFI can work with Multilink PPP (MLP) to send fragmented packets over multiple physical links, Frame Relay, and ATM VCs.

• Compressed Real-Time Protocol (CRTP) offers a compression that can compress the 40-byte IP/UDP/RTP header into a 2- to 5-byte value. CRTP is used on a per- link basis.

• CRTP can be used on serial links with a speed of T1 or less, over HDLC, Frame Relay, PPP, or ISDN.

• CRTP can be used with CEF for maximum efficiency.

Configuration

1. Use LFI with MLP.

   1. Enable MLP on an interface:

      (interface) ppp multilink

      See Section 3-6 for further information about configuring MLP.

   2. Enable packet interleaving:

      (interface) ppp multilink interleave

   3. Set the maximum acceptable delay between fragments:

      (interface) ppp multilink fragment-delay milliseconds

      MLP chooses a fragment size that gives a maximum serialization delay of milliseconds. Voice traffic using RTP typically requires a delay of 20 milliseconds or less.

   4. (Optional) Set up a reserved bandwidth queue for RTP voice traffic:

      (interface) ip rtp reserve lowest-UDP-port UDP-port-range [ max-bandwidth ]

      A special queue with a higher priority can be set up for RTP traffic on the interface. The range of UDP port values for RTP is given as lowest-UDP-port with UDP-port-range number of ports. A maximum reserved bandwidth can be specified to set an upper limit on the RTP bandwidth used. If the actual bandwidth used rises above max-bandwidth (Kbps), the RTP traffic is forwarded in a best-effort fashion.

2. Use Compressed Real-Time Protocol (CRTP) on a serial interface.

   1. (HDLC, PPP, ISDN) Enable CRTP:

      (interface) ip rtp header-compression [ passive ]

      RTP headers are compressed. Be sure to enable compression on both ends of a serial link. The passive keyword allows CRTP to compress outbound headers only if received inbound headers are also compressed.

   2. (Frame Relay) Enable CRTP.

      • Use CRTP for all VCs on the physical interface:

        (interface) frame-relay ip rtp header-compression [ passive ]

        Be sure to enable compression on both ends of the VC. The passive keyword allows CRTP to compress outbound headers only if received inbound headers are also compressed.

      • Use CRTP for a single DLCI:

        (interface) frame-relay map ip ip-address dlci [ broadcast ] rtp header-compression [ active passive ]

        The IP address is mapped to a specific DLCI, and CRTP is enabled. Use active for unconditional RTP header compression and passive for compression only if the far end is sending compressed headers.

   3. (Optional) Define the number of supported CRTP connections:

      (interface) ip rtp compression-connections number

      CRTP keeps a cache of compressed RTP connections occurring on the interface. The maximum number of connections can be changed to number (3 to 1000; the default is 32 connections, or 16 calls). This number specifies unidirectional RTP connections such that a voice call requires two connections (one in each direction). Also, the number of compressed RTP connections should be set to the same value on both ends of a link.

Link Efficiency Mechanism Example

A dialer interface is configured with Multilink PPP and LFI. An LFI fragment delay of 20 milliseconds is used. An IP RTP priority queue is also set up for UDP ports 32768 to 32818, using a maximum bandwidth of 512 kbps. In addition, the serial 1 interface is configured for Compressed RTP headers, with a maximum of 128 RTP connections.

interface dialer 1 description Dialer template for a rotary group ip address 192.168.33.1 255.255.255.0 encapsulation ppp dialer map ... dialer-group 1 dialer-list 1 protocol ip permit ppp multilink ppp multilink interleave ppp multilink fragment-delay 20 ip rtp reserve 32768 50 512 interface serial 1 ip address 192.168.116.71 255.255.255.0 ip rtp header-compression ip rtp compression-connections 128