End-to-End Fragmentation and Reassembly

The fragmentation and reassembly feature reduces excessive delays of Frame Relay packets by breaking them up into smaller fragments and interleaving them with real-time frames. By doing this, real-time and non-real-time data frames can be carried together on lower-speed links without causing excessive delays to the real-time traffic. On receiving the smaller fragments by the peer interface, the fragments are reassembled into their original packet. For example, short delay-sensitive packets, such as packetized voice, can race ahead of larger delay-insensitive packets, such as common data packets.

E Series routers support end-to-end fragmentation according to the FRF.12 Implementation Agreement standard. Unlike UNI and NNI fragmentation, end-to-end supports fragmentation only at the endpoints. End-to-end fragmentation and reassembly are supported only on non-multilink Frame Relay interfaces on cOC12/STM4 and CT3 12 FO modules.

You configure end-to-end fragmentation at the Frame Relay subinterface level. Fragmentation is applied to all PVCs associated with the subinterface. In most cases, fragmentation and reassembly are used together. Fragmentation and reassembly, however, can be configured separately for each map class.

For additional information, see Frame Relay Forum—Frame Relay Fragmentation Implementation Agreement, FRF.12 (December 1997).

Frame Fragmentation

When you enable fragmentation, you can specify a maximum payload size of the resulting fragments. If the maximum payload size is not specified, the default value of 52 bytes is used. When enabled, fragmentation begins when the portion of the packet that has not been transmitted in previous fragments exceeds the configured maximum payload size. The fragmentation process continues until the entire packet has been transmitted. Frames that do not exceed the configured maximum payload size are not fragmented.

If you disable fragmentation, all packets transmitted by the Frame Relay subinterface are transmitted intact.

Frame Reassembly

When reassembly is disabled and a data frame is received, a few scenarios may occur:

If you enable reassembly, then received fragments undergo the reassembly process. Packets that are not fragments are forwarded as normal.

Map Class

Within Frame Relay, a map class acts as a container or context for fragmentation and reassembly parameters. Within the map class context, you can explicitly enable fragmentation and reassembly.

After you define a map class, you can apply it to an unlimited number of subinterfaces. This allows you to change fragmentation and reassembly parameters one time and have the changes immediately reflected in all subinterfaces configured to use that map class.

Configuring End-to-End Fragmentation

You configure end-to-end fragmentation and reassembly on a subinterface in much the same way you configure a standard Frame Relay interface. In this example, end-to-end fragmentation and reassembly is configured on a single subinterface with a 100-byte fragment size (maximum payload size). All tasks are mandatory unless otherwise noted.

Note: The procedure described in this section assumes that a physical interface has been configured. See Before You Configure Frame Relay.

To configure end-to-end fragmentation and reassembly:

  1. Create a map class that you can apply to subinterfaces.
    host1(config)#map-class frame-relay testmap
  2. Specify fragmentation and reassembly for the map class. Optionally, you can specify the maximum payload size of a fragment.
    host1(config-map-class)#frame-relay fragment 100
  3. Enter the physical interface on which you want to configure Frame Relay end-to-end fragmentation and reassembly.
    host1(config-map-class)#interface serial 5/0:4/1
  4. Select Frame Relay as the encapsulation method for the interface.
    host1(config-if)#encapsulation frame-relay ietf
  5. Create a subinterface.
    host1(config-if)#interface serial 5/0:4/1.1
  6. Add a circuit to a subinterface.
    host1(config-subif)#frame-relay interface-dlci 16 ietf
  7. Assign a local IP address to the circuit.
    host1((config-subif)#ip address 42.42.42.41 255.255.255.0
  8. Associate a map class with a subinterface.
    host1(config-subif)#frame-relay class testmap

encapsulation frame-relay ietf

frame-relay class

frame-relay fragment

frame-relay interface-dlci ietf

interface serial

ip address

map-class frame-relay