Avaya Configuring Frame Relay Services User Manual download pdf (Page 37)

Frame Relay Overview

114058 Rev. A 2-13

Random balancing evenly distributes trafﬁc and lets the router use the two lines

efﬁciently. Because packets travel on different paths, they arrive at the destination

out of sequence, and the upper-layer protocols, for example, IP and OSI, have to

resequence the information. Some protocols cannot tolerate packets arriving out

of sequence, so be sure this option is appropriate for your application.

Address-based Distribution

Address-based distribution, as the name implies, determines the data path for

outbound trafﬁc from the source and destination address in each packet. For any

given address pair, the same path is always used.

The router determines whether to route or bridge the packet, and then uses the

appropriate address. It uses the routing-level addresses for routing trafﬁc, and the

MAC-level addresses for bridging trafﬁc.

Address-based distribution ensures that all outbound trafﬁc travels on the same

path, and that packets arrive in sequence. For protocols that cannot receive packets

out of sequence, use this method. Note, however, that this option does not always

distribute trafﬁc evenly across each line.

Congestion Control

Network congestion can degrade network performance. Congestion occurs when

a node receives more frames than it can process, or sends more frames than the

transmission line can handle. The Frame Relay network informs the nodes of

congestion, so that they can reduce the amount of trafﬁc across the network.

In the Frame Relay packet header, there are two bits that the network sets to alert

nodes of network congestion. These bits, as deﬁned by the Frame Relay

speciﬁcation, are the FECN (forward explicit congestion notation) bit and the

BECN (backward explicit congestion notation) bit.

If the network detects congestion, it alerts the router in the same direction as the

received frame by changing the frame’s FECN bit from 0 to 1. For nodes in the

opposite direction of the received frame, it changes the frame’s BECN bit from 0

to 1

(Figure 2-8).

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Avaya Configuring Frame Relay Services User Manual Page 37