Figure 87 Frame Relay Header

1 2 Variable 2 i

Retey Address Header

Data

FC S

Flag

f

Byte 1

Byte 2

-

DLCf

C/R

EA

DLCI

FE CM

BECN

DE*

EA

*|f I here is a proWem, discard the data

*|f I here is a proWem, discard the data

Think of the 10-bit DLCI like the MAC address on a LAN. Both are Layer 2 addresses, and routing re mappings of IP next hops to Layer 2 addresses. The DLCI identifies the local connection. The Command/Response (C/R) bit is application-specific and not modified by the network, and the Ext< bits allow for a 3- or 4-byte header. Current implementations use a 2-byte DLCI, but the EA bits al growth in the future. The next 3 bits are used for congestion control. Figure 8-8 shows a graphical explicit congestion notification (ECN).

Figure 8-8. Frame Relay ECN

Figure 8-8. Frame Relay ECN

BECN - Backward Explicit Congeslion Wolificalion

Notify the source that congestion has occured in the path.

i-fcCN F oiwrd 6 xpl icif Conges! ion Notifies lion

Signal 11-ig nexl neighbor in I he palh loward desiinalmn thai congestion has occurs aioog the pith

FECN is forward explicit congestion notifi cation is that it tells tce receiving end t hat th e congestion the path from the s ource to the destination. oECN is backwa rd explicit congestion n otificdtion in th.

frames traveling in the opposite direction of the congested path. FECN notifies the destination, whe notifies the source. The DE bit is a priority-based DE bit in case the network becomes short of reso frame switch sets the DE bit to 1 when the frame is above your CIR (or committed burst). CPE coul same, but it makes no sense for the CPE (router) to set DE. Why should you volunteer for packet d compared to all the other customers?

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