The Discard Eligibility Bit
When congestion occurs, queues begin to fill, and in some cases, frames must be tail-dropped from
the queues. Switches can (but are not required to) examine the FR Discard Eligibility (DE) bit
when frames need to be discarded, and purposefully discard frames with DE set instead of frames
without DE set.
Both routers and switches can set the DE bit. Typically, a router makes the decision about setting
the DE bit for certain frames, because the network engineer that controls the router is much more
likely to know (and care) about which traffic is more important than other traffic. Marking DE can
be performed with CB Marking, as covered in Chapter 12, “Classification and Marking,” using the
MQC set fr-de command.
Although routers typically mark DE, FR switches may also mark DE. For switches, the marking
is typically done when the switch polices, but instead of discarding out-of-profile traffic, the
switch marks DE. By doing so, downstream switches will be more likely to discard the marked
frames that had already caused congestion.
Table 15-3 summarizes some of the key points regarding Frame Relay’s FECN, BECN, and DE bits.
Frame Relay FECN, BECN, and DE Summary
Bit Meaning When Set Where Set
FECN Congestion in the same direction as this frame By FR switches in user frames
BECN Congestion in the opposite direction of this frame By FR switches or routers in user or
Q.922 test frames
DE This frame should be discarded before non-DE frames By routers or switches in user frames