3Com 4007, Implementation Manual, Page: 194 / 684

194 C
HAPTER
9: B
RIDGE
-W
IDE AND
B
RIDGE
P
ORT
P
ARAMETERS
Frame Processing
All frames that are received on a physical interface and that are not either
discarded or explicitly directed to the module itself are delivered to the
corresponding bridge port. The bridge port either forwards each frame to
another port or discards it.
A module can discard an incoming frame for the following reasons:
■
The destination station is on the same segment as the source station.
■
The receive bridge port is blocked by STP.
■
There is a problem with the frame.
The physical interface does not deliver frames with errors to the bridge
port. Thus, the
rxFrames
fields in the Ethernet statistics display and
bridge statistics display often report different values — that is, the
latter value is lower because it does not count frames in error.
■
A user-defined frame filter (Multilayer Modules only) indicated not to
forward the frame.
■
The frame exceeded the configured broadcast or multicast rate limit
A frame that is forwarded to a bridge port is then transmitted to a
physical interface unless it is discarded. A module can discard a frame at
this point for the following reasons:
■
The transmit bridge port is blocked.
■
The frame is too large for the corresponding physical interface.
■
A user-defined packet filter (Multilayer Modules only) indicated not to
forward the frame.
IP Fragmentation
Standard FDDI allows larger maximum packet sizes than standard
Ethernet. FDDI stations that transmit IP packet sizes larger than
approximately 1500 bytes cannot communicate with stations on an
Ethernet LAN. If a Layer 2 Switching Module receives such packets and
they are destined for one or more Ethernet LANs, it filters them. The same
happens with Multilayer Switching Modules, unless the IP fragmentation
options is enabled.
When you enable IP fragmentation (Multilayer Switching Modules only),
the module breaks up large FDDI packets that is receives into smaller
packets before bridging them to Ethernet ports.