Ethernet Interface (10BASE-T or 10/100BASE-T)
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The SAR contains 15 traffic shapers, each of which can be programmed for
sustained cell rate of transmission (SCR), peak cell rate (PCR), and maximum burst
size (MBS).
The traffic shapers work as follows:
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The packet is segmented using a dual leaky buffer algorithm, whereby the cells
are transmitted from each connection in the shaper at an average rate until the
bucket of token fills up (a token is given to the connection at an average rate if
it has no cells to transmit at that moment).
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The shaper then turns the burst mode on and transmits at the peak rate for a
burst length. Note that the shaper serves every connection independently.
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Cells received from the ATM WAN are switched via the CTX chip to the SAR
queues. You can enable shaping on the SAR queues to slow down the
incoming traffic on the SAR.
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The SAR will then assemble the cells belonging to the connections specified for
it.
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When the cells are assembled, the CPU is given confirmation, and the bridge
function of the CPU examines the packet header, removes the encapsulation,
and—after learning the address and updating the bridge table—forwards the
packet to the proper destination.
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The pointer of the packet to be forwarded is placed in the transmit queue of
the Ethernet port.
Bridge Operation
The PathBuilder S330/S310 switch supports ATM permanent virtual circuits (PVCs)
for access to the ATM network. All the PVC’s configured for a port will form a
virtual Bridged Ethernet network to all other ports at the other end of the PVC.
The bridge operation is equivalent to a multiport bridge. It works as follows:
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The bridge learns and builds forwarding tables for every PVC that is tied to the
Ethernet port.
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When a packet is received on the port or any PVC tied to that port, the source
MAC address is learned and kept in the forwarding table until the aging timer
expires. You can use the Bridge Configuration menu to add static forwarding
addresses that the bridge will not delete after the aging timer expires. See
“Configuring the Bridge” in Chapter 4, for details.
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When the packet arrives, the bridge looks up the destination MAC address to
determine the destination of the packet from the forwarding tables. If a
destination is found, the packet is forwarded to the correct destination. If a
destination is not found, the bridge broadcasts or floods the packet on all
PVC’s that are tied to the Ethernet port. Flooding is performed by forwarding
the packet on all destinations tied to the port.
The PathBuilder S330/S310 switch performs LLC-based multiplexing. Ethernet
frames are bridged as per RFC1483. Frames with NLPID of 0x00 are discarded as
being invalid frames. The switch does not support frames bigger than Ethernet
maximum frame size (1518) or IEEE802.3 frame size (1492) bytes (+ framing
bytes) coming off the ATM network.
