
Switched Fast Ethernet Controller
Revision 1.2
59
Figure 14: Station-to-Station Connectivity Matrix Data Structure
14.3
Monitoring (Sniffer) Mode
The CPU can program the GT-48002A to work in Monitoring mode for one of the two Fast Ethernet ports. In Monitoring
mode, the GT-48002A sends all receive (including local traffic) and transmit packets to the CPU or to a port in one of
the GT-48002A devices which was assigned to be the Sniffer. The packets that are forwarded to the Sniffer are not
necessarily in a linear time order.
Monitoring Mode is enabled by setting bit 2 in the Port Control register. The target sniffer is written into the CPU and
Sniffer Numbers register. Only one port of a single GT-48002A device can work in monitoring mode at a time.
14.4
Spanning Tree Support
The GT-48002A provides the hardware assistance for Bridge Spanning Tree Algorithm implementation. The Spanning
Tree algorithm itself is performed by a management CPU.
The GT-48002A includes a SpanEn bit in the Global Control register and additional SpanEn bits in each of the 8 Port
Control registers. Table 31 summarizes the hardware assistance for the Spanning Tree algorithm.
Note 1: The GT-48002A does not learn MAC addresses during the Spanning Tree ‘Learning’ stage (it is ‘learning’ the
bridge topology while in this mode.) The GT-48002A only learns MAC addresses in the Forward mode.
Note 2: The CPU can send BPDU messages to a port of the GT-48002A which is disabled. The mechanism to send
BPDUs from the CPU to a locked port is to send a BUFFER_REQUEST message like the format shown in Section
10.3.4, but with the LSB bits of the address as 0x58, instead of 0x0. This BUFFER_REQUEST message will cause the
GT-48001A to allocate a buffer regardless of the state of that port.
14.5
Broadcast Storm Filtering
Excessive broadcast packets (broadcast “storms”) can be filtered in a managed switch by setting the DisBroad bit in
the port control register. Broadcast packets can be re-enabled once the loops causing the broadcast storm are elimi-
nated via the spanning tree algorithm.
Table 31: Spanning Tree Enable Bit Definition
Sp a n En
( Gl oba l)
Sp a n En
(P or t)
L ogi c
St at e
Re m a rk s
0
x
Port Enable
No Spanning Tree. Treat BPDUs as regular Multicast.
1
Blocking,
Listening, Learning
Transfer BPDUs to CPU. All receive/transmit packets are
rejected, except BPDU messages from the CPU. Address
learning disabled.
1
0
Forward
Transfer BPDU to the CPU. Accept all packets. Address
learning enabled.
Byte Count
Dest. Addr[16:47]
Src. Addr[32:47]
Dest. Addr[0:15]
Src. Addr[0:31]
.
20
10
0
31
Word0
Word1
Word2
Word3
Word4
Ports
Routed To
7
Type
21