3Com 3C13636 Configuration Manual page 355

3Com Router 3000 Ethernet Family
Configuration Guide
00e0.fcaa.aaaa
00e0.fccc.cccc
Figure 3-7 No matched MAC address is found in the bridging table
III. Eliminating loop
As shown in the following figure, both bridge X and bridge Y are connected with
Ethernet segment 1. Once detecting a broadcasting frame, both bridges will send it to
all ports except the source port on which the frame is detected. That is, both bridge X
and bridge Y will forward this broadcast frame.
Ethernet segment 1
Figure 3-8 Preliminary examination state of bridging loops
As shown in the following figure, the broadcast frame is forwarded over Ethernet
segment 2 and Ethernet segment 3 that are connected with Bridge Z. Upon detecting
two copies of this frame on two different ports, Bridge Z forwards them to Ethernet
segment 3 and Ethernet segment 2 again. Thus, Ethernet segment 2 and Ethernet
segment 3 receive a copy of this frame for the second time. In this way, the frame is
repeatedly forwarded over the network, which is called bridging loop. See the figure
below.
Source address Destination address
00e0.fcaa.aaaa 00e0.fccc.cccc
Bridging table
MAC address
Port
00e0.fcaa.aaaa
1
1
00e0.fcbb.bbbb
Broadcast address
FFFFFFFFFFFF
Bridge X
3Com Corporation
Bridge port 1
Bridge
Bridge port 2
Bridge Y
3-6
Chapter 3 Bridge Configuration
00e0.fcbb.bbbb
Ethernet segment 1
00e0.fcdd.dddd
Ethernet segment 2
Ethernet segment 2
Bridge Z
Ethernet segment 3