Redundancy and Spanning Trees
What if the bridge fails now? All of the stations on any given LAN can speak to each
other, but no frames can flow from LAN to LAN. Figure 13.2 shows two bridges, X and
Y, both connecting trunk 1 to trunk 2. What happens if station A on trunk 1 transmits
a broadcast frame (a frame whose destination MAC address is FF-FF-FF-FF-FF-FF)?
Both bridges would receive the frame, and if both behaved as described in the preceding
section, both would forward the frame to trunk 2.
It is important, at this point, to note something about basic IEEE 802.1D bridges;
they do not alter the forwarded frame. The broadcast frame forwarded by each bridge to
trunk 2 is exactly the same, bit-for-bit, as the original frame transmitted by station A.
Figure 13.2 Simple spanning tree example
x2 y2
Bridge X Bridge Y
Trunk 1
Station A
Station B
Trunk 2
Ethernet Bridging 379
Each bridge sees the frame transmitted by the other bridge. There is nothing in the
frame that indicates the frame was transmitted by a bridge and not by station A; the
frame still has station A as its source address. Therefore, each of the two bridges learns
that station A has moved to trunk 2, based on the frame??™s source address (!) and forwards
the frame back to trunk 1.
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