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Bridging, Switching, and Spanning Tree 167
Figure 4-19
CBPDUs While Bridge 4's MaxAge Expires
A review of the behavior of this network is useful before seeing how it is about to change. For
example, the frame on the Ethernet between Bridges 3 and 4 cannot be forwarded by Bridge 3
because it is blocking on its E1 interface. The instant Bridge 5's E1 port fails, frames can no
longer be forwarded or received on that interface. So, during the period that MaxAge is expiring
on Bridge 4, frames can be sent by hosts on the segment between B4 and B3; then, B4 can
forward the frames, but B5 cannot. If the destination of such frames is on the opposite side of
Bridge 5, the frames are not delivered.
Only Bridge 4's MaxAge expires. The other bridges are still receiving CBPDUs on their root
ports. After MaxAge expires, Bridge 4 will decide the following:
Step 1
My E1 port is no longer my root port.
Step 2
The same root bridge is being advertised in a CBPDU on my E0
port.
Step 3
No other CBPDUs are being received.
E0
E1
E1
E1
Blocking
E0
E0
E0
E0
E1
B1
B2
E1
ID is 0200.2222.2222
Priority 1
Root is 0200.1111.1111
Cost is 100
B5
B3
B4
ID is 0200.3333.3333
Priority 1
Root is 0200.1111.1111
Cost is 200
ID is 0200.4444.4444
Priority 1
Root is 0200.1111.1111
Cost is 110
ID is 0200.1111.1111
Priority 1
Root is 0200.1111.1111
Cost is 0
0200.0000.AAAA
ch04.fm Page 167 Monday, March 20, 2000 5:02 PM