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| 1. 关于stp得sending BID |
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图可能不太清楚，请大家担待着看了  ，这是从lan switching贴下来的，在讲stp的端口角色选举的过程
Lowest Root BID
Lowest Root Path Cost
Lowest Sending BID
Lowest Port ID
对于这个Lowest Sending BID始终没搞明白，以这个图为例：
Second, every non-Root Bridge elects a single Root Port, its port that is closest to the Root Bridge. Cat-B has to choose between three ports: Port 1/1 with a Root Path Cost of 57, Port 1/2 with a cost of 38, or Port 2/1 with a cost of 19. Obviously, Port 2/1 is the most attractive and becomes the Root Port. Similarly, Cat-C chooses Port 2/1. However, Cat-D calculates a Root Path Cost of 38 on both ports—a tie. This causes Cat-D to evaluate the third decision criterion—the Sending BID. Because Cat-B has a lower Sending BID than Cat-C, Cat-D:Port-1/1 becomes the Root Port.
Finally, a Designated Port is elected for every LAN segment (the device containing the Designated Port is referred to as the Designated Bridge). By functioning as the only port that both sends and receives traffic to/from that segment and the Root Bridge, Designated Ports are the mechanism that actually implement a loop-free topology. It is best to analyze Designated Port elections on a per-segment basis. In Figure 7-2, there are five segments. Segment 1 is touched by two bridge ports—Cat-A:Port-1/1 at a cost of zero and Cat-B:Port-2/1 at a cost of 19. Because the directly-connected Root Bridge has a cost of zero, Cat-A:Port-1/1 obviously becomes the Designated Port. A similar process elects Cat-A:Port-1/2 as the Designated Port for Segment 2. Segment 3 also has two bridge ports: Cat-B:Port-1/1 at a cost of 19 and Cat-D:Port-1/1 at a cost of 38. Because it has the lower cost, Cat-B:Port-1/1 becomes the Designated Port. Using the same logic, Cat-C:Port-1/1 becomes the Designated Port for Segment 4. In the case of Segment 5, there are once again two options (Cat-B:Port-1/2 and Cat-C:Port-1/2), however both are a cost of 19 away from the Root Bridge. By applying the third decision criterion, both bridges determine that Cat-B:Port-1/2 should become the Designated Port because it has the lower Sending BID.
在上面的红色部分，CAT-D在选择root port的时候引用Lowest Sending BID的时候，选择的依据是CAT-B与CAT-C之间的比较
而在下面红色部分，Segment5在选择DP的时候，同样也用了Lowest Sending BID，但是这时候选择的依据确是CAT-B与CAT-C本身
那么我想问，这个Lowest Sending BID到底是交换机本身的BID，还是上连的交换机的BID
[lan switching page 210]
附带图片
Tomorrow Is Another Day |
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| 2. Re:关于stp得sending BID |
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看似简单但却又是本质的问题，居然没有高手愿意回答
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| 3. Re:关于stp得sending BID |
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HP : 250 / 1254
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第一个选择是选择上级桥B或者C，D应该比较B和C的BID;
第二个选择是B和C竞争成为 seg 5的designate bridge，所以应该把自己的BID和对方的进行比较，双方达成共识就是由于B的BID比较低因而应成为seg 5的designate bridge。
没有谁能像一座孤岛/在大海里独踞/每个人都像一块小小的泥土/连接成整个陆地/如果一块泥土被海水冲去/欧洲将缺其一隅/这如同一座山岬/也如同你的朋友和你自己/无论谁死了/都是自己的一部分在死去/因为我包含在人类这个概念里/因此我从不问丧钟为谁而鸣/它为我，也为你
No man is an Island, entire of itself; every man is a piece of the Continent, a part of the main; if a clod be washed away by the sea, Europe is the less, as well as if a promontory were, as well as if a man or of thy friends or of thine own were; any man's death diminishes me, because I am involved in Mankind; And therefore never send to know for whom the bell tolls; It tolls for thee.
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| 4. Re: Re:关于stp得sending BID |
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HP : 0 / 367
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多谢麦子指点，那是不是也可以这样理解：
在选举RP的时候，其实是在计算端口接收到的configuration BPDU
而在选举DP的时候，是在计算端口发出的configuration BPDU
因为在书后面还是讲到一点，让我非常confused.
Although the network in Figure 7-13 fails to implement load balancing, it does raise two interesting points. First, notice that it is the non-Root Bridge that must implement load balancing. Recall that all ports on the Root Bridge become Designated Ports and enter the Forwarding state. Therefore, it is the non-Root Bridge that must select a single Root Port and place the other port in a Blocking state. It is precisely this decision process that must be influenced to implement load balancing.
Second, it is the received values that are being used here. Cat-A is not evaluating its own BID and Port ID; it is looking at the values contained in the BPDUs being received from Cat-B.
不知道这样的想法是否成立？
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| 5. Re:关于stp得sending BID |
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HP : 250 / 1254
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EXP : 16%
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除非自己是根桥，否则就需要竞争DP，竞争的时候将自己的BID和其他竞争者的BID比较，看自己是否应当成为segment的DP；对方做同样的比较。
在图7-13，VLAN 2的生成树中B被选为根桥，A的两个端口都连是到根桥的，由于根桥的所有端口都是DP，所以A的端口没有竞争DP的机会，只能计算哪一个端口应当成为根端口（根据收到的BPDU信息），只有根端口可以置于转发状态。我们从图中可以看到，如果另外一个端口也被置于转发状态，那么环路就发生了。
没有谁能像一座孤岛/在大海里独踞/每个人都像一块小小的泥土/连接成整个陆地/如果一块泥土被海水冲去/欧洲将缺其一隅/这如同一座山岬/也如同你的朋友和你自己/无论谁死了/都是自己的一部分在死去/因为我包含在人类这个概念里/因此我从不问丧钟为谁而鸣/它为我，也为你
No man is an Island, entire of itself; every man is a piece of the Continent, a part of the main; if a clod be washed away by the sea, Europe is the less, as well as if a promontory were, as well as if a man or of thy friends or of thine own were; any man's death diminishes me, because I am involved in Mankind; And therefore never send to know for whom the bell tolls; It tolls for thee.
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| 6. Re:关于stp得sending BID |
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HP : 0 / 367
MP : 93 / 4302
EXP : 68%
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名动江湖
成员等级: 15
发表总数: 281
金币总数: 560
所属组别: 中级成员
注册日期: 2005/01/5
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收到，现在可以释怀了
再次感谢麦子 
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