求救２２７Ｑ題庫解答

tj539

求救　！！！這題答案究竟是A 還是 C
  Y3 f! }% o2 h  H# X1 G查遍思科官網，沒有關於MSTP 與 RSTP  compatibility 的內容

daniel@990.net

个人感觉选C

拿RSTP BPDU和MSTP BPDU的格式拿来比较一下就知道了。MSTP报文前面部分和RSTP格式完全兼容（除了版本号不同），MST的信息都以TLV的形式附加在原报文后。

下面这个链接有解释
8 U: t' I8 [: ]# s1 B* x% F. ghttp://blog.ine.com/2010/02/22/understanding-mstp/

8 F' S+ d& ~/ R' t+ H0 D# G摘录如下：
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The IEEE 802.1s implementation does not send BDPUs for every active STP instance separately, nor does it encapsulate VLAN numbers list configuration messages. Instead, a special STP instance number 0 called Internal Spanning Tree (IST aka MSTI0, Multiple Spanning Tree Instance 0) is designated to carry all STP-related information. The BPDUs for IST contain all standard RSTP-style information for the IST itself, as well as carry additional informational fields. Among those fields are configuration name, revision number and a hash value calculated over VLANs to MSTI mapping table contents. Using just this condensed information switches may detect mis-configuration in VLAN mappings by comparing the hash value received from the peer with the local value.

$ z  m/ K% t# q) hM-Records
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By default, all VLANs are mapped to the IST. This represents the case of classic IEEE RSTP with all VLANs sharing the same spanning-tree. Other MSTP instances could be enabled, and they are referred to as Multiple Spanning Tree Instances (MSTIs). Every MSTI assign its own priorities to the switches and use its own link costs to come up with a private logical topology, separate from the IST. Since MSTP does not send MSTI’s information in separate BPDUs, this information is piggybacked into the IST’s BPDUs using special M-Record fields (one for every active MSTI). Using TLV’s (Type-Length-Value) those fields carry root priority, designated bridge priority, port priority and root path cost among others.

tj539

daniel@990.net 发表于 2016-7-11 06:01
% ~( b; R# w# ^. H% a2 B2 Q个人感觉选C
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* Y& a) K0 r' p. m4 t拿RSTP BPDU和MSTP BPDU的格式拿来比较一下就知道了。MSTP报文前面部分和RSTP格式完全兼容 ...

多謝 !!! 我也覺得是C但很多題庫答案都寫A = =

tropikalny

C :)