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久しぶりにGNS3（その29：STP基本）

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Spanning Tree Protocol(STP)の超基本

ループを防ぐSTPの動作確認を行ったときのメモ。

環境

２つの環境を用いた。

その１

L2Switchが２つ。
topology0.png

その２

L2Switchが３つ。
topology.png

設定

特筆すべき項目がないので省略。

検証

その１（L2Switchが２つ）

起動時の状況（長くなるので、一部略、以下同様）。

L2Switch1#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32769
             Address     aabb.cc00.0400
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0400
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Desg FWD 100       128.1    Shr 
Et0/1               Desg FWD 100       128.2    Shr 

L2Switch2#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32769
             Address     aabb.cc00.0400
             Cost        100
             Port        1 (Ethernet0/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0500
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Root FWD 100       128.1    Shr 
Et0/1               Altn BLK 100       128.2    Shr
  • L2Switch1がルートブリッジ（アドレスが最も小さい）
  • L2Switch2のEt0/0がルートポート（ポートが最も小さい）、Et0/1が代替ポート（ブロッキングされ、ループを防ぐため利用されない）

ポートプライオリティを変更

L2Switch1のポートプライオリティを変更し、L2Switch2のルートポートおよび代替ポートの状況を確認する。

L2Switch1(config)#interface ethernet 0/1
L2Switch1(config-if)#spanning-tree port-priority 64

L2Switch1#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32769
             Address     aabb.cc00.0400
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0400
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Desg FWD 100       128.1    Shr 
Et0/1               Desg FWD 100        64.2    Shr 

L2Switch2#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32769
             Address     aabb.cc00.0400
             Cost        100
             Port        2 (Ethernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0500
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Altn BLK 100       128.1    Shr 
Et0/1               Root FWD 100       128.2    Shr
  • L2Switch1のEt0/1のポート優先度を高くする（128→64）
  • すると、L2Switch2のルートポートと代替ポートとが入れ替わる
    • Et0/0が代替ポート、Et0/1がルートポート

その２（L2Switchが３つ）

起動時の状況

L2Switch1#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32769
             Address     aabb.cc00.0100
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0100
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Desg LRN 100       128.1    Shr 
Et0/1               Desg LRN 100       128.2    Shr 

L2Switch2#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32769
             Address     aabb.cc00.0100
             Cost        100
             Port        1 (Ethernet0/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0200
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Root FWD 100       128.1    Shr 
Et0/1               Desg FWD 100       128.2    Shr 

L2Switch3#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32769
             Address     aabb.cc00.0100
             Cost        100
             Port        1 (Ethernet0/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0300
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Root FWD 100       128.1    Shr 
Et0/1               Altn BLK 100       128.2    Shr
  • L2Switch1がルートブリッジ
  • L2Switch2のEt0/0がルートポート、Et0/1が指定ポート
  • L2Switch3のEt0/0がルートポート、Et0/1が代替ポート

パスコスト変更

L2Switch3をターゲットにして、まずは、L2Switch3のEt0/1のパスコストを変更する。

L2Switch3(config)#interface ethernet 0/1
L2Switch3(config-if)#spanning-tree cost 10

L2Switch3#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32769
             Address     aabb.cc00.0100
             Cost        100
             Port        1 (Ethernet0/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0300
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Root FWD 100       128.1    Shr 
Et0/1               Altn BLK 10        128.2    Shr

これだけでは何も変わらない。そこで、L2Switch2のEt0/0のパスコストを変更する。

L2Switch2(config)#interface ethernet 0/0
L2Switch2(config-if)#spanning-tree cost 10

L2Switch2#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32769
             Address     aabb.cc00.0100
             Cost        10
             Port        1 (Ethernet0/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0200
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Root FWD 10        128.1    Shr 
Et0/1               Desg FWD 100       128.2    Shr

L2Switch3#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32769
             Address     aabb.cc00.0100
             Cost        20
             Port        2 (Ethernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0300
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Altn BLK 100       128.1    Shr 
Et0/1               Root FWD 10        128.2    Shr
  • ルートブリッジL2Switch1からL2Switch3のEt0/0へのコストは変わらず「100」のまま
  • L2Switch1からL2Switch3のEt0/1へのコストは「20(=10+10)」となる
    • L2Switch3のEt0/0は代替ポートに変更される
    • Et0/1はルートポートに変更される

強制的なルートブリッジの変更

L2Switch3のプライオリティを上げ、をルートブリッジに強制的に変更する（起動時からの変更）。

L2Switch3(config)#spanning-tree vlan 1 priority 4096

L2Switch3#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    4097
             Address     aabb.cc00.0300
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    4097   (priority 4096 sys-id-ext 1)
             Address     aabb.cc00.0300
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Desg FWD 100       128.1    Shr 
Et0/1               Desg LRN 100       128.2    Shr 

L2Switch1#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    4097
             Address     aabb.cc00.0300
             Cost        100
             Port        2 (Ethernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0100
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Desg FWD 100       128.1    Shr 
Et0/1               Root FWD 100       128.2    Shr

L2Switch2#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    4097
             Address     aabb.cc00.0300
             Cost        100
             Port        2 (Ethernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0200
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Altn BLK 100       128.1    Shr 
Et0/1               Root FWD 100       128.2    Shr
  • L2Switch３のプライオリティを上げ（4096へ）、強制的にルートブリッジに昇格
  • 他のL2Switchのポートなどの状況は、基本ルールに従って設定される

最後に

基本動作の確認はOK。

いくつかの記事で、「GNS」となっていました（「GNS3」ではなく）。あえて、そのままにしておきます。

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