Learning IOS and JUNOS

When you configure PortFast on an access or trunk port, you assure that switch it should not expect a switch on this path. With this assurance, the switch can pass right through forward delay and go directly to forwarding when it gains link. By default, PortFast does not disable STP on the port, but by skipping the listening and learning stats you do increase the probability of creating a loop if a switchin connected. To protect against this situation, you can enable BPDU guard or BPDU filter globally for PortFast port. BPDU guard will error-disable the port if a BPDU is recevied. Switch(config)# spanning-tree portfast bpduguard When the BPDU filter is enabled globally, it causes PortFast ports to stop sending BPDUs. If a BPDU is recevied, the PortFast feature is disabled for that port and normal STP operation resumes. Switch(config)# spanning-tree portfast bpdufilter

There are two types of BPDU's: configuration BPDU's (c-BPDU) and topology change BPDU's (tcn-BPDU). Designated Bridges generate c-BPDU's. Root Ports and BLocked Ports listen for c-BPDU's. c-BPDU's originate from the root bridge and flow out towards the edge of the spanning tree, c-BPDU's are re-generated at every bridging device that receives them. tcn-BPDU's originate from root ports and flow towards the Root Bridge. c-BPDU's are the heartbeat of the Spanning Tree, tcn-BPDU's are the heart attack of the spanning tree.

今天晚上接到一位老同事來電詢問Private VLAN相關的問題，所以我上網找了一篇比較清楚簡單的說明摘錄如下，其中最容易讓大家confused的就是Private VLAN中包含了三種port，我用比較簡單的中文列出它們之間的不同點：   Promiscuous - 在這種Port上通常連結的是這個VLAN中的共用設備，如Gateway或是外部Server Isolated - 在這種Port上通常只能連結至Promiscuous ports，如果在IDC中提供主機代管服務，為了節省IP若是不想切割子網路造成無謂的IP浪費，可以在不同客戶主機使用同一個VLAN(使用同網段IP)但是彼此之間互不相通時就很適合使用這種Port Community - 在這種Port的設備可以直接與相同Community Port上的其他設備互通(比方說某客戶要求三台主機代管，這三台主機使用同網段IP又要互連，但是不跟同網段其他客戶主機互連)，也可以連結至 Promiscuous ports上的Gateway或是外部Server Private VLANs (PVLANs) Until now, I thought PVLANs were a bit  difficult to understand and to implement, like when studying to CCNP that took me a while to digest, and I had some doubts about it, till today! Man... how simple it is, and there´s no much "magic" in that (like our friend Scott Morris usually says)!  Pretty straight-forward and no big deals! The Security Video IPExpert is AWESOME. It´s short, informative, to the point, and solved MANY questions I´ve for a while in minutes! Man! What a nice way to do...

載波檢測多路存取/碰撞避免 (Carrier Sense Multiple Access/Collision Avoidance, CSMA/CA )是802.11無線區域網標準中採用的媒體存取控制(Media Access Control, MAC)方式。 和乙太網MAC層採用的CSMA/CD不同的原因是：無線的環境、不容易確實的偵測是否有碰撞發生，所以修改被動的碰撞偵測的方式為主動的避免碰撞。 CSMA/CA主要使用兩種方法來避免碰撞： 送出資料前，聆聽媒體狀態，等沒有人使用媒體，維持一段時間後，再等待一段隨機的時間後依然沒有人使用，才送出資料。由於每個裝置採用的隨機時間不同，所以可以減少碰撞的機會。 送出資料前，先送一段小小的請求傳送封包(RTS : Request to Send)給目標端，等待目標端回應 CTS: Clear to Send 封包後，才開始傳送。 利用RTS-CTS交握(handshake)程序，確保接下來傳送資料時，不會被碰撞。 同時由於RTS-CTS封包都很小，讓傳送的無效開銷變小。