Learning IOS and JUNOS

如果各位正在上CCNA的課程或是在準備CCNA考試，但是平時可能手邊沒有設備可以練習的話，可以先考慮使用Packet Tracer來練習，以下是相關資源可以下載的地方： http://www.packettracernetwork.com/ Packet Tracer is a powerful network simulator that can be utilized in training for  CCNA TM  and CCNP TM  certification exam by allowing students to create networks with an almost unlimited number of devices and to experience troubleshooting without having to buy real Cisco TM routers or switches. The current version of Packet Tracer supports an array of simulated Application layer protocols (HTTP, DNS, …), as well as basic routing with RIP, OSPF, and EIGRP, to the extent required by the current CCNA TM  curriculum . With the introduction of version 5.3, several new features were added, including  BGP , which is part of the CCNP TM curriculum. While Packet Tracer aims to provide a realistic simulation of functional networks, the application itself utilizes only a small number of features found within the actual hardware ru...

本週在上ICND2的時候，有同學突然問了我一個問題，他在測試debug eigrp時出現了一個 log message 02:36:26: EIGRP: Packet from ourselves ignored 一下子我還真的無法回答上來，後來趁著休息時間 上網找了一些資料，終於在 CCIE Practical Studies: Security (CCIE Self-Study) 這本書中找到相關說明(感謝Google圖書館)。 原來是因為在Router上建立的loopback interface啟動了EIGRP，EIGRP Router也會在loopback interface上發送hello的封包，然後又被自己接收到，EIGRP知道這個hello封包是由自己送出的，因此忽略它不再嘗試去建立neighbor關係。 解決方案就是如果有類似的情況又不想看到這樣的訊息，可以試著加上passive-interface的指令來讓EIGRP router不要在loopback interface上送出hello封包。

• Autonomous switching With this type of switching, an incoming packet matches an entry in the autonomous-switching cache located on the interface processor. Autonomous switching provides faster packet switching by allowing the ciscoBus controller to switch packets independently without having to interrupt the system processor. It is available only on Cisco 7000 series routers and in AGS+ systems with high-speed network controller cards. • SSE switching.  With this type of switching, an incoming packet matches an entry in the silicon-switching cache located in the silicon switching engine (SSE) of the Silicon Switch Processor (SSP) module. This module is available only on Cisco 7000 series routers. Silicon switching provides very fast, dedicated packet switching by allowing the SSE to switch packets independently without having to interrupt the system processor. 

The Configuration Register is a 16 bit field that is converted into a hexadecimal value for convenience.  As you know each hexadecimal character represents 4 bits.  Hexadecimal numbers go from 0 to F and represent decimal numbers from 0 to 15.  Hexadecimal numbers generally are preceded by a 0x to clue us in on the fact that we are looking at a hexadecimal number.  By default a Cisco router has a Configuration Register setting of 0×0101 so the actual 16 bits would look like 0000000100000001 so now you see that finally there is a use for the hexadecimal numbering system!  The bits in the Configuration Register are numbered 0 to 15 and you start from the far right: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 So for every 4 bits we translate that into hex, observe the following typical Configuration Register: 2 1 0 2 0 ...

說實話，關於這個問題我自己也常常覺得很納悶，剛好最近PacketLife.net(我真的愈來愈喜歡這個網站了，只要上過我課程的學生應該不陌生，給各位同學的cheatsheet都是從PacketLife上抓下來的)把這個issue提出來並且作了一份packet analyze報告，請參考! Traceroute timeouts Posted by stretch in Networking on Monday, 29 Dec 2008 at 2:26 a.m. GMT If you spend a lot of time performing traceroutes to Cisco routers you've probably noticed that they usually end like this: R1# traceroute 10.0.34.4 Type escape sequence to abort. Tracing the route to 10.0.34.4 1 10.0.12.2 16 msec 8 msec 12 msec 2 10.0.23.3 16 msec 16 msec 16 msec 3 10.0.34.4 16 msec * 20 msec Notice that the second reply from the last hop has timed out. This is easily repeated with subsequent traceroutes, and it is always the second attempt which times out. Strange, eh? The reason for this is IOS' default ICMP rate limiting. Back in May I wrote an article explaining the common "U.U.U" response that results from pinging an unreachable destination, and the same logic is at work here. Inspecting the d...

以下這篇文章是轉載 PacketLife 的一篇文章，因為我發現有很多人事實上在準備CCNA/CCNP的過程中有時只著重在筆試的重點上，但是卻忽略了更重要的基礎理論，像這一篇就是介紹MSS(Maximum Segement Size)與MTU(Maximum Transmission Unit)差別，雖然沒有談很多，但是一圖勝過千言萬語，只要把底下那張圖片記在腦海裏，就不會搞不清楚MSS跟MTU的差別了。 Posted by stretch in Networking on Wednesday, 5 Nov 2008 at 2:26 a.m. GMT The Maximum Transmission Unit (MTU) is the maximum length of data that can be transmitted by a protocol in one instance. For example, the MTU of Ethernet (by default 1500) is the largest number of bytes that can be carried by an Ethernet frame (excluding the header and trailer). MTUs are found at various layers of the OSI model, and can often be tweaked to more efficiently transport large volumes of data. Ethernet The default Ethernet MTU is 1500 bytes, not including the header or trailer. Sometimes a slightly higher MTU is preferable to accommodate Q-in-Q tunneling or other encapsulation. The MTU can be raised on Cisco IOS with the system mtu command under global configuration: Switch(config)# system mtu ? ...

Posted by stretch in  Networking  on Thursday, 2 Oct 2008 at 3:36 a.m. GMT There are two major classes of routing protocol: distance vector and link-state. It's easy to remember which protocols belong to either class, but comprehending their differences between the two classes takes a bit more effort. Distance vector  routing is so named because it involves two factors: the  distance , or metric, of a destination, and the vector , or direction to take to get there. Routing information is only exchanged between directly connected neighbors. This means a router knows from which neighbor a route was learned, but it does not know where that neighbor learned the route; a router can't see beyond its own neighbors. This aspect of distance vector routing is sometimes referred to as "routing by rumor." Measures like split horizon and poison reverse are employed to avoid routing loops. Link-state  routing, in contrast, requires that all routers know about the paths reachable by...