May 13, 2009

我的 CCENT 證書和證明卡

從來沒有想過要考這個CCENT,剛好今年拿到All-You-Can-TEST Voucher Number,所以就找個空檔考一下,也是想看看現在的證書及那張證明卡長什麼樣子。不過我覺得內容相當活用各種ICND1課程中教授的技巧,對於第一次考Cisco筆試的同學可能會不太習慣,有不少題組是以一個網路架構圖為基礎來發問,題目難度不會太深,主要是考基本觀念及理解,死背的題目類型幾乎沒有。但是個人覺得有少部份題目及答案的組合有點怪怪的,找不到真正適合的答案(也就是題意有點模稜兩可,答案可能兩個都可以但只能選一個),如果各位真的遇到這種題目就只能猜猜看了。

WRR(Weighted Round Robin) vs SRR(Shared/Shaped Round Robin)

在舊版的QoS 2.2中只有提到WRR(Weighted Round Robin),但是在實務中,SRR已經漸漸在愈來愈多的Switch平台中使用,其中WRR與SRR最大的差別就是在於它們同時使用權重但是Scheduling的方式不同。

以下圖而言(暫不考慮Strict Priority Queue),如果是用WRR的話,Queue 3的權重為4,所以每一輪都可以同時送出4個封包;Queue 2的權重為2,所以每一輪都可以同時送出2個封包;Queue 1的權重為1,所以每一次都可以同時送出1個封包。

如果是用SRR(Shared or Shaped Round Robin)的話,Queue 3的權重為4,Queue 2的權重為2,Queue 1的權重為1。因此Q3,Q2,Q1各出1個封包,接著Q3,Q2各出1個封包,再接著Q3出2個封包,這樣子才算是一輪。

所以SRR的好處在於每一個Queue很平均地送出封包前後穿插進入FIFO Queue(Hardware Queue),而不是像WRR那樣一次出清每個Queue權重所佔比例數量的封包,這樣子下來的結果會讓不同Queue中的封包排程更加smooth,不會互相排擠。

SRR is a scheduling service for specifying the rate at which packets are dequeued. With SRR there are two modes, Shaped and Shared (default).

Shaped mode is only available on the egress queues. Shaped egress queues reserve a set of port bandwidth and then send evenly spaced packets as per the reservation.

Shared egress queues are also guaranteed a configured share of bandwidth, but do not reserve the bandwidth. That is, in Shared mode, if a higher priority queue is empty, instead of the servicer waiting for that reserved bandwidth to expire, the lower priority queue can take the unused bandwidth.

Neither Shaped SRR nor Shared SRR is better than the other. Shared SRR is used when one wants to get the maximum efficiency out of a queuing system, because unused queue slots can be used by queues with excess traffic. This is not possible in a standard Weighted Round Robin (WRR). Shaped SRR is used when one wants to shape a queue or set a hard limit on how much bandwidth a queue can use. When one uses Shaped SRR one can shape queues within a ports overall shaped rate.

As stated earlier, SRR differs from WRR. With WRR, queues are serviced based on the weight. Q1 is serviced for Weight 1 period of time, Q2 is served for Weight 2 period of time, and so forth. The servicing mechanism works by moving from queue to queue and services them for the weighted amount of time.

With SRR weights are still followed; however, SRR services the Q1, moves to Q2, then Q3 and Q4 in a different way. It doesn't wait at and service each queue for a weighted amount of time before moving on to the next queue. Instead, SRR makes several rapid passes at the queues, in each pass, each queue may or may not be serviced. For each given pass, the more highly weighted queues are more likely to be serviced than the lower priority queues. Over a given time, the number of packets serviced from each queue is the same for SRR and WRR. However, the ordering is different.

With SRR, traffic has a more evenly distributed ordering.
With WRR one sees a bunch of packets from Q1 and then a bunch of packets from Q2, etc. With SRR one sees a weighted interleaving of packets.

SRR is an evolution of WRR that protects against overwhelming buffers with huge bursts of traffic by using a smoother round-robin mechanism.

Reference URL:
http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps5023/prod_white_paper09186a00801b096a.html




On your queuing thoughts there: WRR, or weighted round robin, is the method that 3550s use to allocate and manage the hardware queues on interfaces. SRR (which can mean shared round robin or shaped round robin, depending on what commands you use) is the method that a 3560 uses for the same management.

There are some important differences when it comes to allocation and how things are configured! In the WRR system, things are linear in their logic. By default:

Queue 1 serves CoS values 0 and 1
Queue 2 serves CoS values 2 and 3
Queue 3 serves CoS values 4 and 5
Queue 4 serves CoS values 6 and 7

Queue 4 may be set up as a strict priority queue. Numerically, this makes sense; operationally, it doesn't really!

On the 3560, things default to a more operational logic:

Queue 1 serves CoS value 5
Queue 2 serves CoS values 0 and 1
Queue 3 serves CoS values 2 and 3
Queue 4 serves CoS values 4, 6 and 7

Queue 1 may be set up as a strict priority queue. Operationally, this makes much more sense. Numerically, it does not!

--

Scott Morris, quadruple CCIE, JNCIE and all-around uber-geek, can often be seen traveling around the world consulting and delivering CCIE training. He recently accepted a new Senior CCIE Instructor position with Internetwork Expert! For more information on him check out http://www.uber-geek.net or for CCIE training check out http://www.internetworkexpert.com. You can contact Scott via editor@tcpmag.com. You can contact Scott about "What's the Deal with Cisco 3560 Switches? " at editor@tcpmag.com.

Reference URL:
http://tcpmag.com/qanda/article.asp?EditorialsID=368

What is the meaning of "AKA" or "a.k.a " ?

在國外的技術文件中常常會看到這幾個英文字元 AKA or a.k.a (尤其是Juniper),如果對口語英文不熟悉的人常常是丈八金剛摸不到腦袋,通常在上課時講師也不會特別解釋這個縮寫,所以我想趁此機會介紹一下這個詞語的用法:

aka= also known as

EX:
SVI's (Switched Virtual Interfaces), aka "interface vlan" are the hooks at Layer3 into a VLAN for routing purposes.

May 12, 2009

最後一次的母親節

2009/05/10 是一個陽光普照的好日子,中午連同我大哥一同請我母親到石門活魚吃完一頓豐盛的大餐之後,下午就回到台北老婆的娘家,準備晚上帶岳母去吃中山捷運站旁的衣蝶百貨7F新開張的勝博殿。兩個兒子一聽到要去吃他們最喜歡的豬排飯,在車上都興喜若狂,不過我和我老婆一點都感覺不到快樂的feeling,因為我們心知肚明,這一次的母親節將會是我們陪岳母渡過最後一次的母親節。

隨著年紀增長,感情變得愈來愈脆弱,很容易因為感動而落淚(尤其是看到小孩與動物有關的社會新聞或電影情節)。每次回到娘家看到岳母,就會讓我不得不想起我的父親肝癌末期的樣子,兩頰深陷,臉色暗黃如土色。大學住在台北時,約三個月才回一次家,那一次回家是因為母親來電告知我這個晴天霹靂的消息,隨後就立刻趕回家中,一進到大門就看到我母親旁邊坐了我幾乎已經認不出來的父親,看了約三秒鐘我才猛然發現這個貌似陌生人的中年人竟然是我深愛的父親(原本80幾公斤的臉龐是有如彌勒佛的笑容,現在那張熟悉的臉龐竟然像洩了氣的氣球般的蠟黃色…現在我終於了解何謂"蠟黃色" - 永遠無法從我記憶中抹去的蠟黃色臉龐…)

從去年開始,每次回到娘家,心中就一直有一股無法抹去的哀愁,大家都心有戚戚焉,只能避而不談,儘量利用談笑風生及小朋友的吵鬧聲來掩蓋這一切的不如意與未知數。說實話,我的岳母已經算是很幸運的肝癌患者,從健康檢查到有肝腫瘤到現在已經歷經了超過6年以上的時間,期間歷經了N次的栓塞手術,但是不幸的是每過一段時間就會再度復發,所以這幾年來大家都有了一個默契,接下來只有兩種結果,一種就是奇蹟,另一種則是大家最不希望但是看來最有可能的結果。

「我這次也要點兒童餐!」我的小兒子一進到勝博殿的門口,看到櫥窗裏的玩具就直覺地要求跟哥哥一樣的待遇,平常我們通常不會允許,因為鐵定又是花錢買玩具而又要我們幫忙解決他的兒童餐,這次看在母親節的份上,老婆就直接點了兩份兒童餐。幾份餐點很快地就上桌了,小朋友還在努力地磨著芝蔴,岳父岳母也是,可是我的心裏一直想著這樣的情景不知道還有幾次…看著岳母等餐時面無表情發呆的表情,我依稀想我父親因肝昏迷導致意志不清被五花大綁地綁在急診室的病床上,臉部表情猙獰痛苦,身體不斷掙扎扭動,口中一直喊著「為什麼要把我綁起來? 為什麼要把我綁起來?」,這一幕真的讓我無法相信這是我從小對我呵護有加,對雙親至孝,對兄弟姐妹疼愛有加的父親,現在我只希望我的岳母不要發生類似的情境,這對親人來說真的是一種痛苦的心理負擔。

往常,我和老婆至少每個人都一定會多叫一碗飯,這一次,我們都吃不出來食物的美味,吃了一碗飯已經沒有胃口,岳母也是吃得很少因為腹水嚴重宛如四五個月的孕婦,我想就算有胃口也沒有空間可以擠在她瘦小的身軀中。這讓我想起來"越獄"中Lincoln Burrows要坐電椅行刑前要求了一份他常常作給他兒子吃的藍莓派,我想他當時的感受應該就像這樣吧。
不忍心再看我岳母的樣子以免我不爭氣的眼淚在家人面前流下,低著頭快速地把我自己的餐點清空,再忙著控管大小兒子好動的身子轉移我的注意力。

用完餐後,我們和岳父及老婆攙扶著岳母離開衣蝶緩緩地走向中山捷運站,準備一起搭捷運回到民權捷運站旁的老家。短短的一小段路,岳母必須花費比平常人多一倍的時間來行走,可以看得出來有點虛弱與吃力。

小朋友仍不知道什麼叫真正的死亡,他們認為PSP中三國無雙的角色(如果在路上聽到有小朋友大喊「我是呂布!」,不要懷疑那就是我小兒子,極度崇拜近似戰神的呂布這個角色),無論歷經多久永遠不會變老,如果不小心被敵人圍攻失血過多不支倒地,大不了重新按Start就好,這就是"死亡"。

和信的醫生早已給我們心理準備,岳母已經無法再次承受化療或手術,如果…再發生任何腫瘤破裂的情況,也許就是我們分離的時刻。或許我的兒子們就快要真的地了解到,永遠再也無法感受那雙從小幫他們洗澡呵護的溫暖雙手再度擁抱他們的感覺!