2.10
1.BGP-4 與早期版本的主要不同之處是什麼？
CIDR
2. CIDR 解決了哪兩個問題？
Internet 路由表爆炸
B類地址空間的逐漸耗盡
3. 有類別與無類別IP路由器之間的區別是什麼？
無類別路由器直接進行最長匹配查找操作
有類別路由器首先查找主類網絡地址，再其主類下的子網列表中尋找匹配項
4. 有類別與無類別IP路由協議之間的區別是什麼？
無類別路由宣告地址前綴 like /20
5.請解釋彙總是如何隱藏網絡的不穩定性的。
192.168.1.128/25 匯聚地址
192.168.1.176/28
更爲精確的子網發生任何變化都不會通過路由器通告出去
6. 請解釋彙總是如何產生不對稱流量模型的 。
彙總路由隱藏了路由背後的互聯網絡細節信息
7. 不對稱流量是不被期望的嗎？
是非期望的
８. 什麼是NAP ？ network access point
9. 什麼是路由服務器
一臺unix 工作站或運行了 BGP的服務器
10. 什麼是與提供商向獨立的地址空間？擁有此地址的好處是什麼？
不屬於ISP的CIDR 地址快，這樣ISP1 and ISP2 都可以負載均衡，然後互不干擾的宣告用戶的地址
11.擁有一個與提供商獨立的/21地址爲何會有問題。
Some national service providers do not accept IP prefixs longer than a /19, as a result, the /21 might not be advertised to all parts of the Internet.
12. 什麼是路由策略
路由策略實際上就是通過控制路由及其特性來控制互聯網中的流量模型的設置和配置方法。
重分發 redistribution，路由過濾器route filters，路由映射route map等等都是
A routing policy is a predefined set of rules for handling incoming and outgoing routes, Typical tools for setting routing policies are redistribution, route filters and route maps.
13. BGP用以可靠連接其鄰居的底層協議是什麼？
TCP 179
14. BGP的4種消息類型分別是那些，沒種消息類型優勢如何使用的。
open keepalive update notification
Open messages are used to initially identify a BGP speaker to its neighbor and befin a peering session.
Keepalives maintain the peer connection.
Updates are used to advitise routes and Notification messages advise peers of errors.
15. 什麼狀態下，BGP對等體自建可以交換update消息。
BGP peers can exchanges update message only when both are in the established state.
What is the underlying protocol that BGP uses to reliably connect to its neighbors?
BGP uses tcp port 179
16.什麼是NLRI
Network Layer Reachability Information is the IP address prefix or prefixes advertised in a BGP update.
17. 什麼是路徑屬性 path attribute
路徑屬性是路由器所宣告BGP 路由的特性
18. BGP的4種路徑屬性是什麼/
周知強制屬性 Well-know mandatory
周知自選屬性 well-know discretionary
可選傳遞屬性 optional transitive
可選非傳遞屬性 optional onotransitive
19. AS_PATH 屬性的作用是什麼
防環 和選擇最短路徑
influence the incoming traffic.
The AS_PATH attribute describes the AS numbers that a received update ahs crossed after it left the originating router, This information can be used to determine the shortest inter-AS path and it is also for detecting routing loops.
20. AS_PATH 包括哪些不同類型的屬性
21. NEXT_HOP屬性的作用是什麼
嚇一跳路由器（位於所宣告目的地的路徑之上）的IP地址。
describes teh IP address of the next-hop router on the path to the advertised destination.
22. LOCAL_PREF 屬性的作用是什麼
如果內部BGP發言者接收到多條去往同一目的地的路由，那麼將比較這些路由的local_pref 屬性，
local_pref 最高的路由 將被選中
23. MULTI_EXIT_DISC 屬性的作用是什麼
local_pref 僅影響離開的流量，對入站流量，need Multi_exit_disc med
24. BGP 發言者發起一條聚合路由是哪種或那些屬性會有用
在確定最佳路徑時，路由器總是選擇更精確的路徑
同時宣告精確路由和較不精確路由
僅宣告精確路由
聚合兩條路由並宣告聚合路由
僅宣告不精確路由
兩條路由都不宣告
25. 什麼是BGP的管理性權值
weight cisco 專有BGP 參數，僅用於單個路由器內的路由。
單個對等體有效
26. 假設去往同一目的地有兩條路由，一條是EBGP，另一條是IBGP，請問BGP會優選哪條路由
如果weight,local_pref. as_path origin and MED都相等。
EBGP > IBGP
27. 路由器有兩條去往同一目的地的路由，路徑A 的LOCAL_Pref 爲300，且AS_PATH中有3個AS號，路徑B的LOCAL_PREF爲200，且AS_PATH中有2個AS 號，假定其他情況完全一致，請問路由器會有選哪條路徑
28， 什麼是路由阻尼
路由阻尼（route dampening)是一種阻止不穩定路由傳播到整個互聯網絡的方法。
29，請給出路由阻尼中的懲罰之，抑制門限，重用門限以及半衰期的定義
當某條路由出現翻動是，將會給該路由分配一個懲罰值，翻動次數越多，累加的懲罰值就越大，
suppress limit 抑制門限
當懲罰值超出了預期的門限，該路由就被抑制，不再對外宣告。
重用門限
懲罰值 1000/路由翻動
抑制門限： 2000
重用門限：750
半衰期： 15分鐘
最大抑制時間：60分鐘
什麼是IGP 同步，爲什麼同步很重要。
IGP sync is a rule whereby a BGP router cannot advertise a transit route to an EBGP peer unless the route is found in the IGP routing table.
If a BGP router forwards a transit packet to an IBGP peer via an IBGP router, and the IBGP router does not know the route, the packet is dropped.
什麼情況下可以安全的關閉IGP同步機制。
IBGP peers in an AS are fully meshed, or when the AS is not a transit AS.
什麼是BGP 對等地組
A BGP peer group is a group of BGP peers that have been identified on a single router to share common routing policies, Peer groups simplify configuration by allowing route policies to be applied to the gourp rather than to each individual member.
什麼是BGP團體
a BGP community is a group of routes that share common routing policies.
they work by setting a common COMMUNITY attribute in the routes;
peers receiving these routes can recognize the COMMUNITY attribute and apply the appropriate.
什麼是路由反射器？什麼是路由反射客戶，什麼是路由反射族
Arouter is configured as a route reflector(PR) and other IBGP routers known as clients, peer with the PR only, rather than with every other IBGP routers
route cluster include PR and with clients.
路徑屬性ORIGIATOR_ID and CLUSTER_LIST 的作用是什麼？
prevent routing loops.
The ORIGINATOR_ID is a 32-bit value created by a route refector, the value is a router ID of the originator of the route in the local AS, if the originator sees it RID in the ORIGINATOR_ID of a received route, it knows that a loop has occurred and the route is ignored.
CLUSTER_LIST is a sequence of route reflection cluster IDs through which the route has passed, if a route reflector see its local cluter ID in the CLUSTER_LIST of a received route, it knows that a loop has occoured, and the route is ignored.
什麼是BGP 聯盟
A BGP confederation is a large AS that has been subdivided into a group of smaller autonomous system for easier management.
路由反射器能否作用於聯盟內
Yes.
next-hop-self功能的作用是什麼？是否有其他實現該功能的替代方案
Next-hop-self tells a router to change the NEXT_HOP attribute of routes received from an external peer to its own IP address, This function is used when the IGP has no knownledge of th external next-hop address.
An alternative method is to run the IGP passively on the external link so that it knows the subnet on which the external next-hop address resides.