2.10
1.BGP-4 与早期版本的主要不同之处是什么？
CIDR
2. CIDR 解决了哪两个问题？
Internet 路由表爆炸
B类地址空间的逐渐耗尽
3. 有类别与无类别IP路由器之间的区别是什么？
无类别路由器直接进行最长匹配查找操作
有类别路由器首先查找主类网络地址，再其主类下的子网列表中寻找匹配项
4. 有类别与无类别IP路由协议之间的区别是什么？
无类别路由宣告地址前缀 like /20
5.请解释汇总是如何隐藏网络的不稳定性的。
192.168.1.128/25 汇聚地址
192.168.1.176/28
更为精确的子网发生任何变化都不会通过路由器通告出去
6. 请解释汇总是如何产生不对称流量模型的 。
汇总路由隐藏了路由背后的互联网络细节信息
7. 不对称流量是不被期望的吗？
是非期望的
８. 什么是NAP ？ network access point
9. 什么是路由服务器
一台unix 工作站或运行了 BGP的服务器
10. 什么是与提供商向独立的地址空间？拥有此地址的好处是什么？
不属于ISP的CIDR 地址快，这样ISP1 and ISP2 都可以负载均衡，然后互不干扰的宣告用户的地址
11.拥有一个与提供商独立的/21地址为何会有问题。
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. 什么是路由策略
路由策略实际上就是通过控制路由及其特性来控制互联网中的流量模型的设置和配置方法。
重分发 redistribution，路由过滤器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用以可靠连接其邻居的底层协议是什么？
TCP 179
14. BGP的4种消息类型分别是那些，没种消息类型优势如何使用的。
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. 什么状态下，BGP对等体自建可以交换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
16.什么是NLRI
Network Layer Reachability Information is the IP address prefix or prefixes advertised in a BGP update.
17. 什么是路径属性 path attribute
路径属性是路由器所宣告BGP 路由的特性
18. BGP的4种路径属性是什么/
周知强制属性 Well-know mandatory
周知自选属性 well-know discretionary
可选传递属性 optional transitive
可选非传递属性 optional onotransitive
19. AS_PATH 属性的作用是什么
防环 和选择最短路径
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 包括哪些不同类型的属性
21. NEXT_HOP属性的作用是什么
吓一跳路由器（位于所宣告目的地的路径之上）的IP地址。
describes teh IP address of the next-hop router on the path to the advertised destination.
22. LOCAL_PREF 属性的作用是什么
如果内部BGP发言者接收到多条去往同一目的地的路由，那么将比较这些路由的local_pref 属性，
local_pref 最高的路由 将被选中
23. MULTI_EXIT_DISC 属性的作用是什么
local_pref 仅影响离开的流量，对入站流量，need Multi_exit_disc med
24. BGP 发言者发起一条聚合路由是哪种或那些属性会有用
在确定最佳路径时，路由器总是选择更精确的路径
同时宣告精确路由和较不精确路由
仅宣告精确路由
聚合两条路由并宣告聚合路由
仅宣告不精确路由
两条路由都不宣告
25. 什么是BGP的管理性权值
weight cisco 专有BGP 参数，仅用于单个路由器内的路由。
单个对等体有效
26. 假设去往同一目的地有两条路由，一条是EBGP，另一条是IBGP，请问BGP会优选哪条路由
如果weight,local_pref. as_path origin and MED都相等。
EBGP > IBGP
27. 路由器有两条去往同一目的地的路由，路径A 的LOCAL_Pref 为300，且AS_PATH中有3个AS号，路径B的LOCAL_PREF为200，且AS_PATH中有2个AS 号，假定其他情况完全一致，请问路由器会有选哪条路径
28， 什么是路由阻尼
路由阻尼（route dampening)是一种阻止不稳定路由传播到整个互联网络的方法。
29，请给出路由阻尼中的惩罚之，抑制门限，重用门限以及半衰期的定义
当某条路由出现翻动是，将会给该路由分配一个惩罚值，翻动次数越多，累加的惩罚值就越大，
suppress limit 抑制门限
当惩罚值超出了预期的门限，该路由就被抑制，不再对外宣告。
重用门限
惩罚值 1000/路由翻动
抑制门限： 2000
重用门限：750
半衰期： 15分钟
最大抑制时间：60分钟
什么是IGP 同步，为什么同步很重要。
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.
什么情况下可以安全的关闭IGP同步机制。
IBGP peers in an AS are fully meshed, or when the AS is not a transit AS.
什么是BGP 对等地组
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.
什么是BGP团体
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.
什么是路由反射器？什么是路由反射客户，什么是路由反射族
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.
路径属性ORIGIATOR_ID and CLUSTER_LIST 的作用是什么？
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.
什么是BGP 联盟
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于联盟内
Yes.
next-hop-self功能的作用是什么？是否有其他实现该功能的替代方案
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.