1、 1、 What is BGP synchronization? A: The rule of synchronization states the following: Before a route learned from an IBGP neighbor is entered into the IP routing table or is advertised to a BGP peer, the route must first be known via IGP. 2、 which BGP attributes will influence inbound flow? A: MED、
2、AS-PATH， ORIGIN 3、 BGP Administrative distance A： BGP has a distance bgp command that allows you to set different administrative distances for three route types: external,internal, and local. External route 20 Internal route 200 Local route 200 4、 Which picture is Traffic Shape, why ? A: Policing ty
3、pically drops excess traffic. Shaping smooth traffic to a specific rate using a buffer. 5、 Unequal cost load-sharing A: Router E will select both B and C as next hop to reach network Z.Unequal cost load sharing happening. The variance command is used to determine which routes are feasible for unequa
4、l-cost load sharing. Variance defines a multiplier by which a metric may differ, or vary, from the metric of the lowest-cost route. Any route whose metric exceeds the metric of the lowest-cost route, multiplied by the variance, will not be considered a feasible route. The default variance is one, me
5、aning that the metrics of multiple routes must be equal, to load balance. Variance must be specified in whole numbers. The following three conditions must be met for a route to be included in unequal-cost load sharing: The maximum-paths limit must not be exceeded as a result of adding the route to a
6、 load-sharing group. The next-hop router must be metrically closer to the destination. That is, its metric for the route must be smaller than the local routers metric. A next-hop router, being closer to the destination, is often referred to as the downstream router.(加入 负载均衡的下一跳的路由器必须要满足 FC，即 AD Loca
7、l_pref - as-path - med Weight prefers local preference, local preference prefers as-path, as-path prefers med. 43. What is an autonomous system? A: Depending on the usage, an autonomous system can be defined as a network under a common administrative domain or a single routing domain. 什么是 BGP 自治系统？
8、A: An autonomous system is a network controlled by a single technical administration entity. BGP autonomous systems are used to divide global external networks into individual routing domains where local routing policies are applied. This organization simplifies routing domain administration and sim
9、plifies consistent policy configuration. 44. 什么是原子聚合？ A: ATOMIC_AGGREGATE is a well-known discretionary attribute that is used to alert downstream routers that a loss of path information has occurred. 补充： Each path attribute falls into one of four categories: Well-known mandatory Well-known discreti
10、onary Optional transitive Optional nontransitive 45. 给了两段 documentation 里的描述，问这个时候发那种类型的 ICMP 包。 The interface on which the packet comes into the router is the same interface on which the packet gets routed out. The subnet or network of the source IP address is on the same subnet or network of the n
11、ext-hop IP address of the routed packet. 答： ICMP 类型 5 重定向消息 ICMP type 5 redirect message. 46. 问 ip precedence 有几个 bit ？ 答： 3 bit 47. 问 10.1.1.0/24 10.1.2.0/24 可以将这两条路由汇总成最短的掩码是？ 答： It can be summarized as a route with /22 netmask. 48. 给出了一段描述，问是什么机制用于拥塞避免，适当解释一下？ （有可能是考 CBWRED） A： WRED 。 WRED(Weight
12、ed Random Early Detection), the Cisco implementation of RED for congestion avoidance, combines the capabilities of the RED algorithm with IP Precedence to provide preferential traffic handling for higher priority packets. It can selectively discard lower priority traffic when a queue begins to get c
13、ongested and provide differentiated performance for different classes of service. WRED（ Weighted Random Early Detection） combines the RED algorithm with IP precedence when a queue is congested.It discard lower priority packets and provide different services for different classes 补充： Tail drop: Tradi
14、tionally, packets arriving at the queue when the queue reaches its maximum queue length are dropped. This behavior continues until the queue decreases because of a packet transmission. This queue management technique is called tail-drop. RED: RED is a mechanism that randomly drops packets before a q
15、ueue is full, preventing congestion and avoiding tail drop. RED operates by increasing the rate at which packets are dropped from queues as the average queue size increases. 49. 两个路由，直连，建立 EBGP，使用了一条 nei 命令，问你为啥不通 ？ 答：此题答案有很多，考生可根据自身情况作答。主要从底层问题造成直连不通，注意 BGP open 消息格式 和 协商的具体内容， BGP 配置问题，策略抑制。 常见的答案
16、： The neighbors remote-as Configuration errors Blocked by TCP 179 communicate EBGP multi-hop configuration errors EBGP directly connect is not communicate 50. 问有哪些路由协议在路由条目发生变化时只做部分更新，并且使用比较低的链路带宽？ 答： EIGRP 51. 在配 EIGRP 的时候（ frame-relay 的 NBMA ）命令 router(config-router)#neighbor 192.168.1.129 是什么意思呢？
17、 答： specify a neighbor router to establish EIGRP peer with via unicast. 52. ip bandwidth-percent eigrp 200 20是什么意思 ? A: The maximum bandwidth EIGRP 200 can utilize is 20 percent of the configured bandwidth of the interface (not the physical bandwidth). 摘自文档： Router(config-if)# ip bandwidth-percent e
18、igrp percent Configures the percentage of bandwidth that may be used by EIGRP on an interface. By default, EIGRP packets consume a maximum of 50 percent of the link bandwidth, as configured with the bandwidth interface configuration command. 53. EIGRP 中的 SIA 是什么意思？ A： If a route lose on a router and
19、 no feasible successor is found for the route,the router will begins a diffusing computation.The router will set the route in active state and send queries to all of its neighbors,and meanwhile the router will set a active timer.If all expected replies are not received before the Active time expires
20、, the route is declared stuck-in-active (SIA). The neighbors that did not reply will be removed from the neighbor table, and the diffusing computation will consider the neighbor to have responded with an infinite metric. 补充： EIGRP DUAL算法工作过程： （ 1） When an EIGRP router is performing no diffusing comp
21、utations, each route is in the passive state. （ 2） A router will reassess its list of feasible successors for a route any time an input event occurs. （ 3） The first step in its reassessment is a local computation in which the distance to the destination is recalculated for all feasible successors. T
22、he possible results are： If the feasible successor with the lowest distance is different from the existing successor, the feasible successor will become the successor. If the new distance is lower than the FD, the FD will be updated. If the new distance is different from the existing distance, updat
23、es will be sent to all neighbors. While the router is performing a local computation, the route remains in the passive state. （ 4） If a feasible successor cannot be found in the topology table, the router will begin a diffusing computation and the route will change to the active state. Until the dif
24、fusing computation is completed and the route transitions back to the passive state, the router cannot Change the routes successor Change the distance it is advertising for the route Change the routes FD Begin another diffusing computation for the route （ 5） A router begins a diffusing computation b
25、y sending queries to all of its neighbors . The query will contain the new locally calculated distance to the destination. Each neighbor, upon receipt of the query, will perform its own local computation: If the neighbor has one or more feasible successors for the destination, it will send a reply t
26、o the originating router. The reply will contain that neighbors minimum locally calculated distance to the destination. If the neighbor does not have a feasible successor, it too will change the route to the active state and will begin a diffusing computation. （ 6） For each neighbor to which a query
27、 is sent, the router will set a reply status flag (r) to keep track of all outstanding queries. The diffusing computation is complete when the router has received a reply to every query sent to every neighbor. 54. EIGRP 中 traffic-share balanced 命令是什么意思？ Traffic is distributed inversely proportionall
28、y to metrics of routes. Routes that have higher metrics represent less-preferable routes and get less traffic. 55. 考官描述了一段文字，问描述的是什么协议？ 答： ICMP v6 。 Internet Control Message Protocol (ICMP) in IPv6 functions the same as ICMP in IPv4ICMP generates error messages, such as ICMP destination unreachable
29、messages, and informational messages, such as ICMP echo request and reply messages. Additionally, ICMP packets in IPv6 are used in the IPv6 neighbor discovery process, path MTU discovery, and the Multicast Listener Discovery (MLD) protocol for IPv6. MLD is used by IPv6 routers to discover multicast
30、listeners (nodes that want to receive multicast packets destined for specific multicast addresses) on directly attached links. MLD is based on version 2 of the Internet Group Management Protocol (IGMP) for IPv4. A value of 58 in the Next Header field of the basic IPv6 packet header identifies an IPv
31、6 ICMP packet. 补充： In IPv6, several mechanisms and functionalities of the protocol use ICMPv6 messages: Replacement of the Address Resolution Protocol (ARP) A mechanism used on local-link scope to replace ARP in IPv4. Nodes and routers keep track of their neighbors. New ICMPv6 messages are defined i
32、n IPv6 for that specific use. Stateless autoconfiguration The autoconfiguration functionality allows nodes to configure their IPv6 addresses by themselves using the prefixes advertised on the local links by routers. Prefix advertisement and stateless autoconfiguration use new ICMPv6 messages. Duplic
33、ate Address Detection (DAD) At the boot and during the stateless autoconfiguration process, each node verifies the existence of a tentative IPv6 address before using it. This function is also performed using new ICMPv6 messages. Prefix renumbering Prefix renumbering is a mechanism used when an IPv6
34、prefix on a network is changed to new one. Like prefix advertisement, prefix renumbering uses new ICMPv6 messages. Path MTU discovery (PMTUD) A mechanism by which a source node detects the largest MTU value along a delivery path to a destination host. ICMPv6 messages are also used to perform this ta
35、sk. 56. IPv6报头中的个别字段，问和 IPv4报头相比差了哪几个字段？ 57. ARP和反向 ARP啥意思？ ARP is IP4 address resolution protocol. It works by mapping IP network addresse to the hardware address used by a data link protocol. ARP is using to request hardware address based IP address. Reverse arp is contrary to ARP. It works by map
36、ping hardware address used by a data link protocol to IP network address. RARP is using to request IP address based hardware address. ARP和 RARP的区别： ARP:request hardware address based IP address; RARP:request IP address based hardware address. 58.什么是水平分割 A: There are two categories of split horizon:
37、simple split horizon and split horizon with poisoned reverse. The rule for simple split horizon is, when sending updates out a particular interface, do not include networks that were learned from updates received on that interface. The rule for split horizon with poisoned reverse is, when sending up
38、dates out a particular interface, designate any networks that were learned from updates received on that interface as unreachable. 60. OSPF OE1 和 OE2 的区别 : Type 1 external paths (E1) are to destinations outside the OSPF domain. Type 1 external paths have a cost that is the sum of this external cost
39、plus the cost of the path to the ASBR. Type 2 external paths (E2) are also to destinations outside the OSPF domain, but do not take into account the cost of the path to the ASBR. 61 Auto RP mapping agent 是什么 ,224.0.1.39 是起什么作用？ Mapping agent is the router that make decision which router in the multi
40、cast domain is RP. Mapping agent receives the RP announcement messages from candidate RPs and arbitrates conflicts. The RP mapping agent then sends the consistent group-to-RP mappings using discovery messages to all other routers by dense mode flooding. The multicast address 224.0.1.39 is used by Ca
41、ndidate-RP to send RP-announce message as the destination IP address, and the mapping agent is listening for the address. 62. what is the routing family which the router has to build map for the routing domain? OSPF 63 what is the difference between the HSRP and VRRP? A: （ 1） VRRP is an IEEE standar
42、d (RFC 2338) for router redundancy; HSRP is a Cisco proprietary protocol. （ 2） VRRP allows actual router IP address to be the same with the virtual IP,and if the same the router which get the IP will become Master router of the VRRP group;HSRP do not allow actual router IP address to be the same wit
43、h the virtual IP. （ 3） HSRP uses the multicast address 224.0.0.2 which represent all router in the subnet , while VRRP uses the multicast address 224.0.0.18;HSRP uses UDP port 1985 while VRRP uses a protocol number of 112. （ 4） HSRP may have one active router,one standby router,multiple listening ro
44、uter.While VRRP may have one master router,multiple backup router. （ 5） HSRP exchange three kinds of message, hello message,coup message ,resign message. VRRP exchange only one kind of message, advertisement message. （ 6） In VRRP， preempt is enable by default.But in HSRP preemt is disable by default
45、. 补充： HSRP Messages and States Routers configured with HSRP exchange three types of multicast messages: HelloThe hello message conveys to other HSRP routers the HSRP priority and state information of the router. CoupWhen a standby router wants to assume the function of the active router, it sends a
46、coup message. ResignA router that is the active router sends this message when it is about to shut down or when a router that has a higher priority sends a hello or coup message. associated interface is not up. HSRP groups configured on other routers on the network that are learned via snooping are
47、displayed as being in the Init state. Locally configured groups with an interface that is down or groups without a specified interface IP address appear in the Init state. 64. Why you use Spanning-tree? Difference between PVST all routes belong to this community by default. Received routes belonging
48、 to this community are advertised freely. NO_EXPORT (4294967041, or 0xFFFFFF01) Routes received carrying this value cannot be advertised to EBGP peers or, if a confederation is configured, the routes cannot be advertised outside of the confederation. NO_ADVERTISE (4294967042, or 0xFFFFFF02) Routes r
49、eceived carrying this value cannot be advertised at all, to either EBGP or IBGP peers. LOCAL_AS (4294967043, or 0xFFFFFF03) RFC 1997 calls this attribute NO_EXPORT_SUBCONFED. Routes received carrying this value cannot be advertised to EBGP peers, including peers in other autonomous systems within a confederation. 73. what is the purpose of classification? A： The purpose of classification is to sort packets into different traffic types, so that different policies then can be applied . 74. what does three entries of th
