BGP路由协议-CISCO设备.ppt

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1、BGPBGP路由路由协议BGP Autonomous Systems An AS is a collection of networks under a single technical administration.(自治系统经典定义:在单一技术下管理下,采用同一种内部网关协议和统一度量值在AS内转发数据包,并采用一种外部协议将数据包转发到其他AS的一组路由器.)IGPs operate within an AS.BGP is used between Autonomous Systems.Exchange of loop-free routing information is guaran

2、teed.(BGP的主要目标是提供一种能够保证自治系统间无环路的路由选择信息交换的域间路由系统.)BGP Characteristics(特征)nBGP is a distance-vector protocol with the following enhancements:uReliable updates:BGP runs on top of TCP(port 179;可靠通信.)uIncremental,triggered updates only(增量更新,触发更新)uPeriodic keepalive messages to verify TCP connectivity(周期性

3、通过keepalive包发送消息.)uRich metrics(called path vectors or attributes,丰富的计量值,一一比较,BGP中称为属性;类似于IGP中的开销值.)uDesigned to scale to huge internetworks(e.g.,the Internet;专门为大型互联网络而设计.)u支持VLSM可变长子网掩码。BGP Databases(数据表)uNeighbor table vList of BGP neighbors(BGP对等体,邻居,交换路径信息,与RIP类似.)v可以直连可以不直连.v邻居关系建立在TCP连接之上.uBG

4、P forwarding table/databasevList of all networks learned from each neighborvCan contain multiple pathways to destination networks vDatabase contains BGP attributes for each pathwayuIP routing tablevList of best paths to destination networks(列出了到目标网络的最佳路径)BGP表运行BGP的路由器保存着一个独立于IP路由表的BGP表.SHOW IP BGPSt

5、atus codes:s suppressed,d damped,h history,*valid,best,i-internalOrigin codes:i-IGP,e-EGP,?-incomplete Network Next Hop Metric LocPrf Weight Path*10.0.0.0 10.1.1.100 0 0 65200 iBGP Message TypesBGP defines the following message types:Open：(版本号4,AS号,保持时间,路由器ID)Includes holdtime and BGP router IDKeepa

6、live：(类似HELLO包)Update：(撤消路由,路径属性,网络层可达信息)Information for one path only(could be to multiple networks)Includes path attributes and networksNotification：(通告消息,检测到出错条件时,发送通知消息)When error is detectedBGP connection is closed after sentBGP StatesWhen establishing a BGP session,BGP goesthrough the followin

7、g steps:Idle(闲置状态):Router is searching routing table to see if a route exists to reach the neighbor.Connect(连接状态):Router found route and has completed three-way TCP handshake.Open sent(发送消息):Open message sent with the parameters(参数)for the BGP session.Open confirm(确认消息):Router received agreement on

8、the parameters for establishing session.Established(建立邻居,开始路由):Peering is established;routing begins.Peers=Neighbors A BGP peer,also known as a BGP neighbor,is a specific term that is used for BGP speakers that have established a neighbor relationship.Any two routers that have formed a TCP connectio

9、n to exchange BGP routing information are called peers or neighbors.Internal BGPIGBP refers to the presence of BGP neighbors within the same AS.The neighbors do not have to be directly connected.External BGP When BGP neighbors belong to different autonomous systems they are called EBGP.EBGP neighbor

10、s,by default,need to be directly connected.BGP PeeringRouterA#show ip bgp summaryBGP table version is 23,main routing table version 2310 network entries and 11 paths using 1242 bytes of memory4 BGP path attribute entries using 380 bytes of memoryBGP activity 23/13 prefixes,38/27 paths0 prefixes revi

11、sed.Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd10.1.1.100 4 65200 211 211 13 0 0 00:01:53 5192.168.1.18 4 65101 214 226 23 0 0 00:00:13 1192.168.1.34 4 65101 214 226 23 0 0 00:00:09 1192.168.1.50 4 65101 214 225 23 0 0 00:00:06 3BGP Commandsrouter bgp autonomous-system(64512-6

12、5535)Router(config)#This command,with no subcommands,does not activate BGP.(还需要其他子命令配合.)Only one instance of BGP can be configured on the router at a single time.(仅一个BGP进程,边界是链路.)The autonomous system number identifies the autonomous system to which the router belongs.The autonomous system number in

13、 this command is compared to the autonomous system numbers listed in neighbor statements to determine if the neighbor is an internal or external neighbor.(确定邻居关系)BGP neighbor Commandneighbor ip-address|peer-group-name remote-as autonomous-systemRouter(config-router)#The neighbor command activates a

14、BGP session with this neighbor.(只有指定邻居双方,才能建立会话.)The term remote-as shows what AS this neighbor is in.This AS number is used to determine if the neighbor is internal or external.(邻居AS号一样,为IBGP;不一样为EBGP.)This command is used for both external and internal neighbors.The IP address specified is the des

15、tination address of BGP packets going to this neighbor.(TCP连接或环回口地址.)This router must have an IP pathway to reach this neighbor before it can set up a BGP relationship.Example:BGP neighbor CommandBGP network Command network network-number mask network-maskRouter(config-router)#This command tells BGP

16、 what network to advertise,not how to advertise the network.(宣告什么网络,将此路由注入BGP;也可使用再发布.)The command does not activate the protocol on an interface.Without a mask option,it advertises classful networks.If a subnet of the classful network exists in a routing table,the classful address is announced if a

17、uto summary is enabled.Auto summary is enabled by default.BGP looks for an exact match in the local routing table before announcing this route.(宣告前必须保证路由是可用的.)在IOS 12.0之前的版本中,每台BGP路由器有200条NETWORK限制;现在这个限制已经取消了;路由器的内存决定了我们可以使用的NETWORK命令的最大数量.MASK命令可以处理超网和子网;自动汇总缺省是自动的.NETWORK命令列表必须包括AS中我们想要通告的所有网络,而不

18、仅仅是那些本地连接在路由器上的网络.案例:在多个AS间配置BGP客户要求:配置BGP来与两家服务供应商交换路由信息;以实现较强的容错的能力.方法:1,保证路由器直连.2,配置ISP1,ISP2路由器.(实际中不可操作)Router bgp 100Neighbor 192.168.1.6 remote-as 243,配置公司的边界路由器:Router bgp 24Neighbor 192.168.1.5 remote-as 100Neighbor 172.16.1.5 remote-as 2004,show ip bgp观察到什么情况?PING可达吗?5,请在多台路由器上实现BGP的互联,看还需

19、要什么条件?BGP Example Configuration1.RouterB(config)#router bgp 650002.RouterB(config-router)#neighbor 10.1.1.2 remote-as 645203.RouterB(config-router)#neighbor 192.168.2.2 remote-as 65000IBGP and RedistributionA transit AS should run IBGP on all routers because the full Internet routing table is too la

20、rge to redistribute into an IGP.(由互联网络上的路由表太大了,如果发布到IGP中,会使网络崩溃,所以在转接AS中所有路由器都要运行BGP)IBGP Split Horizon RuleBy default,routes learned via IBGP are never propagated to other IBGP peers.(缺省情况下,路由器从IBGP中学到的路由不会再传给其它IBGP邻居;.)Partial Mesh IGPRouting Issues without Fully Meshed IBGPRouter C will drop the

21、packet to network 10.0.0.0.Router C is not running IBGP;therefore,it has not learned about the route to network 10.0.0.0 from router B.In this example,router B and router E are not redistributing BGP into OSPF.所以我们必须为每个连接指定BGP邻居;或者将BGP发布到IGP中.1.理解bgp同步1.BGP同步规则的定义:在bgp同步打开的情况下,一个BGP路由器不会把那些通过ibgp邻居学

22、到的bgp路由通告给自己的ebgp邻居;除非自己的igp路由表中存在这些路由,才可以向ebgp路由器通告.如下例:同步开启时,A不会将B告诉它的告诉E;除非A和B的IGP路由表中也存在172.16.0.0.这条路由,A才会向E宣告.也即A,B同时运行BGP和IGP时,要打开同步.(但此图中没有匹配的IGP路由172.16.0.0;如同步打开,按同步规则就会出现IBGP不会传递IBGP路由.)Example:BGP Synchronization If synchronization is on(the default),then:路由器B将通过IBGP向AS65500中的其他路由器通告到的路由

23、.路由器B将使用到的路由,并且将它放置到它的路由表中.Routers A,C,and D would not use or advertise the route to 172.16.0.0 until they receive the matching route via an IGP(也即,因为没有运行IGP,所以这些路由器将永远不使用或通告此条路由).Router E would not hear about 172.16.0.0.If synchronization is off,then:Routers A,C,and D would use and advertise the ro

24、ute they receive via IBGP;router E would hear about 172.16.0.0.If router E sends traffic for 172.16.0.0,routers A,C,and D would route the packets correctly to router B.2.同步规则的基本需求2.BGP同步规则的目的:为防止一个AS(不是所有的路由器都运行bgp)内部出现路由黑洞,即向外部通告了一个本AS不可达的虚假的路由.3.BGP同步规则的基本需求如果一个AS内部存在非bgp路由器,那么就出现了bgp和igp的边界,需要在边界

25、路由器将bgp路由发布到igp中,才能保证AS所通告到外部的bgp路由,在AS内部是连通的.实际上是要求bgp路由和igp路由的同步.3,然而4.满足BGP同步规则的基本需求的而产生的严重结果如果将bgp路由发布到igp中,由于bgp路由主要是来自AS外部的路由(来自internet),那么结果是igp路由器要维护数以万计的外部路由,对路由器的cpu和memeory以及AS内部的链路带宽的占用将带来巨大的开销.4.结论:-同步目的之一,缺省情况下,同步打开,不会将IBGP路由通告给EBGP邻居.(BGP+IGP+重发布+开同步)-同步目的之二,防止内部出现路由黑洞,向外界公布虚假路由.-同步目

26、的之三,当BGP和IGP同时存在时,为了保证向外界公告的路由是真实的,理论上要求同步.-但是,BGP路由主要来自于互联网,IGP不能承受数以万计的外部路由;我们一般不建议将BGP路由注入IGP中.通常bgp协议的运行需要关闭同步,5,因此:在实际运用中，如下情况要关闭同步：1，当一个AS内的所有转接路由器都在运行全网状互联的IBGP时；内部可达性可以保证；关闭同步。（外部EBGP可以通过IBGP自动转接）2，当AS不是一个转接型AS时，即不转接其他AS间的数据流时，关闭同步。在未来的IOS版本中,缺省情况下BGP同步将被关闭,因为大多数都在所有的路由器上运行BGP.二.BGP同步规则的总结1.

27、在所有的方案中,既要保证传递bgp路由,还要保证bgp路由的连通性.2.关闭同步能够实现bgp路由的传递,不一定能保证as内部连通性,除非as内所有路由器都运行bgp才可以保证连通性;否则,仍然需要路由再发布(bgp+igp).3.最后,在as内部一般需要部署igp来维持AS内部网络路径的连通性,以保证as内部的所通告的bgp路由的下一跳的可达性.这样bgp网络就具有更好的灵活性和扩展性.4,在某种程度上来讲,同步是一个陈旧的特性,它承担将路由重发布到IGP的任务.三.BGP同步的解决方案1.full mesh ibgpAS内部的所有路由器都运行full mesh ibgp,就可以关闭所有路由

28、器的同步,而不影响路由的通告和连通性.问题:当as内部路由器数量很多时,需要建立N*(N-1)/2个ibgp会话,带来过度的系统开销,扩展性不好.15个路由器的AS,需要建立15(15-1)/2=105个ibgp会话Full-Mesh IBGP 拓扑示意:Full mesh ibgp解决方案三.bgp同步的解决方案2.路由反射器AS内部的所有路由器都运行bgp,在AS内部部署路由反射器,构建hub and spoke的ibgp(会话数为N-1),然后关闭所有bgp路由器的同步.问题:此方案可以使bgp路由器传递ibgp路由到ebgp,并保证bgp路由的连通性.但是对物理拓扑有很大的限制(要求是

29、星型拓扑)15个路由器的AS,具有冗余的RR方案(33个ibgp会话)Bgp路由反射器设计拓扑:路由反射器解决方案三.bgp同步的解决方案3.bgp联盟:AS内部的所有路由器都运行bgp,把一个原始的AS基于网络拓扑划分为若干个sub-AS(又称联盟AS),联盟AS之间的bgp邻居叫做联盟ebgp,不需要full mesh bgp会话;在每个联盟AS内部运full mesh ibgp或者hub and spoke反射器,然后就可以关闭所有路由器的bgp同步功能.结论:bgp联盟结合路由反射器的方式,较好的解决了bgp的同步规则带来的需求,是最为有效的解决方案.Bgp联盟命令：bgp confederation identifier 1200Bgp confederation peers 65533 65535bgp联盟拓扑示意图:BGP联盟拓扑示意图: