HC120112002BGP路徑選擇

1、Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. BGP路徑選擇Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page1前 言BGP作為一個策略工具，主要作用是實現(xiàn)AS間的路由信息傳遞。BGP就是結(jié)合豐富的路徑屬性，很好的控制路由信息的傳遞，從而實現(xiàn)路徑的選擇。Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page2培訓(xùn)目標(biāo)學(xué)完本課程后，您應(yīng)該

2、能：l知道什么是路徑屬性l了解BGP常用路徑屬性l了解BGP的選路原則Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page3目 錄 BGP路徑屬性介紹 BGP路徑選擇Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page4目 錄 BGP路徑屬性介紹路徑屬性介紹 BGP路徑選擇Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page5BGP的

3、路徑屬性BGP路徑屬性是一組描述BGP前綴特性的參數(shù)BGP路徑屬性可以被分為四大類：l公認(rèn)必遵 (Well-known mandatory)l公認(rèn)任意 (Well-known discretionary)l可選過渡 (Optional transitive)l可選非過渡 (Optional non-transitive)Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page6BGP的路徑屬性 (續(xù))公認(rèn)屬性是所有BGP路由器都必須識別的屬性l公認(rèn)必遵 (Well-known mandatory)n所有BGP路

4、由器都可以識別，且必須存在于Update消息中。如果缺少這種屬性，路由信息就會出錯l公認(rèn)任意 (Well-known discretionary)n所有BGP路由器都可以識別，但不要求必須存在于Update消息中，可以根據(jù)具體情況來決定是否添加到Update消息中Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page7BGP的路徑屬性 (續(xù))可選屬性不需要都被BGP路由器所識別l可選過渡 (Optional transitive)nBGP路由器可以選擇是否在Update消息中攜帶這種屬性。接收的路由器如果不識

5、別這種屬性，可以轉(zhuǎn)發(fā)給鄰居路由器，鄰居路由器可能會識別并使用到這種屬性l可選非過渡 (Optional non-transitive)nBGP路由器可以選擇是否在Update消息中攜帶這種屬性。在整個路由發(fā)布的路徑上，如果部分路由器不能識別這種屬性，可能會導(dǎo)致該屬性無法發(fā)揮效用。因此接收的路由器如果不識別這種屬性，將丟棄這種屬性，不必再轉(zhuǎn)發(fā)給鄰居路由器Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page8Copyright 2012 Huawei Technologies Co., Ltd. All rig

6、hts reserved. Page9常見BGP路由屬性1、Origin2、AS_PATH3、Next hop4、MED5、Local-Preference6、Atomic-Aggregate7、Aggregator8、Community9、Originator-ID10、Cluster-List11、MP_Reach_NLRI12、MP_Unreach_NLRI13、Extended_CommunitiesCopyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page10常見BGP路由屬性 (續(xù))| BGP屬性類別

7、1. Origin2. AS_Path3. Next_Hop4. Multi_Exit_Disc5. Local_Pref6. Atomic_Aggregate7. Aggregator8. Community9. Originator ID10. Cluster List(well-known mandatory)(well-known mandatory)(well-known mandatory)(optional non-transitive)(well-known discretionary)(well-known discretionary)(optional transitive

8、)(optional transitive)(optional non-transitive)(optional non-transitive)Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page11起源 (Origin) 屬性一般的，具體的實現(xiàn)按如下方式?jīng)Q定一條路由的Origin屬性l某條路由是直接而具體的注入到BGP路由表中的，則origin屬性為IGPn通過network命令注入BGP的路由l通過EGP(RFC904)學(xué)到的路由，則origin屬性為EGPl其他情形下，Origin屬性都為 Incom

9、pleten通過import命令注入BGP的路由Origin屬性值默認(rèn)情況下不被任何路由器修改Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page12Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page13起源 (Origin) 屬性 (續(xù))RTBdisplay bgp routing-table Total Number of Routes: 2 BGP Local router ID is 192.168.2.1 S

10、tatus codes: * - valid, - best, d - damped, h - history, i - internal, s - suppressed, S - Stale Origin : i - IGP, e - EGP, ? - incomplete Network NextHop MED LocPrf PrefVal Path/Ogn * 192.168.1.0 10.1.1.1 0 0 100i * 192.168.2.0 10.1.1.1 0 0 100iCopyright 2012 Huawei Technologies Co., Ltd. All right

11、s reserved. Page14AS路徑(AS_PATH)屬性18.0.0.0/8AS200AS400RTANLRI 18.0.0.0/8AS_PATH (400 300 200)NLRI 18.0.0.0/8AS_PATH (500 200)AS 100AS300AS500NLRI 18.0.0.0/8AS_PATH (500 200)Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page15Copyright 2012 Huawei Technologies Co., Ltd. All rights

12、reserved. Page16問題10.0.0.0/8AS 387NLRI 10.0.0.0/8AS_PATH (213)RTARTCRTBNLRI 10.0.0.0/8AS_PATH(387 213)AS 213RTA會接受這條路由嗎？Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page17預(yù)先規(guī)劃AS_PATH10.0.0.0/8AS 462AS 3872 Mbps64 kbps2 MbpsAS 123NLRI 10.0.0.0/8AS_PATH (123)NLRI 10.0.0.0/8AS_PATH

13、 (123 123 123)NLRI 10.0.0.0/8AS_PATH (462 123)2 Mbps10.0.0.0/8RTARTBRTCRTDCopyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page18下一跳 (Next Hop) 屬性18.0.0.0/820.0.0.0/8RTARTCRTBRTD19.0.0.0/821.0.0.221.0.0.110.0.0.210.0.0.310.0.0.1AS100AS200IBGPIBGPEBGP RTB可經(jīng)下一跳10.0.0.2到達(dá)18.0.0.0/8可經(jīng)下一

14、跳10.0.0.3到達(dá)20.0.0.0/8 RTA可經(jīng)下一跳10.0.0.2到達(dá)18.0.0.0/8可經(jīng)下一跳10.0.0.3到達(dá)20.0.0.0/8可經(jīng)下一跳21.0.0.1到達(dá)19.0.0.0/8 RTC 可經(jīng)下一跳10.0.0.1到達(dá)19.0.0.0/8可經(jīng)下一跳10.0.0.3到達(dá)20.0.0.0/8Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page19Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page20本

15、地優(yōu)先級屬性 (Local-Preference)RT1RT2ISP1RT4RT3210.52.83.0/24210.52.82.0/24RT5 Prefix/mask Local-Pref210.52.83.0 100210.52.82.0 200 Prefix/mask Local-Pref210.52.83.0 200210.52.82.0 100InternetISP2ISP0Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page21Copyright 2012 Huawei Technologies

16、 Co., Ltd. All rights reserved. Page22MED (Multi-Exit-DISC) 屬性RT1RT2RT3AS200AS100MED是可選非過渡屬性區(qū)別到達(dá)同一鄰居AS的多條入口鏈路(MED值越小，鏈路越優(yōu)先)通過EBGP發(fā)送MED值給對等體 Prefix/mask MED210.52.83.0 50210.52.82.0 100 Prefix/mask MED210.52.83.0 100210.52.82.0 50Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page2

17、3Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page24問題在RTC上為什么不一定會選擇通往RTA的鏈路（MED值較小）作為主鏈路去往目標(biāo)網(wǎng)段192.10.0.0/16？201.1.1.2/24160.1.1.2/24201.1.1.1/24160.1.1.1/24RTCRTBRTAAS500AS600AS700192.10.0.0/16E0E0MED10MED20Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page2

18、5團體 (Community) 屬性什么是團體屬性l團體是一組有相同性質(zhì)的目的地址路由。目的就是將路由信息編組，通過組的標(biāo)識決定路由傳遞的策略。團體屬性l屬性類型：8l可選，過渡屬性Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page26團體 (Community) 屬性團體屬性是由一系列4字節(jié)(0 x000000000 xFFFFFFFF)數(shù)值所組成l保留的團體屬性：n0 x000000000 x0000FFFFn0 xFFFF00000 xFFFFFFFFl公認(rèn)團體屬性：nNO_EXPORT (0 xF

19、FFFFF01) nNO_ADVERTISE (0 xFFFFFF02) nNO_EXPORT_SUBCONFED (0 xFFFFFF03) l私有團體屬性：nAS(2B):Number(2B)Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page27Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page28目 錄 BGP路徑屬性介紹 BGP路徑選擇路徑選擇Copyright 2012 Huawei Technologies Co., Ltd. All rights reserved. Page29BGP路徑選擇過程1，如果此路由的下一跳不可達(dá)，忽略此路由2，Pre