BGP路由策略之經典文章(2)

 

《 BGP routing policies in ISP networks 》 IEEE Network Magazine 2005  1 ways to configure local policyThere are three classes of “knobs” that can be used to controlimport and export policies:1） Preference(decision process)2） Filter(eliminate certain route)3） Tag(community) 2 BGP policy common practice and design pattern1) Business relationship(1)inbound: assign local-preference to influence the BGP decision process.Often an ISP will achieve this byassigning a non-overlapping range of LocalPref values to eachtype of peering relationship; for example LocalPref values inthe range 90-99 might be used for customers, 80-89 for peers,70-79 for providers, and 60-69 for backup links. LocalPref canthen be varied within each range to do traffic engineering withoutviolating the constraints associated with the business relationship.那麼如果我站在一個自治系統或者ISP的角度能夠通過SNMP等方式獲得該自治系統的邊界路由器的路由表，那麼就可以針對同一個prefix的多個路由的local-preference值來推測出與鄰居自治系統的商業關係。(2)outbound: Controlling route export匯出策略好像很難通過單個自治系統的方式來進行推導,不過也不是說要和策略配置的方式一一驗證和對應起來，其實只要能夠把商業關係推斷正確就可以了。 2) Traffic engineering(1)outbound traffic control(by changing local-preference and IGP costs)   Outbound traffic的控制其實就是inbound策略的控制，我覺得在單個自治系統的角度也是可以做到的，比如你從不同的邊界路由器的路由表中發現到達相同的prefix的local-preferce不同，可能就是做了負載平衡；不同的metric來發現不同的hot-potato地區，不同的AS-Path長度啦等等。(2) Inbound traffic control (by AS prepending and MED):       Inbound traffic的控制其實就是outbound的策略，即如何到達自己自治系統的首碼。主要包括對於相鄰自治系統在多串連(multiple link)下所使用的MED值，以及遠端AS控制的自治系統號添加兩種方式。但是這些我覺得從其鄰居自治系統那裡來採集資料進行分析可能會更加直接些。 (3)Remote control (by changing community attributes):Remote controlprovides more flexibility than MED because it allows controlof inputs to earlier steps of the decision process like Local-Pref, as shown in the example above.Moreover,MED can onlychange the relative preference of routes, while remote controlcan be configured to filter routes, or perform AS prepending.However, ISP’s neighbors mustagree in advance to accept community attributes from the otherpeer.Also, the highly expressive nature of community attributesintroduces potential for misconfiguration. 3)Scalability Limiting routing table size (by filtering and using the community attribute): Limit the number of routing changes (by suppressing routes that flap)4)Security Discarding invalid routes (by import filtering) Protect integrity of routing policies (by rewriting attributes) Securing the network infrastructure (by export filtering) Blocking denial-of-service attacks (by filtering and damping)    《 Practical Verification Techniques for Wide-Area Routing》 sigcomm CCR 20041 verify five aspect of correcteness: validity (the existence of a route impliesthe existence of a corresponding path), visibility (the existence ofa path implies the existence of a corresponding route), safety (theexistence of a stable, unique path assignment), determinism (bestroute selection is independent of message ordering and the presenceof sub-optimal routes), and information-flow control (the protocolconforms to a specified information flow policy; that is, it does not“leak” information).2 verification methodology:Static analysisparses configuration statements to detect errors that are evidentfrom the configuration commands themselves. A sandbox can determinewhether (and under what circumstances) a seemingly correct

configuration can produce incorrect behavior