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Border Gateway Protocol

Border Gateway Protocol

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Border Gateway Protocol

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  1. Border Gateway Protocol Chapter 8

  2. Learning Objectives • Explain when and where you would use BGP, and how you can avoid using BGP • Describe how BGP operates • Explain how BGP aggregates routes

  3. BGP Background • An Exterior Gateway Protocol (EGP) • Routes packets between Autonomous systems • Based on distance-vector routing protocols • Does not require hierarchical design • Current version BGP-4 contains enhancements, is more stable, and uses less bandwidth and CPU time

  4. IBGP and EGBP • May be used both inside and outside Autonomous System • Used inside, it is called Internal BGP (IBGP) • Exchanges information about external Autonomous Systems • Can exchange routing information between different portions of very large Autonomous System • Used outside, it is called External BGP (EBGP)

  5. Routing Between Autonomous Systems • Autonomous System is cohesive unit with clear definition of routes it contains • EGP will be unable to route packets to all sections in Autonomous System if it is not contiguous • BGP uses Autonomous System numbers to avoid routing loops • Internet Assigned Numbers Authority (IANA) ultimately responsible for allocating numbers • IANA delegated responsibility to number of regional organizations

  6. Autonomous System Numbers • Autonomous System numbers are limited • AS Number field is 16 bits long • Maximum of 65,535 possible AS numbers • Some ranges of numbers have been set aside • From 64,512 to 65,535 • BGP called path vector routing protocol because it stores routes that AS passes through

  7. Using BGP • BGP adds complexity to routing • Consider carefully before using BGP • Reasons to use BGP • Your AS is multihomed (has multiple connections to other Autonomous Systems) • Other Autonomous Systems use your AS as a transit AS • You need to control traffic entering and leaving your AS

  8. Reasons to Avoid BGP • Your AS has a lone connection to the Internet or to another AS • Your routers do not have enough memory or a CPU to handle thousands of routes • You do not need to worry about routing policy or controlling traffic as it enters and leaves your AS • You do not have much bandwidth between Autonomous Systems • You do not understand BGP well

  9. How to Avoid Using BGP • Use static or default routes and redistribute them • See Figure 8-1 • Customer uses default route propagated through its OSPF AS with the default-information originate always command • ISP uses static route to get to customer’s AS

  10. Connecting to an ISP with a Default Route

  11. Using Two Simultaneous Internet Connections • Two routers may connect to two different Internet Service Providers • See Figure 8-2 • Disadvantage is asymmetric routing • Packets take different routes to same destination • Some applications cannot handle packets arriving out of order • Makes troubleshooting more difficult

  12. Connecting to Two Different Internet Service Providers

  13. Avoiding Asymmetric Routing • Use one ISP as primary ISP and other as back-up ISP • Use floating static route • May not provide enough flexibility • Cannot do load balancing • Can use BGP to get around limitations

  14. BGP Protocol Operation • BGP routers must communicate with each other • Establish neighbor relationships • Exchange routing information • When routers exchange information, they are called neighbors or peers

  15. BGP Neighbor Relationships • BGP routers, also known as BGP speakers, use TCP port 179 to communicate • All communication between two BGP speakers will be reliable • Different types of messages used to exchange BGP peer routing information

  16. BGP Messages • Each BGP message has header • 16-byte Marker field used to authenticate messages from other routers • 2-byte Length field indicates length of entire message, from 19 bytes to maximum of 4096 bytes • 1-byte Type field indicates message type • See Figure 8-3

  17. BGP Message Header Format

  18. Four Types of BGP Messages • Open messages used to initiate neighbor relationships and negotiate BGP parameters • Notification messages used to reset peer relationships between two routers • Update message used to send information about a single route • Keepalive messages used to indicate that router accepted an open message and to make sure neighbors are still responding; similar to a hello packet

  19. Establishing Neighbor Relationships • BGP router opens TCP connection and sends open message to find potential BGP peer • IBGP Peers can be anywhere within the AS • EBGP are typically adjacent to each other • If not adjacent, they require additional configuration • See Figure 8-4

  20. Format of Open Message

  21. Fields in Open Message • Version - 1-byte field identifying version of BGP • My AS number - 2-byte field containing AS number of sending router • Hold time - maximum time in seconds that router waits between keepalive or update packets • BGP Identifier - 4-byte field identifying BGP router • Optional parameter length field - 1-byte field containing length of optional parameters field or set to 0 indicates no optional parameters • Optional parameters - only current one is authentication; more may be added in future

  22. States During Neighbor Negotiation • BGP routers pass through several states during process of becoming neighbors • Idle - BGP router refuses all incoming until a Start event is initiated • Connect - Waits for TCP connection to complete • Active - Attempts to initiate connection with peer • OpenSent - Has sent message; waits for reply • OpenConfirm - Waits for keepalive or notification message from neighbor • Established - Has completed negotiations with its peer

  23. BGP Neighbors • If router jumps between Connect and Active states, it usually means two routers have a communication problem • BGP does not allow for dynamic discovery of neighbors • Must manually configure them • Prevents unwanted routers from injecting routes into BGP routing tables • Makes Internet more stable

  24. Notification Messages • If BGP router encounters error while establishing peer relationships, it may send a notification message • Router closes TCP connection to peer • Notification message is 1-byte Error Code field and variable-length Data field • See Figure 8-5 for format of notification message • See Table 8-1 for BGP error codes

  25. Notification Message Format

  26. BGP Error Codes continued

  27. BGP Error Codes

  28. Exchanging Routing Information • BGP peers exchange full routing tables only when they first become peers • Afterwards, they send incremental routing updates

  29. Update Messages • May contain information about single route or withdrawn route(s) that are no longer reachable • May contain Network Layer Reachability Information (NLRI) • 1-byte field shows length of prefix followed by prefix with trailing bits to end on octet boundary • See Figure 8-6 • May contain path attributes • Various information about a route such as NEXT_HOP

  30. Update Message Format

  31. Update and Withdrawal Messages • Withdrawal Routes Message has 1-byte Length field followed by route prefix • Length field must also include length field of path attributes • If Length field of path attributes is set to 0, it indicates no path attributes • In theory, minimum size of update message packet is 23 bytes • 19 bytes for header • 2 bytes for each length field

  32. Path Attributes • Path attributes determine which routes to include in routing tables and which routes to filter • Numerous path attributes • All BGP-4 routers agree on some path attributes • See Figure 8-7 for format of path attributes within an update message

  33. Format of Path Attributes

  34. Attribute Flags Field • Attribute Flags field indicate four possible classes of each attribute • Well-known mandatory attributes • Well-known discretionary attributes • Optional transitive attributes • Optional nontransitive attributes • See Table 8-2

  35. Attribute Flags Field

  36. Path Attributes • Three parts for each attribute • Attribute type—contains 1-byte attribute flag and 1-byte attribute type code • See Table 8-3 • Attribute length • Attribute value

  37. Attribute Type Codes

  38. BGP Table • BGP router tracks various protocol-specific parameters in BGP table • Includes information about attributes of each path • BGP table version increases by one each time the table changes • Provides some indication of stability of BGP sessions • May change frequently because of Internet changes

  39. Types of Path Attributes • BGP routers make routing decisions and filter routes based on path attributes • Some common attributes include • ORIGIN Attribute • AS_PATH Attribute • NEXT_HOP Attribute • MED Attribute • Weight Attribute

  40. ORIGIN Attribute • Mandatory attribute that indicates source of particular route • Three possible values • EGP - route learned from another AS through Exterior Gateway Protocol; indicated with “e” • IGP - route learned from same AS through Interior Gateway Protocol; indicated with “I” • Incomplete - route’s source is unknown or learned through another means; indicated with “?”

  41. AS_PATH Attribute • Indicates path a particular route took to reach router • Shows whether route is loop free • Each AS prepends or places its own AS number at beginning of route’s AS_PATH attribute • If router sees its own AS number in AS_PATH attribute, route is a loop and will be rejected • Also contains information about summarized routes • See Figure 8-8

  42. BGP Routers Add AS Numbers to AS_PATH Attribute

  43. NEXT_HOP Attribute • In EBGP, indicates next hop router • Several factors affect this attribute • EBGP routers consider next hop peer that sent update about route • IBGR routers must keep NEXT_HOP attribute for routes learned from EBGP, even when advertising them to IBGR peers • See Figure 8-9

  44. Setting NEXT_HOP Attribute

  45. Third-party Next Hop • Third-party next hop specifies the destination router on multiaccess network to avoid extra hop • See Figure 8-10

  46. NEXT_HOP Attribute on Multiaccess Networks

  47. MED Attribute • MULTI_EXIT_DISC attribute helps routers distinguish between multiple connections to same external AS • Optional nontransitive attribute; also called the BGP metric • Propagated to neighboring Autonomous Systems but not beyond • Used by routers outside AS to decide how to enter AS • Controls how traffic leaves AS • Must be configured manually • See Figure 8-11

  48. MED Attribute Advertised Between Autonomous Systems

  49. Other Attributes • LOCAL_PREF – discretionary attribute used by routers inside an AS to choose an exit path • ATOMIC_AGGREGATE – discretionary attribute used in BGP’s handling of CIDR address blocks • AGGREGATOR – optional transitive attribute router may add to summarized routes

  50. COMMUNITY Attribute • COMMUNITY Attribute – gives BGP routers a mechanism to filter routes by tagging a group of routes into a BGP community • Optional transitive attribute defined in RFC 1997 • Consists of one or more community values consisting of 32-bit fields • Values may be well known or private