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How BGP Works

In this webinar, we cover how Border Gateway Protocol works. Starting from key concepts, you'll learn about Autonomous Systems, the BGP protocol, AS Path, learning and advertising routes, RIBs and route selection. See the webinar recording at https://www.thousandeyes.com/webinars/how-bgp-works

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How BGP Works

  1. 1. 1© 2017 ThousandEyes Inc. All Rights Reserved. How BGP Works Young Xu, Product Marketing Manager
  2. 2. 2© 2017 ThousandEyes Inc. All Rights Reserved. About ThousandEyes Network Intelligence platform that gives you a complete picture from users to internal and cloud-based applications Routing! User App End-to-End Performance Data App Performance! User Experience! Network Topology! Routing Topology! Enterprise, Endpoint and Cloud Agents Network Connectivity! And Route Monitors! Surface insights from a global data set Lightweight, flexible data collection Unified view of diverse performance data Solve issues across shared infrastructure See any network like it’s your own
  3. 3. 3© 2017 ThousandEyes Inc. All Rights Reserved. Border Gateway Protocol BGP-4 RFC4271 Where do I forward traffic to reach an IP address in an external network? How can I control the route and composition of inbound traffic to my network?
  4. 4. 4© 2017 ThousandEyes Inc. All Rights Reserved. •  Autonomous System –  Collection of IP prefixes –  Common routing policy to other ASes –  Registered by an RIR (regional Internet registry) –  Denoted by a name and number •  A Private AS can be used when a single upstream exists –  External routing policy is identical IP Blocks and Autonomous Systems AS 200 2.2.2.0/24 2.2.3.0/24 Autonomous System
  5. 5. 5© 2017 ThousandEyes Inc. All Rights Reserved. •  Inter-Network – Used for routing between networks (Autonomous Systems), or within large networks •  Reachability – BGP defines how one AS can reach another, described as a path vector (AS Path) •  Policy-Based – BGP makes it possible for an AS to apply policies (e.g. multi- homing, failover, commercial terms) •  Decentralized – Each AS makes policy decisions autonomously, using BGP to coordinate and share routes Key BGP Concepts
  6. 6. 6© 2017 ThousandEyes Inc. All Rights Reserved. •  Neighboring routers, within or between ASes, establish a TCP connection on port 179. •  BGP messages include: –  Open –  Update –  Notification –  Keep Alive BGP the Protocol AS 100 1.1.1.0/24 AS 300 3.3.3.4/22 AS 400 4.4.0.0/16 AS 200 2.2.2.0/24 Border Router Origin Autonomous System Internal Router BGP peers exchange routes, within and among ASes
  7. 7. 7© 2017 ThousandEyes Inc. All Rights Reserved. •  An update message may advertise routes, withdraw routes, or both •  Any number of routes may be withdrawn •  Any number of routes may be advertised –  They must all share the same attributes –  These attributes include AS Path and Origin •  Therefore, you’ll expect at least one Update message for each Origin AS BGP Update Message BGP Header Withdrawn Routes (n Prefix/Length tuples) Path Attributes 7 well-known attributes: Origin, AS Path, Next Hop, MED, Local Pref, Atomic Aggregate, Aggregate Optional attributes: Community, Originator Network Layer Reachability Info (n Prefix/Length tuples)
  8. 8. 8© 2017 ThousandEyes Inc. All Rights Reserved. AS 300 3.3.3.4/22 •  Describes available routes using a path vector •  Each AS will prepend itself onto the AS Path •  Associated with an origin AS and prefix •  Avoids routing loops by rejecting any AS Path containing the local AS AS Path AS 100 1.1.1.0/24 AS 400 4.4.0.0/16 AS 200 2.2.2.0/24 Origin Autonomous System 300 100 100100 200 100
  9. 9. 9© 2017 ThousandEyes Inc. All Rights Reserved. The Update Process Loc-RIB Routes with policies applied Adj-RIB-In Unprocessed routes Adj-RIB-Out Routes to advertiseNewly learned routes Newly advertised routes FIB Routes with next-hop and interfaces IP Routing Table Routes aggregated across protocols Next-hops resolved Interfaces calculated Locally learned routes added 1.  Preference calculated (PIB) 2.  Route selection •  Ensure resolvability •  Break ties 3.  Route dissemination •  Aggregation Incorporated with IS-IS, OSPF, etc. by Administrative Distance
  10. 10. 10© 2017 ThousandEyes Inc. All Rights Reserved. The routing application builds a Routing Information Base (RIB) to map learned prefixes and routes Example of the routing table for AS100 (show ip bgp) * = valid; > = best Routing Tables (RIBs) Network Next Hop Metric (MED) Local Pref Weight Path *> 1.1.1.0/24 10.1.12.2 0 0 i *> 2.2.2.0/24 10.1.14.4 0 0 200 i *> 2.2.3.0/24 10.1.14.4 0 0 200 i *> 3.3.3.4/22 10.1.16.6 0 0 300 i *> 4.4.0.0/16 10.1.16.6 0 32768 300 400 i * 4.4.0.0/16 10.1.14.4 0 0 200 400 i
  11. 11. 11© 2017 ThousandEyes Inc. All Rights Reserved. •  Highest weight (de facto standard) –  Set by the local router •  Highest local preference –  Set by the local AS, typically based on commercial relationships •  Shortest AS Path –  The route that traverses the fewest ASes •  Origin type –  Internal-learned (IGP) routes preferred •  Multi-Exit Discriminator (MED) –  A preference set by the origin AS •  Additional tiebreaking and multipath criteria… Route Selection
  12. 12. 12© 2017 ThousandEyes Inc. All Rights Reserved. •  Match the most specific prefix –  If none available, then the prefix is not reachable •  Forward traffic to the correct interface –  Based on information placed in the FIB, learned from BGP (and other protocols) •  Thus, a forwarding decision is influenced by: –  Specificity of IP prefix –  Internal routes –  BGP routes, their attributes and the local routing policy Making Forwarding Decisions
  13. 13. 13© 2017 ThousandEyes Inc. All Rights Reserved. •  Generally, BGP speakers within an AS must communicate with one another in a full mesh, each updating one another •  But this can be hard to scale in large ASes •  Alternatives to full meshes exist, including: Coordinating Within an AS Route reflection (hub-spoke) Confederations (AS subdomains)
  14. 14. 14© 2017 ThousandEyes Inc. All Rights Reserved. •  Communities –  Communicate to neighbors how to advertise routes they learn from you, and vice versa –  ISPs publish community definitions on how advertisements will be handled –  Often used for local preference, no-export, prepending, geographic or peering limitations •  MED (Multi-Exit Discriminator) –  Communicate preferred inbound paths to a neighbor •  Prepending –  Inserting the local AS to the AS Path multiple times to lengthen the path and reduce its preference by others Coordinating Between ASes
  15. 15. 15© 2017 ThousandEyes Inc. All Rights Reserved.© 2017 ThousandEyes Inc. All Rights Reserved. Demo
  16. 16. 16© 2017 ThousandEyes Inc. All Rights Reserved. BGP Route Visualization Shows Preferred Routes WV Fiber Switch Level 3
  17. 17. 17© 2017 ThousandEyes Inc. All Rights Reserved. Routes Are Reflected in Traffic Paths WV Fiber Switch Level 3
  18. 18. 18© 2017 ThousandEyes Inc. All Rights Reserved.© 2017 ThousandEyes Inc. All Rights Reserved. Watch the webinar:
 www.thousandeyes.com/webinars/how-bgp-works

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