1 / 35

Routing: Exterior Gateway Protocols and Autonomous Systems

Routing: Exterior Gateway Protocols and Autonomous Systems. Border Gateway Protocol (BGP) Reference D. E. Comer, Internetworking with TCP/IP, ISBN 1-13-018380-6, 4 th Ed., Vol. 1, Ch. 15. Autonomous System (AS).

oswald
Download Presentation

Routing: Exterior Gateway Protocols and Autonomous Systems

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Routing: Exterior Gateway Protocols and Autonomous Systems • Border Gateway Protocol (BGP) Reference D. E. Comer, Internetworking with TCP/IP, ISBN 1-13-018380-6, 4th Ed., Vol. 1, Ch. 15. Network Architecture and Design

  2. Autonomous System (AS) • AS = collection of networks under a single technical administration & sharing the same routing policy • AS# can be 1 to 65535 (64512 – 65535 private) • Internal Gateway Protocols (IGPs) operate within an AS to ensure IP connectivity within it • Exterior Gateway Protocols (EGPs) run between ASs to enable routing policies between them Network Architecture and Design

  3. BGP Necessity Q: how will the traffic between AS1 and AS20 flow Q: will AS 2 allow this traffic Q: how would you solve this problem with OSPF or EIGRP Network Architecture and Design

  4. BGP Characteristics • Distance-vector protocol with enhancements: • Reliable updates (TCP port 179) • Incremental, triggered updates only • Full BGP tables exchanged after conn. setup • Only changes (delta) sent afterwards • Rich metrics (called path attributes) • Periodic keepalives to verify TCP connectivity • Designed to scale to huge internetworks • Full Internet Routing Table (FIRT) = 100.000 routes, 7000 ASs, 40MB approx. Network Architecture and Design

  5. BGP Characteristics • BGP session = TCP connection port 179 • Two routers with BGP session established are called peers or neighbors • No periodic updates • Triggered updates are batched and rate-limited (every 5 seconds for internal peer, every 30 seconds for external peer) • Needs an IGP to provide TCP connectivity between BGP peers Network Architecture and Design

  6. BGP Table and Routing Table Routing Table BGP Table • BGP has its own table, in addition to the IP routing table • Information can be exchanged between the two IGP/Static BGP Network Architecture and Design

  7. Sample BGP configurations • Customer connected to Internet Service Provider (ISP) • Customer connected to several Service Providers (multi-homed) • Service Provider networks (transit autonomous systems) • Service Providers exchanging traffic at an exchange point (e.g. AIX, MAE-East,…) • Backbone of large enterprise networks Network Architecture and Design

  8. Configuration #1 Large customer or small ISP connecting to the Internet Network Architecture and Design

  9. Configuration #2 Customer connecting to several service-providers (multi-homed customer) • BGP is mandatory in this case • Public AS# needed for the customer • Provider-independent address space for the customer Network Architecture and Design

  10. Configuration #3 Service providers exchanging traffic at exchange points (*IX) Network Architecture and Design

  11. Configuration #4 Transit AS (carrying traffic from other AS’es) Network Architecture and Design

  12. BGP Message Types • OPEN • Initialize communication • UPDATE • Advertise or withdraw routes • NOTIFICATION • Response to an incorrect message • KEEPALIVE • Actively test peer connectivity Network Architecture and Design

  13. BGP Path Attributes • BGP metrics are called path attributes • Part of the BGP Update Packet • Implemented as TLVs (Type-Length-Value) • Used by BGP peers as route selection criteria • Well-known vs Optional attributes Network Architecture and Design

  14. Well-known Attributes • Well-known attributes • must be recognized by all compliant implementations • Well-known mandatory attributes • must be present in all update messages • Well-known discretionary attributes • could be present in update messages • All well-known attributes are propagated to other neighbors Network Architecture and Design

  15. Optional Attributes • Optional attributes • recognized by some implementations (could be private), expected not to be recognized by everyone • Optional transitive attributes • propagated to other neighbors if not recognized • Optional non-transitive attributes • discarded if not recognized • Recognized optional attributes are propagated to other neighbors based on their meaning (not constrained by transitive bit) Network Architecture and Design

  16. Well-known Mandatory Attributes • Origin Code • Specifies the origin of a BGP route • IGP = route originated in an IGP (network command) • EGP = route originated in an EGP (and redistributed) • Incomplete (?) = route redistributed into BGP from IGP/static • AS_Path • sequence of AS numbers through which the network is accessible • Next_Hop • IP address of the next-hop router Network Architecture and Design

  17. Well-known Discretionary Attributes • Local preference • Used for consistent routing policy within AS • Atomic aggregate • informs the neighbor AS that the originating router aggregated routes Network Architecture and Design

  18. AS-Path Attribute • The AS-path attribute is empty when a local route is inserted in the BGP table • The sender’s AS number is prepended to the AS-path attribute when the routing update crosses AS boundary • The receiver of BGP routing information can use the AS-path to determine through which AS the information has passed • An AS that receives routing information with its own AS number in the AS-path silently ignores the information Network Architecture and Design

  19. AS-Path Attribute Network Architecture and Design

  20. Next Hop Attribute • Next-hop attribute indicates the next-hop IP address used for packet forwarding • Usually set to the IP address of the sending BGP router • Can be set to a third-party IP address to optimize routing Network Architecture and Design

  21. Next Hop Processing in Shared Media • If the receiving BGP router is in the same subnet as the current next-hop, the next-hop address is not changed to optimize packet forwarding Network Architecture and Design

  22. Next Hop Processing in NBMA • BGP next-hop processing can break connectivity with improper network designs over partially-meshed WAN networks • Correct use of subinterfaces and subnets alleviates the problem Network Architecture and Design

  23. BGP Session Establishment • BGP does not auto-discover Neighbors - they must be configured manually • Configuration must be done on both sides of the connection • Both routers will attempt to connect to the other with a TCP session on port number 179 • Only one session will remain if both connection attempts succeed • Source IP address of incoming connection attempts is verified against a list of configured neighbors Network Architecture and Design

  24. Example Network Network Architecture and Design

  25. BGP Neighbors – Idle State Initially all BGP sessions to the neighbors are idle as123>show ip bgp sum BGP table version is 1, main routing table version 1 Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State 2.3.4.5 4 21 0 0 0 0 0 never Idle 3.4.5.6 4 37 0 0 0 0 0 never Idle Network Architecture and Design

  26. BGP Neighbors – Steady State All neighbors shall be up (no state info) Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State 2.3.4.5 4 21 17 22 10 0 0 0:01:47 3.4.5.6 4 37 11 17 10 0 0 0:07:07 Network Architecture and Design

  27. External BGP - EBGP • BGP neighbors in different AS Router B: router bgp 110 network 150.10.0.0 neighbor 131.108.10.1 remote-as 109 Router A: router bgp 109 network 131.108.0.0 neighbor 131.108.10.2 remote-as 110 AS 109 131.108.0.0 A .1 131.108.10.0 .2 B 150.10.0.0 AS 110 Network Architecture and Design

  28. External BGP - EBGP Network Architecture and Design

  29. Internal BGP - IBGP AS 109 • BGP Neighbors in same AS Router B: router bgp 109 network 131.108.0.0 neighbor 131.108.30.1 remote-as 109 neighbor 131.108.30.1 update-source lo0 Router A: router bgp 109 neighbor 131.108.10.2 remote-as 109 neighbor 131.108.30.2 update-source lo0 • Update source is loopback0 to make sessions insensitive to topology changes within the AS A 131.108.30.1 B 131.108.30.2 Network Architecture and Design

  30. Internal BGP - IBGP Network Architecture and Design

  31. BGP Attributes – AS Path • AS-Path • Contains the list of AS’s traversed by the update • Sending router updates AS with its own AS • Used for loop detection: • if a router receives an update containing its own AS, then it discards it Network Architecture and Design

  32. BGP Attributes – AS Path 1983 192.2.0.0/24 • AS-Path • Ordered list of AS’s traversed by route update • AS-Set • Route update traversed one or more members of a set 1981 192.2.1.0/24 1980 192.2.2.0/24 1982 192.2.3.0/24 192.2.0.0/24, 1980 1983 192.2.1.0/24, 1980 1981 192.2.2.0/24, 1980 192.2.3.0/24, 1980 1982 192.2.0.0/22 {1980, 1981, 1982, 1983} Network Architecture and Design

  33. BGP Attributes – AS Path • AS-Path – Loop detection 1. Router A sends update for 146.124.54.0/24 with AS_PATH: 1560 B 670 2. Router B sends update for 146.124.54.0/24 with AS_PATH: 670 1560 (pre-pends its AS path) 1560 A 146.124.54.0/24 4. Router A detects its own AS in the update’s AS_PATH and discards it 210 3. Router C sends update for 146.124.54.0/24 with AS_PATH: 210 670 1560 (pre-pends its AS path) C Network Architecture and Design

  34. BGP Attributes – Local Preference • Local Preference • Is sent only to routers in the same AS • Path with highest local-preference is preferred • Default is 100 65500 65002 65001 A Local Pref 120 Need to go to 65500 65003 65000 B Network Architecture and Design

  35. End of Fifth Lecture Network Architecture and Design

More Related