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

2 Network Architecture and Design Autonomous System (AS) AS = collection of networks under a single technical administration & sharing the same routing policy AS# can be 1 to (64512 – 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

3 Network Architecture and Design 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

4 Network Architecture and Design 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) = routes, 7000 ASs, 40MB approx.

5 Network Architecture and Design 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

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

7 Network Architecture and Design 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

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

9 Network Architecture and Design 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

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

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

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

13 Network Architecture and Design 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

14 Network Architecture and Design 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

15 Network Architecture and Design 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)

16 Network Architecture and Design 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

17 Network Architecture and Design 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

18 Network Architecture and Design 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

19 Network Architecture and Design AS-Path Attribute

20 Network Architecture and Design 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

21 Network Architecture and Design 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

22 Network Architecture and Design 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

23 Network Architecture and Design 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

24 Network Architecture and Design Example Network

25 Network Architecture and Design 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 never Idle never Idle

26 Network Architecture and Design BGP Neighbors – Steady State All neighbors shall be up (no state info) Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State :01: :07:07

27 Network Architecture and Design External BGP - EBGP BGP neighbors in different AS Router B: router bgp 110 network neighbor remote-as 109 Router A: router bgp 109 network neighbor remote-as 110 AS 109 AS A B

28 Network Architecture and Design External BGP - EBGP

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

30 Network Architecture and Design Internal BGP - IBGP

31 Network Architecture and Design 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

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

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

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

35 Network Architecture and Design End of Fifth Lecture