PART III. Implementing Inter-Network Relationships with BGP

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1 PART III Implementing Inter-Network Relationships with BGP ICNP 2002

2 Routing Protocols Autonomous System

3 BGP-4 BGP = Border Gateway Protocol Is a Policy-Based routing protocol Is the de facto EGP of today s global Internet Relatively simple protocol, but configuration is complex and the entire world can see, and be impacted by, your mistakes : BGP-1 [RFC 1105] Replacement for EGP (1984, RFC 904) 1990 : BGP-2 [RFC 1163] 1991 : BGP-3 [RFC 1267] 1995 : BGP-4 [RFC 1771] Support for Classless Interdomain Routing (CIDR) 3

4 Routing problems YouTube outage underscores big Internet problem BGP (Border Gateway Protocol) routing

5 BGP Operations (Simplified) Establish session on TCP port 179 AS1 Exchange all active routes BGP session AS2 Exchange incremental updates While connection is ALIVE exchange route UPDATE messages 5

6 Two Types of BGP Neighbor Relationships External Neighbor (ebgp) in a different Autonomous Systems AS1 Internal Neighbor (ibgp) in the same Autonomous System ebgp ibgp is routed (using IGP!) ibgp AS2

7 Internet inter-as routing: BGP BGP (Border Gateway Protocol): the de facto interdomain routing protocol glue that holds the Internet together BGP provides each AS a means to: ebgp: obtain subnet reachability information from neighboring ASs. ibgp: propagate reachability information to all AS-internal routers. determine good routes to other networks based on reachability information and policy Allows subnet to advertise its existence to rest of Internet: I am here Network Layer 4-7

8 BGP basics BGP session: two BGP routers ( peers ) exchange BGP messages Advertising paths to different destination network prefixes ( path vector protocol) Exchanged over semi-permanent TCP connections When AS3 advertises a prefix to AS1: AS3 promises it will forward datagrams towards that prefix AS3 can aggregate prefixes in its advertisement other networks 3b 3c AS3 3a 1a AS1 BGP message 1c 1d 1b 2a 2c AS2 2b other networks Network Layer 4-8

9 BGP Session Border router Internal router BGP Managed Session is used to 1) Distribute externally learned routes internally 2) Distribute routes to external destinations

10 BGP messages BGP messages exchanged between peers over TCP connection BGP messages: OPEN: opens TCP connection to peer and authenticates sender UPDATE: advertises new path (or withdraws old) KEEPALIVE: keeps connection alive in absence of UPDATES; also ACKs OPEN request NOTIFICATION: reports errors in previous msg; also used to close connection announcement = prefix + attributes values

11 BGP FSM Event/Action List 1 Request for Connection 2 Connection Succeeded 3 Connection Failed 4 "Open" Received 5 "KeepAlive" Received 6 "Update" Received 7 "Notification" Received 8 "Open" Sent 9 "KeepAlive" Sent 10 "Update" Sent 11 "Notification" Sent 12 Close connection 13 Any Error, including: TCP operation error, Packets format error, Packets type error. 14 KeepAlive Timer Expire 15 KeepAlive Timer Reset 16 HoldOn Timer Reset 17 HoldOn Timer Expire / 3/12 Idle Any State Other than Idle/Active/ Connect 13 / 12 13/12 7 / 12 Active Connect 13 / 11,12 4/8, 9 Idle -/8 Open Confirmed 17/11,12 4/9 5/16 -/10 Open Sent 5/- (15,16) Established 14 / 15,9 Established 1/- 2/- 6/-

12 BGP basics: distributing path information Using ebgp session between 3a and 1c, AS3 sends prefix reachability info to AS1 1c can then use ibgp do distribute new prefix info to all routers in AS1 1b can then re-advertise new reachability info to AS2 over 1b-to-2a ebgp session When router learns of new prefix, it creates entry for prefix in its forwarding table other networks 3b AS3 3a 1a AS1 1c 1d ebgp session ibgp session 1b 2a 2c AS2 2b other networks Network Layer 4-12

13 BGP routing policy W A B C X legend: provider network customer network: Y A,B,C are provider networks X,W,Y are customer (of provider networks) X is dual-homed: attached to two networks X does not want to route from B via X to C.. so X will not advertise to B a route to C 4-14

14 BGP routing policy (2) W A B C X legend: provider network customer network: Y A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C? No way! B gets no revenue for routing CBAW since neither W nor C are B s customers B wants to force C to route to w via A B wants to route only to/from its customers!

15 Path attributes & BGP routes Advertised prefix includes BGP attributes prefix + attributes = route Two important attributes AS-PATH: contains ASs through which prefix advertisement has passed: e.g., AS 67, AS 17 NEXT-HOP: indicates specific internal-as router to next-hop AS (may be multiple links from current AS to next-hop-as) Gateway router receiving route advertisement uses import policy to accept/decline e.g., never route through AS x policy-based routing Network Layer 4-16

16 Attributes are Used to Select Best Routes /24 pick me! /24 pick me! /24 pick me! /24 pick me! Given multiple routes to the same prefix, a BGP speaker must pick at most one best route (Note: it could reject them all!)

17 BGP Route Processing Open ended programming. Constrained only by vendor configuration language Receive BGP Updates Apply Policy = filter routes & tweak attributes Based on Attribute Values Best Routes Apply Policy = filter routes & tweak attributes Transmit BGP Updates Apply Import Policies Best Route Selection Best Route Table Apply Export Policies Install forwarding Entries for best Routes. IP Forwarding Table

18 BGP Design Distance vector protocol Implements policies to enable politics and traffic engineering Decision process (in each router): updates updates in-policy filter in-policy filter best path selection out-policy filter (modified) updates 19

19 Route Selection Summary Highest Local Preference Enforce relationships Shortest ASPATH Lowest MED i-bgp < e-bgp Lowest IGP cost Traffic engineering Lowest router ID Throw up hands and break ties

20 BGP Next Hop Attribute AS 6431 AT&T Research AS 7018 AT&T AS RIPE NCC RIS project /16 Next Hop = /16 Next Hop = Every time a route announcement crosses an AS boundary, the Next Hop attribute is changed to the IP address of the border router that announced the route. 21

21 Join EGP with IGP For Connectivity /16 Next Hop = / AS Forwarding Table /30 destination next hop / Forwarding Table destination next hop EGP destination next hop / / AS 2

22 LOCAL PREFERENCE Local preference used ONLY in ibgp AS 4 local pref = 80 local pref = 90 AS 3 local pref = 100 Higher Local preference values are more preferred AS 2 AS /16 25

23 ASPATH Attribute /16 AS Path = AS 1129 Global Access /16 AS Path = AS 1755 Ebone /16 AS Path = Sprint AS /16 AS Path = AS RIPE NCC RIS project /16 AS Path = 6341 AS 6341 AT&T Research /16 Prefix Originated AS7018 AT&T /16 AS Path = /16 AS Path = AS 3549 Global Crossing 28

24 Shorter Doesn t Always Mean Shorter Mr. BGP says that path 4 1 is better than path Duh! AS 4 AS 3 AS 2 AS 1

25 Interdomain Loop Prevention AS 7018 BGP at AS YYY will never accept a route with ASPATH containing YYY Don t Accept! /16 ASPATH = AS 1 30

26 Traffic Often Follows ASPATH /16 ASPATH = AS /16 AS 2 AS 3 AS 4 IP Packet Dest =

27 But It Might Not AS /16 ASPATH = /25 ASPATH = 5 AS 2 AS 2 filters all subnets with masks longer than /24 AS /16 ASPATH = AS /16 IP Packet Dest = AS /25 From AS 4, it may look like this packet will take path 3 2 1, but it actually takes path 3 2 5

28 AS Graphs Depend on Point of View 1 3 peer provider peer customer

29 Implementing Backup Links with Local Preference (Outbound Traffic) AS 1 primary link backup link Set Local Pref = 100 for all routes from AS 1 AS Set Local Pref = 50 for all routes from AS 1 Forces outbound traffic to take primary link, unless link is down We ll talk about inbound traffic soon 34

30 Why different Intra- and Inter-AS routing? Policy Inter-AS: admin wants control over how its traffic routed, who routes through its net. Intra-AS: single admin, so no policy decisions needed Performance Intra-AS: can focus on performance Inter-AS: policy may dominate over performance 4-35

31 Import Routes provider route peer route customer route ISP route From provider From provider From peer From peer From customer From customer

32 Export Routes provider route peer route customer route ISP route To provider From provider To peer To peer To customer To customer filters block

33 Basic BGP commands Basic Commands router bgp <as-number> neighbor <ip address> remote-as <as-number> Examples router bgp 400 neighbor remote-as 300 neighbor remote-as 3 network mask show commands show ip bgp show ip bgp neighbors

34 Protocoles et Interconnexions Course Overview and Introduction Dario Vieira Department of Computer Science EFREI

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