CS 268 Computer Networking L 3 BGP Outline BGP ASes Policies BGP Attributes BGP Path Selection iBGP 2 Autonomous Systems ASes Autonomous Routing Domain Glued together by a common administration policies etc Autonomous system is a specific case of an ARD ARD is a concept vs AS is an actual entity that participates in routing Has an unique 16 bit ASN assigned to it and typically participates in inter domain routing Examples MIT 3 CMU 9 AT T 7018 6341 5074 UUNET 701 702 284 12199 Sprint 1239 1240 6211 6242 How do ASes interconnect to provide global connectivity How does routing information get exchanged 3 Nontransit vs Transit ASes ISP 2 ISP 1 Traffic NEVER flows from ISP 1 through NET A to ISP 2 At least not intentionally NET A Nontransit AS might be a corporate or campus network Could be a content provider IP traffic 4 Customers and Providers provider provider customer IP traffic customer Customer pays provider for access to the Internet 5 The Peering Relationship A peer provider B peer customer C Peers provide transit between their respective customers Peers do not provide transit between peers traffic allowed traffic NOT allowed Peers often do not exchange 6 Peering Wars Peer Reduces upstream transit costs Can increase end to end performance May be the only way to connect your customers to some part of the Internet Tier 1 Don t Peer You would rather have customers Peers are usually your competition Peering relationships may require periodic renegotiation Peering struggles are by far the most contentious issues in the ISP world Peering agreements are often confidential 7 Routing in the Internet Link state or distance vector No universal metric policy decisions Problems with distance vector Bellman Ford algorithm may not converge Problems with link state Metric used by routers not the same loops LS database too large entire Internet May expose policies to other AS s 8 Solution Distance Vector with Path Each routing update carries the entire path Loops are detected as follows When AS gets route check if AS already in path If yes reject route If no add self and possibly advertise route further Advantage Metrics are local AS chooses path protocol ensures no loops 9 BGP 4 BGP Border Gateway Protocol Is a Policy Based routing protocol Is the 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 1989 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 10 BGP Operations Simplified Establish session on TCP port 179 AS1 BGP session Exchange all active routes AS2 Exchange incremental updates While connection is ALIVE exchange route UPDATE messages 11 Interconnecting BGP Peers BGP uses TCP to connect peers Advantages Simplifies BGP No need for periodic refresh routes are valid until withdrawn or the connection is lost Incremental updates Disadvantages Congestion control on a routing protocol Inherits TCP vulnerabilities Poor interaction during high load 12 Four Types of BGP Messages Open Establish a peering session Keep Alive Handshake at regular intervals Notification Shuts down a peering session Update Announcing new routes or withdrawing previously announced routes announcement prefix attributes values 13 Policy with BGP BGP provides capability for enforcing various policies Policies are not part of BGP they are provided to BGP as configuration information BGP enforces policies by choosing paths from multiple alternatives and controlling advertisement to other AS s Import policy What to do with routes learned from neighbors Selecting best path Export policy What routes to announce to neighbors Depends on relationship with neighbor 14 Examples of BGP Policies A multi homed AS refuses to act as transit Limit path advertisement A multi homed AS can become transit for some AS s Only advertise paths to some AS s Eg A Tier 2 provider multi homed to Tier 1 providers An AS can favor or disfavor certain AS s for traffic transit from itself 15 Export Policy An AS exports only best paths to its neighbors Guarantees that once the route is announced the AS is willing to transit traffic on that route To Customers Announce all routes learned from peers providers and customers and self origin routes To Providers Announce routes learned from customers and selforigin routes To Peers Announce routes learned from customers and selforigin routes 16 Import Routes provider route peer route From provider customer route ISP route From provider From peer From peer From customer From customer 17 Export Routes provider route peer route To provider customer route ISP route From provider To peer To peer To customer To customer filters block 18 BGP UPDATE Message List of withdrawn routes Network layer reachability information List of reachable prefixes Path attributes Origin Path Metrics All prefixes advertised in message have same path attributes 19 Path Selection Criteria Information based on path attributes Attributes external policy information Examples Hop count Policy considerations Preference for AS Presence or absence of certain AS Path origin Link dynamics 20 Important BGP Attributes Local Preference AS Path MED Next hop 21 MED Typically used when two ASes peer at multiple locations Hint to R1 to use R3 over R4 link Cannot compare AS40 s values to AS30 s 180 10 0 0 MED 50 R1 AS 10 R3 180 10 0 0 MED 120 R2 AS 40 180 10 0 0 MED 200 R4 AS 30 26 MED MED is typically used in provider subscriber scenarios It can lead to unfairness if used between ISP because it may force one ISP to carry more traffic SF ISP1 ISP2 NY ISP1 ignores MED from ISP2 ISP2 obeys MED from ISP1 ISP2 ends up carrying traffic most of the way 27 Route Selection Process Highest Local Preference Enforce relationships Shortest ASPATH Lowest MED i BGP e BGP Traffic engineering Lowest IGP cost to BGP egress Lowest router ID Throw up hands and break ties 28 Internal vs External BGP BGP can be used by R3 and R4 to learn routes How do R1 and R2 learn routes Option 1 Inject routes in IGP Only works for small routing tables Option 2 Use I BGP AS1 R1 R3 E BGP R4 AS2 R2 29 Internal BGP I BGP Same messages as E BGP Different rules about re advertising prefixes Prefix learned from E BGP can be advertised to I BGP neighbor and vice versa but Prefix learned from one I BGP neighbor cannot be advertised to another I BGP neighbor Reason no AS PATH within the same AS
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