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The Border Gateway Protocol (BGP) Sharad Jaiswal.

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1 The Border Gateway Protocol (BGP) Sharad Jaiswal

2 Prologue Internet is divided into Autonomous Systems (ASs) AS - a collection of one or more networks under a single technical administration technical administration- refers to aspects of the n/w like, routing policies etc.

3 Contd. Intra-AS routing driven mostly by performance considerations Inter-AS routing depends on policy issues, economics etc

4 BGP (basic info) BGP is the de facto Inter-AS routing protocol Used for exchanging route information between ASs Conveys information about AS path topology


6 BGP (basic info contd.) Current version, BGP 4, defined in RFC 1771 Runs over TCP (port 179) Path Vector protocol –Exchange entire path information –Prevents loops Incremental

7 How does BGP work? How is it configured? How is route information exchanged using BGP? What are the attributes of the exchanged routes? How is this information processed? And further distributed?

8 BGP Configuration Identify BGP “speakers”, assign AS number and define neighbors (peers) External BGP peers are directly connected Not necessary for Internal BGP peers

9 BGP Message Exchange BGP peers form a TCP connection, use the OPEN message to establish BGP connection; Connections kept open by KEEPALIVE messages; Initially exchange routing table, further modifications are incremental;

10 BGP Message Exchange (II) Modifications (Route additions and withdrawls) are made by UPDATE messages; Errors are reported by NOTIFICATION messages; Most interesting stuff happens in the processing of UPDATE messages.

11 The UPDATE message Used to transfer routing information between peers Advertise new routes, withdraw unfeasible paths Contains Path Attributes info

12 Routing Information Storage Adj-RIBs-In –Store messages that have been learned from inbound UPDATE messages Loc-RIB –Contains local routing information of a BGP speaker Adj-RIBs-Out –Store routing information local router has chosen for distribution

13 Path Attributes Origin –defines the origin of a path AS_PATH NEXT_HOP MULTI_EXIT_DISC LOCAL_PREF Community

14 Composed of a sequence of AS path segments or the set of ASs traversed

15 IP address of the Border router that should be used for the next stop


17 Used to inform other BGP speakers in the same AS about preferences for a particular route

18 Used to discriminate among multiple exit points in neighboring ASs

19 Community Attribute Optional Attribute Method to group destination in a certain community Range 0, 4294,967,200 Apply routing destinations (accept,prefer,redistribute) using these communities

20 Decision Process Phase I –Calculating the degree of preference for each route Phase II –Choosing the best route and installing it into Loc-RIB Phase III –dissemination,route aggregation and information reduction

21 Applying Policy in the Decision Process Policy based on AS path, community information Accept / Reject routes Set Attributes to influence path selection

22 Phase I For each received update, local speaker shall calculate the degree of preference Degree of preference calculated based on the LOCAL_PREF attribute if received from within AS Calculated from pre-configured policy information if received from neighboring AS

23 Phase 2 Route selection –highest degree of preference of any route to to the same set of destinations –is only route to the destination –use tie breaking techniques Install route in Loc-RIB

24 Phase 3 Route Dissemination to peers in neighboring ASs –when routes in a Loc-RIB to local destinations have changed All routes processed in Adj-RIBs-OUT Aggregation of routes may occur here

25 Overlapping Routes BGP speaker may transmit routes with overlapping NLRI Information Overlap occurs when a set of destinations are identified in non- matching routes Destinations are always identified by IP prefixes More specific prefix route gets precedence.

26 Internal BGP (iBGP) Same protocol as BGP; Used when AS_PATH is supposed to be intact between different eBGP peers; iBGP nodes are fully meshed; No re-advertisement of route updates to prevent looping;


28 iBGP Scaling BGP Confederations –Divide AS into Sub-ASs to reduce size of mesh –Still present a unified front to the outside world Route Reflectors –Relax no re-advertisement rule –Single iBGP peer reflects routes to sub-ordinate peers within a cluster –No peering between clusters

29 Other Issues! Achieving Stability –Using loopbacks –BGP/IGP interaction –Peer Groups –Route Flap dampening BGP and CIDR

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