Lixin Gao ECE Dept. UMASS, Amherst Introduction to BGP Lixin Gao ECE Dept. UMASS, Amherst 9/20/2018 ECE697F: Special Topics-Internet Routing

Border Gateway Protocol (BGP) Dynamic Inter-AS Routing Protocol Goals: Exchange reachability information between ASes Apply local policies for selecting route receiving and announcing route ECE697F: Special Topics-Internet Routing

ECE697F: Special Topics-Internet Routing BGP Sessions Used for Exchange Reachability Information iBGP : fully meshed, might not directly connected, Alternative to fully meshed: confederation, route reflector eBGP eBGP iBGP AS701 MCI AS7018 AS1 AT&T BBN ECE697F: Special Topics-Internet Routing

Route Information Exchange Route Announcement Local Policies 1.2.0.0/16 (1) eBGP 1.2.0.0/16 (7018,1) iBGP AS701 1.2.0.0/16 (1) 1.2.0.0/16 (1) 1.2.0.0/16 (7018,1) 1.2.0.0/16 (7018,1) MCI 1.2.0.0/16 ( ) AS7018 AS1 1.2.0.0/16 (1) 1.2.0.0/16 (1) 1.2.0.0/16 AT&T BBN ECE697F: Special Topics-Internet Routing

ECE697F: Special Topics-Internet Routing Route Announcement Network Prefix: 12.0.0.0/8 Attributes: for reaching network prefix AS path Next hop : next hop IP addr Local Preference Multiple Exit Discriminator (MED) Community Set Origin code Atomic Aggregate Aggregator ….. ECE697F: Special Topics-Internet Routing

ECE697F: Special Topics-Internet Routing AS Path Sequence of ASes traversed Loop avoidance Path selection: Path Length Policy: AS7018 in AS path is more preferred ECE697F: Special Topics-Internet Routing

ECE697F: Special Topics-Internet Routing Next Hop Next hop IP address to reach network prefix EBGP: next hop IP addr of announcer iBGP From same AS:IP address of announcer From diff AS: next hop unchanged AS7018 AS1 2.3.4.1 3.4.5.2 1.2.0.0/16 (1) 2.3.4.1 1.2.0.0/16 (1) 2.3.4.1 1.2.0.0/16 1.2.0.0/16 ( ) 3.4.5.2 ECE697F: Special Topics-Internet Routing

ECE697F: Special Topics-Internet Routing Next hop Determine next hop in forwarding table Combing Inter-AS and Intra-AS routing table Recursive route lookup BGP abstract topology in AS level ECE697F: Special Topics-Internet Routing

ECE697F: Special Topics-Internet Routing Local Preference Local to AS carry in iBGP session but not eBGP session Used to influence path selection Select the highest Local Pref path eBGP iBGP AS701 1.2.0.0/16 (1) 100 1.2.0.0/16 (1) 100 1.2.0.0/16 (7018,1) 200 1.2.0.0/16 (7018,1) 200 MCI 1.2.0.0/16 ( ) 300 AS7018 AS1 1.2.0.0/16 (1) 90 1.2.0.0/16 (1) 90 1.2.0.0/16 AT&T BBN ECE697F: Special Topics-Internet Routing

Multiple Exit Discriminator (MED) Used for path selection select an entry point among several to an AS (smaller MED is preferred) meaningful among paths from the same AS may use Intra-AS routing cost metrics Non-transitive AS701 1.2.0.0/16 (1) 200 1.2.0.0/16 (1) 100 AS1 1.2.0.0/16 ECE697F: Special Topics-Internet Routing

ECE697F: Special Topics-Internet Routing Community Set Used for grouping destination Simplify policy setting AS701 1.2.0.0/16 (1) 1000 1.2.0.0/16 (1) 1000 3.0.0.0/18 (7018) 1000 3.0.0.0/16 (7018) 1000 MCI 3.0.0.0/8 AS7018 AS1 1.2.0.0/16 AT&T BBN ECE697F: Special Topics-Internet Routing

ECE697F: Special Topics-Internet Routing Origin Code IGP interior to the originating AS. EGP learned via EGP (Exterior Gateway Protocol) Incomplete unknown ECE697F: Special Topics-Internet Routing

ECE697F: Special Topics-Internet Routing Atomic Aggregate Aggregation that loses information ECE697F: Special Topics-Internet Routing

ECE697F: Special Topics-Internet Routing Aggregator Optional For route that is an aggregation ECE697F: Special Topics-Internet Routing

Announcement Processing Input Policy Engine Receive from neigboring Ases new announcements Attribute Manipulation and Filter announcement Store announcements (Incremental protocol) Path Selection among announcements Output Policy Engine Attribute Manipulation Send to neighboring Ases new announcements ECE697F: Special Topics-Internet Routing

ECE697F: Special Topics-Internet Routing Input Policy Engine Set local preference Filter routes with the same AS number (loop avoidance) Filter bogus routes ECE697F: Special Topics-Internet Routing

Path Selection Process For each prefix, Prefer largest Local Preference Prefer route this router originates Prefer shorter AS path prefer the lowest origin code (IGP<EGP<INCOMPLETE). Prefer path with lowest MED Prefer route with lowest IP addr ECE697F: Special Topics-Internet Routing

ECE697F: Special Topics-Internet Routing Output Policy Engine Send to neighbors only the best path Send to neighbors only the best path changes Set MED Set Community set Set Atomic Aggregate Set next hop Append AS number ECE697F: Special Topics-Internet Routing

Routing Information Storage Adj-RIBs-In routes received Input to route selection Loc-RIB Routes selected by selection process Output of route selection Adj-RIBs-Out announcements to neighbor Output of Output Policy Engine ECE697F: Special Topics-Internet Routing

BGP Session Implementation OPEN message establish TCP session KEEPALIVE message keep session going Update message announcement or withdrawal Notification message error in announcement ECE697F: Special Topics-Internet Routing