1. Border Gateway Protocol (BGP4) Rizwan Rehman, CCS, DU

2. Border Gateway Protocol (BGP) Routing/Forwarding basics Building blocks Exercises BGP protocol basics Exercises BGP path attributes Best path computation Exercises

3. Autonomous System (AS) Collection of networks with same policy Single routing protocol Usually under single administrative control IGP to provide internal connectivity AS 100

4. Autonomous System(AS)... Identified by ‘AS number’ Public & Private AS numbers Examples: – Service provider – Multi-homed customers – Anyone needing policy discrimination

5. Routing flow and packet flow For networks in AS1 and AS2 to communicate: AS1 must announce routes to AS2 AS2 must accept routes from AS1 AS2 must announce routes to AS1 AS1 must accept routes from AS2 accept announce accept AS 1 AS2 packet flow Routing flow egress ingress

6. Egress Traffic Packets exiting the network Based on – Route availability (what others send you) – Route acceptance (what you accept from others) – Policy and tuning (what you do with routes from others) – Peering and transit agreements

7. Ingress Traffic Packets entering your network Ingress traffic depends on: – What information you send and to who – Based on your addressing and ASes – Based on others’ policy (what they accept from you and what they do with it)

8. Types of Routes Static Routes – configured manually Connected Routes – created automatically when an interface is ‘up’ Interior Routes – Routes within an AS Exterior Routes – Routes exterior to AS

9. What Is an IGP? Interior Gateway Protocol Within an Autonomous System Carries information about internal prefixes Examples—OSPF, ISIS, EIGRP…

10. What Is an EGP? Exterior Gateway Protocol Used to convey routing information between ASes De-coupled from the IGP Current EGP is BGP4

11. Why Do We Need an EGP? Scaling to large network – Hierarchy – Limit scope of failure Define administrative boundary Policy – Control reachability to prefixes

12. Interior – Automatic discovery – Generally trust your IGP routers – Routes go to all IGP routers Exterior Specifically configured peers Connecting with outside networks Set administrative boundaries Interior vs. Exterior Routing Protocols