W8 L1 - Interdomain Routing

Intra vs Inter Domain

Intra-Domain (Routing among routers within same AS’s):

• All routers in AS (Autonomous Systems aka Domains) run same intra-domain protocol

• Routers in different AS can run different intra-domain protocols

• Gateway Router - At edge of its AS and has link to router in other AS

Inter-Domain (Routing among seperate AS’s):

• Gateways perform inter-domain routing

Inter-AS Routing: BGP

BGP - Border Gateway Protocol

• Allows subnet to advertise its existence, and the destinations it can reach, to the internet

BGP allows each AS to:

• Obtain destination network reachability info from neighbouring AS’s

• Determine routes to other networks based on reachability information

• Propogate reachability information to all AS-internal routers

• Advertise destination reachability info

BGP Advertised Route: Prefix + Attributes

Prefix: Destination being advertised

Attributes:

• AS-PATH: list of AS’s through which prefix advertisement has passed

• NEXT-HOP: indicates specific internal-AS router to next-hop AS

Tiers of Inter-AS

Tier 3 AS’s - Minor ISPs or Customer networks

Tier 2 AS’s - National or Regional ISPs

Tier 1 AS’s - International (global) ISPs

Why different Intra vs Inter-AS routing?

Policy:

• Inter-AS - Admin wants control over how its traffic routed, who routes routes through its network

• Intra-AS - Single admin, so policy less of an issue

Scale:

• Hierarchical routing saves table size, reduced update traffic

Performance:

• Intra-AS - Can focus on performance

• Inter-AS - Policy dominates over performance