IXPs

Question 1

What is a Route Collector?

Answer 1

Usually a router or Unix box running BGP

Gathers routing information from service provider routers at an IXP

Does not forward packets

Question 2

Purpose of a Route Collector

Answer 2

To provide a public view of the Routing Information available at the IXP

• Useful for existing members to check functionality of BGP filters
• Useful for prospective members to check value of joining the IXP

Question 3

What is a Route Server?

Answer 3

All the features of a Route Collector
But also:

Announces routes to participating IXP members according to their routing policy definitions

Implemented using the same specification as for a Route Collector

Question 4

Why is a Route Server necessary?

Answer 4

Helps scale routing

Simplifies Routing Processes on ISP Routers

Insertion of RS Autonomous System Number in the Routing Path

Uses Policy registered in IRR (optional)

Question 5

Briefly describe BGP (Border Gateway Protocol)

Answer 5

BGP Operation
BGP is the de facto Inter-AS routing protocol

Used for exchanging route information between ASs

Conveys information about AS path topology

Question 6

How is BGP path chosen?

Answer 6

1. Local preference value attribute: A policy-based decision is made regardless of the shortest path. If an AS is configured to not favour an AS it will attempt to find other paths at all costs.
2. The Shortest AS-PATH is chosen: If no relevant policies override decision.
3. Closest NEXT-HOP router: If there is no shortest AS-PATH, then instead the closest NEXT-HOP router is chosen. The closest NEXT-HOP router will be the closest router of the ‘tie-breaker’ within the same AS. (This is known as hot potato routing :) )
4. Else, any other criteria that are specific to the router or AS’s BGP implementation.

Question 7

State and differentiate the three main means of interconnecting an Autonomous System with another Autonomous System.

Answer 7

IXP - Peer with fellow members and share Routing information

IP Transit Provider - AS pays Transit Provider to forward traffic for them

Private Network Interconnect - Peering between two Autonmous systems directly

Question 8

Describe Assymetric Routing briefly

Answer 8

Traffic does not follow the same bi-directional path between two networks
• Not necessarily a bad thing - multiple transit providers plus IX connections
• Sometimes by design
• Sometimes a mis-configuration - hard to detect

Question 9

How does trace route work?

Answer 9

Use TTL to discover path
• Routers decrement the TTL value by 1 on each hop
• When it reaches 0, that router sends an ICMP Time Exceeded (with its own IP)
• Traceroute sends packets with an incrementing TTL until it reaches the target