IXP, BGP and the Internet

Question 1

Explain the purpose of mtr utility and what it typically shows.

Answer 1

mtr (my traceroute) is a computer program which combines the functions of the traceroute and ping programs in one network diagnostic tool. It provides real-time analysis of the path packets take to reach a destination, showing detailed information about latency and packet loss at each hop.

When an IP datagram is sent, its TTL is set (no. hops/routers it can pass through before the packet is discarded). As the packet passes through a router the TTL is decremeted until, when the TTL reaches zero, the packet is destroyed and an ICMP “time exceeded” message is returned to the sender.

traceroute + ping, TTL

Question 2

What does settlement free mean?

Answer 2

benefit to both parties, no charge/cost between the two.

Question 3

Explain what a PNI is.

Answer 3

Private Network Interconnect

a direct interconnect and peering between only two networks, across a layer 1 or 2 medium that offers dedicated capacity that is not shared by any other parties.

Question 4

What does it mean when two networks peer?

Answer 4

peering is a voluntary interconnection of administratively separate Internet networks for the purpose of exchange traffic between the users of each network.

Question 5

Explain what an IP transit is.

Answer 5

An Internet Transit is the service allowing network traffic to cross or “transit” a computer network, usually used to connect a smaller Internet Service Provider (ISP) to the larger Internet.

It facilitates the advertisement of customer routes to other ISPs, attracting inbound traffic towards the customer. At the same time, it advertises routes from other ISPs to the customer, soliciting outbound traffic from the customer to these networks.

Question 6

What is the Default Free Zone?

Answer 6

the collection of all internet AS that do not require a default route to route a packet to any destination. // It is the “full” routing table, no default routes.

Question 7

What is an Autonomous System?

Answer 7

a collection of Internet Protocol (IP) routing prefixes (address blocks) under the control of an Internet Service Provider (ISP). // It is a network managed by a single entity, uniquely identified by an ASN (AS number)

Question 8

What is the Internet?

Answer 8

A network of interconnected networks. Each IP block/prefix is routed by a single network (AS). A network may have many IP prefixes. Each network is described by an ASN.

Question 9

Explain what BGP is

Answer 9

a routing protocol that allows one network (AS) to signal to other networks the IP prefies that can be reached through it.

Question 10

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

Answer 10

1. Transit: you are the customer to an upstream ISP (tier 1, wholsale ISP e.g. hurrican electric)
2. Peerings: normally settlement free; IXPs, PNIs
3. Customers: you are the ISP providing downstream connectivity.

Question 11

What are the key parameters that are required for a BGP peering session?

Answer 11

• A layer 2 connectivity between routers (physical link, IXPs etc)
• a layer 3 subnet for IP communication
• prefixes (routes to advertise over BGP
• ASN
• security (inbound/outbound prefix filters, AS path filters, next hop verification etc).

Question 12

In terms of a BGP what does “shortest path” mean?

Answer 12

the least amount of AS you have to pass through

Question 13

What are some of the preferences when choosing the shortest path in BGP? mention 2 tie-breakers.

Answer 13

• prefer the path with the shortest AS_PATH
• prefer the path with the highest LOCAL_PREF (def:100)
• prefer the path with the lowest MED (multi exit discriminator)
• prefer the oldest path.

tie-breakers:
- prefer the path from the router with the lower router-id
- prefer the path that comes from the lowest neighbor address

Question 14

Why was INEX founded?

Answer 14

the internet connectivity in ireland was extremely expensive, the speed of access was low and the local traffic was being routed to London/Amsterdam which made it inefficient

Question 15

Explain what mandates are included in INEX.

Answer 15

• provide high-speed reliable and resilient IP traffic exchange facilities
• keep Irish traffic in Ireland
• no ‘feature creep’ into other business areas
• allow our national and international members route traffic more efficiently

Question 16

What are IXPs?

Answer 16

Internet Exchange Point
a network facility that enables the interconnection and exchange of internet traffic between more than 2 autonomous sytems (only AS). It doesn’t require the internet network passing between any two AS to pass through any third AS, nor does it alter or otherwise interfere with such traffic.

Question 17

What is Irelands IXP?

Answer 17

Internet Neutral Exchange Association (INEX)

Question 18

What are some advantages of IXPs?

Answer 18

• IXPs operate with a smaller and more manageable number of prefixes and routes compared to the broader internet, ensuring streamlined and efficient data transmission paths.
• the proximity of networks at the IXP significantly reduces the time it takes for data to travel between them, resulting in faster and more responsive internet services.
• traffic remains local as it is not being routed through distant servers, minimising unnecessary data travel

Question 19

What are some disadvantages of IXPs?

Answer 19

-IXPs require specialised physical infrastructure (such as data centers), which can be costly to set up and maintain. Smaller networks might face barriers to entry if they cannot afford to participate.
- security risks: a successful attack on an IXP, such as a Dedicated Denail of Service (DDoS) attack can potentially disrupt multiple networks that rely on it for connectivity
- as traffic volumes increase, an IXP can become a bottleneck if the infrastructure foesn’t scale appropriately. Heavy traffic can cause congestion especially if the exchange point has limited bandwidth or insufficient capacity.

Question 20

What happens when an AS joins INEX?

Answer 20

INEX Operations assigns you an IP address (one for each IXP in Dublin and one for Cork). They do a quarantine procedure. Your first BGP peering session is with the route collector (purely for diagnostic and monitoring purposes but mandatory).

Question 21

Explain what a Route Server is, what function it performs and why it is necessary.

Answer 21

A Route Server is a specialized BGP often used at IXPs. It facilitates the exchange of routing information between multiple networks (AS) without requiring each network to establish a direct BGP sessions with every other network at the IXP.
It does NOT:
-Change advertised routes in any way
-Put its own ASN in the AS path
The next-hop address is the other member’s router -route servers do not route traffic
It must be reliable, secure and trust-worthy.

Question 22

What is a BGP Anycast?

Answer 22

a network addressing and routing methodology in which a single destination address has multiple routing paths to two or more end point destinations. It is reliable and latent because if one network or service instance goes offline, your router’s BGP best-path algorithm will pick up the next best (closest) anycast advertisement.

Question 23

What are ISLs?

Answer 23

An InterSite Links is a connection established between two network switches to enable them to communicate and share data.
Multi-mode fibre (10GB @ 300m)
Single-mode fibre (10 GB @ 120km)

Question 24

Describe what D(WDM) is.

Answer 24

Wavelength division multiplexing is when you use lasers to send light through fibre systems. For example, when light bends when sent through a prism or a diffraction grating. different wavelengths bend by a different amount. We can run 1/10/100 G links on different colours (wavelengths).

Question 25

Explain how traceroute works and what it shows.

Answer 25

Traceroute is a network diagnostic tool that maps the path packets take from source to destination across an IP network. It provides details about each step along the way, including the time it takes (Round-trip Time /RTT) to reach each network device (hop),the IP address of each router, and whether packets are lost.

It uses the TTL Mechanism (Time To Live) which is a counter that determines how many hops a packet can traverse before being discarded. Traceroute sends packets with incrementally increasing TTL values, starting with 1. If the TTL reaches 0, the router discards the packet and sends back an ICMP “time exceeded” message to the source. It then increments the TTL to 2 and tries again.

Question 26

When running an /ip route print command on a router in GNS3, what does the route entry for destination 0.0.0.0/0 mean and why is this route entry particularly important?

Answer 26

it is the default route, used to direct packets to a gateway when no specific route for the destination exists in the routing table. It is particularly important because it ensures connectivity to external networks, including the internet, but acting as a “catch-all” route.

Question 27

When running an /ip route print command on a router in GNS3, how can you tell if an IP range is in the same AS or in a different AS?

Answer 27

in the flags section, the “b” tells you that the route is learned via BGP, which is used to exchange routing information between different AS.

Question 28

When running an /ip route print command on a router in GNS3, what is the meaning of the GATEWAY value shown and how might this affect a routing decision?

Answer 28

GATEWAY: represents the next-hop IP address/interface that a packet should use to reach a specific destination network. It affects the routing decision as it determines the exact next device the router will use to send packets along the path to their destination.