Introduction to Internet Routing & Functions

Before moving into Internet Routing and Functions let me start off with a small overview of routing protocols. A routing protocol specifies how routers communicate with each other to distribute information. We don’t need to configure every route on each router in the network manually. (like in static route).


IGP and EGP

IGP stands for Interior Gateway Protocol which is used by gateways to exchange routing information within an AS (Autonomous System). An AS is also known as a routing domain. This could be like a Corporate network, Campus Network, ISP (Internet Service Provider) network.

EGP stands for Exterior Gateway Protocol, used by gateways to exchange routing information between ASs. In EGP, routing path selection is based on network policies and network administrator configured rule-sets. Currently the Internet uses BGP4 (Border Gateway Protocol version 4).


IGP

There are two main categories of IGP.

  • LS (Link-State) routing protocols

  • DV (Distance-Vector) routing protocols

To get an idea of IGP, these are some of the examples. You may have heard of them.

  • OSPF (Open Shortest Path First)

  • RIP (Routing Information Protocol)

  • IS-IS (Intermediate System to Intermediate System)

  • EIGRP (Enhanced Interior Gateway Routing Protocol)

Among these LS routing protocols and OSPF are used most frequently.


Let’s head over to BGP;

BGP can be divided into eBGP (external BGP) and iBGP (internal BGP). BGP security has an issue since BGP routers commonly belong to different ISPs, therefore,

  • Each router may use different encryption and security schemes.

  • Routers and gateways are administrated and managed by different ISPs, so security coordination is difficult.


BGP routers need to exchange setup and updates information with each other. So if the coordination is poor it can cause to security vulnerabilities. When different encryption and security schemes are used, it can be difficult to authenticate and protect against spoofed BGP messages and malware attacks.


OSPF (Open Shortest Path First)

Most widely used IGP routing protocol in the internet. It is used for IPv4, IPv6 and CIDR addresses.

Internet Gateways and Routers used OSPF. And OSPF uses LSR (Link State Routing) algorithm.

OSPF Operations:

The way that it operates is ;

  1. Routers collect LS information from other routers in the AS network.

  2. Then the network connection map is made including Cost values of links.

  3. LS routing algorithm is used to setup a Loop-Free SPT (Shortest Path Tree) routing path from source node to all destination nodes in the network. (SPT selects the Minimum Cost routing path using Dijkstra’s algorithm from source to each destination node in the network)

  4. Gateways and routers will setup/update their routing tables (based on SPT routing paths)

  5. When changes in the network are detected, steps 1-4 are repeated.


OSPF Link Cost Factors:

  • Distance of a router

  • RTT (Round Trip Time)

  • Number of hops (routers/switches) to reach the destination

  • Throughput (bits/packets)

  • Availability

  • Reliablity

Moving into the SPT , this is how it works,

Shortest Path Tree Algorithm

  1. M is the set of nodes connected to the SPT

  2. Initially M includes only the source node (S)

  3. Find the least cost route (by adding up link cost values on the routing path) from S to node X (that no in set M yet), using only nodes in M as intermediate nodes

  4. Add node X to set M

  5. Record the least cost route from S to X

  6. Repeat steps 3 ~ 5 until all nodes are in M


OSPF Router Types:

  • IR (Internal Router) : All routing interfaces belong to the same network area

  • ABR (Area Border Router) : Connects subarea networks to the backbone network

  • BR (Backbone Router) : Connects to the backbone network

  • ASBR (Autonomous System Boundary Router) : Connects between ASs using multiple routing protocols

Routers may have multiple type functionality.


ARP (Address Resolution Protocol)

This protocol is used to map an IPv4/IPv6 address to a device's DLL (Data Link Layer) address. For an example an ethernet MAC Address can be mapped to an IPv4 address. Ipv6 networks use NDP (Neighbor Discovery Protocol) for ARP functionality.

ARP Packet Parameters

  • PLEN (Protocol Address Length)

  • Operation

  • SHA (Sender Hardware Address)

  • SPA (Sender Protocol Address)

  • THA (Target Hardware Address)

  • TPA (Target Protocol Address)


Hope you got an overall idea about routing functions.

Thank you!

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