Network Routing Protocols

This post summarizes gateway protocols or routing protocols on computer networks.

All of them are within OSI layer 3.

List of Network Routing Protocols

Overall list of network routing protocols:

Interior routing protocols
- Distance-vector routing protocols
  - IGRP
  - EIGRP
  - RIP
- Link-state routing protocols
  - OSPF
  - IS-IS
Exterior routing protocols
- Path vector protocols
  - BGP

The two main categories of network routing protocols are:

Interior protocols
Exterior protocols

Interior Protocols

Interior protocols makes the best decision only taking into account the next hop.

They are typically used within an autonomous system.

Categories of interior protocols

Distance-vector protocols
Link-state routing protocol

Distance-vector Protocols

Distance-vector protocols use metrics including the direction traffic and distance in hops to the destination to make decision about the next hop routing.

Distance-vector protocols featured on this post:

IGRP
EIGRP
RIP

IGRP

Interior Gateway Routing Protocol (IGRP).

It does not support Variable Length Subnet Mask (VLSM).

EIGRP

Enhanced IGRP (EIGRP) replaces IGRP.

It does support VLSM.

RIP

Routing Information Protocol (RIP).

It does support VLSM.

Link-state Routing Protocol

Link-state routing protocol gathers routing characteristics (such a speed latency error rates and actual monetary cost) and uses this information to decide the next hop routing.

Link-state routing protocols:

OSPF
IS-IS

OSPF

Open Shortest Path First (OSPF) is a routing protocol that keeps a map of all connected remote networks and uses that map to select the shortest path to a remote destination.

Each router sends periodically to its neighbors a packet that contains the list of the neighbors recognized by the router.

It uses the Dijkstra algorithm.

It does support Variable Length Subnet Mask (VLSM).

It uses multicast IP for sending and receiving packets.

It operates in the OSI layer 3 network.

IS-IS

Intermediate System to Intermediate System (IS-IS) does not belong to OSI layer 3.

It uses the Dijkstra algorithm, as well as OSPF. It uses CLNS (Connectionless Network Service) to encapsulate its packets

It operates in the OSI layer 2 data link, though it is used to take decisions on the OSI layer 3 network.

It is defined in the standard ISO/IEC 10589:2002.

Exterior Routing Protocols

Exterior protocols makes a decision based on the entire remaining path to the destination, instead of the next hop only.

External routing protocols:

Path-vector Protocol

Path-vector routing Protocol

Path-vector routing protocols:

BGP

Border Gateway Protocol (BGP) is a path vector protocol that is primarily used for exchanging routing and reachability information between autonomous systems (AS) on the internet.

It is sometimes referred as BGP-4, because of its version.

It is considered an application layer protocol in the OSI model.

IBGP refers to interior BGP.

EBGP refers to exterior BGP.

A prefix learnt from a neighbor using I-BGP cannot be reissued to another neighbor through I-BGP.

Messages used by BGP:

OPEN
KEEPALIVE
NOTIFICATION
UPDATE

External References

Chapman; “CISSP Study Guide 9th Edition”, chapter 11 “Secure Network Architecture”, section “OSI Model”, p. 503; Wiley, 2021