Routing Protocols Explained


In this paper I will explain many different routing protocols and give some

basic details and features about them. This paper in no way should be considered

a source of full detail about any of the listed protocols. For detailed

information about routing protocols I would suggest looking into the Request For

Comments (RFC) for that protocol, goggling them, or maybe visiting different

vendor’s websites such as Cisco, Juniper, or Bay networks. Most vendors’ web

sites have detailed explanations about all the Protocols there equipments

support features. Before you can understand routing protocol you will have to

have a basic understanding or IP networks, Variable Length Subnet Masks (VLSM),

Network Topologies (bus, star, Hub, etc..), and the OSI model. If you are not

familiar with any of the above subjects you may want to read up and learn about

them before moving into routing protocols. Routing protocols are what makes the

internet work by moving traffic from network to network. With out routing

protocols the internet could not work because networks would just be separate

LANs with no connection to others LANs. The internet in basic terms is just a

lot of LANs wired together to make a huge WAN.

Protocol types:

Distance Vector: distance vector routing is a type of

routing protocol that discovers routes on interconnected networks. The Distance

Vector routing algorithm is the based on the Bellman-Ford algorithm. Examples of

distance-vector routing protocols include RIP (Routing Information Protocol),

Cisco’s IGRP (Internet Gateway Routing Protocol) try Google to see all the

different protocols in the Distance Vector family. Distance Vector protocols are

suitable for smaller networks as many of the protocols in this family aren’t

scalable in lager complex networks as they are limited. The main limit to

Distance Vector Protocols is there method that requires each router simply

inform its neighbors of its routing table. This routing table update to other

members is bandwidth intensive to lager networks.

RIP (Routing Information Protocol):

Routing Information Protocol (RIP) is one of the first protocols to be used in

networking and is classified as a distance vector routing protocol. RIP uses

broadcast User Datagram Protocol (UDP) data packets to exchange routing

information. There are two versions RIP V1 & V2. RIP Version 1 is the original

version and has many limitations. The metric that RIP uses to rate the value of

different routes is hop count. The hop count metric works by assigning static

routes with a value of 0 and all other routers values are set by the number of

hops (up to 15) that the data must travel though to get to an end point. RIP

Version 2 supports plain text and MD5 authentication, route summarization,

classless inter-domain routing (CIDR), variable-length subnet masks (VLSMs),

Multicast support. Some vendors support other non-standard features for RIP but

be careful as many vendor centric features are not compatible in a mixed vendor


IGRP (Inter Gateway Routing Protocol):

Interior Gateway Routing Protocol (IGRP) is a distance vector routing protocol

which is a proprietary and invented by Cisco. It is used by routers to exchange

routing data within an autonomous system (AS). IGRP supports multiple metrics

for routes, including bandwidth, load, delay and MTU. This improves reliability

over RIP because IGRP uses advanced metrics to compare two routes into a

combined route. The two routes together are combined into a single metric, using

a formula which can be adjusted via command line. The maximum hop count of IGRP

is 255 which is an improvement over RIPs 15 hop max. Keep in mind that IGRP is a

Cisco proprietary protocol and can not be used in a mixed vendor network.

Link State: Link State routing protocol requires each

router (peer) to maintain at least a partial map of the network. When a network

link changes state (up to down, or vice versa), a notification, called a link

state advertisement (LSA) is flooded throughout the network. All the routers

note the change, and recompute their routes accordingly. This method is more

reliable, easier to debug and less bandwidth-intensive than Distance-Vector. It

is also more complex and more compute- and memory-intensive. Link state routing

protocols are found in many lager networks and provide Scalable solutions for

more complex networks.

OSPF (Open Shortest Path First):

Open shortest path first (OSPF) is a link-state routing protocol that calls for

the sending of link-state advertisements (LSAs) to all other routers within the

same hierarchical area or autonomous system (AS). An AS can be divided into a

number of areas, which are groups of contiguous networks and attached hosts.

Information on attached interfaces, metrics used, and other variables are

included in OSPF LSAs. As OSPF routers accumulate link-state information, they

use the SPF algorithm to calculate the shortest path to each node.

(IS-IS) Intermediate System-to-Intermediate System:

Intermediate System-to-Intermediate System (IS-IS) is a routing protocol

developed by the ISO and is natively an ISO Connectionless Network Service or

CLNS protocol so it does not use IP to carry routing information messages. It

uses OSI protocols to deliver its packets and establish its adjacencies. IS-IS

has been enhanced to carry IP (Internet Protocol) and this is called Integrated

IS-IS. Integrated IS-IS supports VLSM and converges rapidly. It is also scalable

to support very large networks and is the key protocol in many larger ISP’s.

Hybrid: Hybrid routing Protocols are a combination or

both Distance Vector and Link state protocols and only one protocol fits into

this field. EIGRP is Cisco Systems Proprietary protocol based on their original

IGRP. For more information on Hybrid Protocols look below to the section on “EIGRP”.

EIGRP (Enhanced Inter Gateway Routing Protocol):

Enhanced Inter Gateway Routing Protocol (EIGRP) is Cisco Systems Proprietary

protocol based on their original IGRP. EIGRP is a balanced hybrid IP routing

protocol, with optimizations to minimize both the routing instability incurred

after topology changes, as well as the use of bandwidth and processing power in

the router. EIGRP has Protocol-Dependent Modules that can deal with AppleTalk

and IPX as well as IP. The advantage with this is that only one routing process

need run instead of a routing process for each of the protocols. EIGRP provides

loop-free operation and almost instant simultaneous synchronization of routers.

Inter-Autonomous System: Inter-Autonomous System routing protocols are designed

to connect lager networks or Autonomous Systems (AS) together and allow for

muitiple Autonomous Systems to network. One example for the need of an

inter-autonomous system protocol is to connect two or more Internet Service

Providers (ISP) together so there customer can connect to each other. Without

getting in to too much detail both “Link State & Distance Vector” protocols are

considered intra-autonomous system protocols as they are designed to just route

traffic in a singal AS. Inter-Autonomous System protocols main goal is to

propergate the intra-autonomous system information between different autonomous


BGP4 (Border Gateway Protocol Version 4):

Border Gateway Protocol is the backbone routing protocol for most of the

internet and allows for peering and carrier networks to connect. BGP is

explained as a path vector protocol. With BGP the policy or attributes for

making the actual route selections among the interconnected autonomous systems

is based on Weight ,Local preference, Multi-exit discriminator, Origin, AS path,

Next hop, & Community. BGP information is propagated through the network by

exchanges of BGP messages (4 types: Open, Update, Notification, & Keep Alive)

between peers. Another key feature to BGP is that is supports Classless Inter

Domain Routing (CIDR) with the support of CIDR BGP can reduce the size of the

Internet routing tables. BGP neighbors exchange full routing information when

the TCP (port 179) connection between neighbors is first established. When

changes to the routing table accrue, the BGP routers send to their neighbors

only those routes that have changed. BGP routers do not send periodic routing

updates and advertise only the optimal paths to a destination.