Routing Protocols Explained

Preface:

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 are not
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 although 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
network.

IGRP (Inter Gateway Routing Protocol):

The Internal 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 through 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 packages 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
systems.

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 automatic systems
is based on Weight, Local preference, Multi-exit discriminator, Origin, AS path,
Next hop, & Community. BGP information is propagated through the network by
exports of BGP messages (4 types: Open, Update, Notification, & Keep Alive)
between peers. Another key feature to BGP is that 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.



Source by Brian Wilson

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