How Routing Protocol Works? - Explained Routing Protocols

Routing Protocol :

A routing protocol is an algorithm that gives a Router the ability to learn (dynamically) about specific existing networks around it (or even the Internet). Practically, these routing protocols help routers learn dynamically about each other without the need for an administrator to intervene.

How Routing Protocol Works?
In all network the Routing Protocol perform below functions:

1.    Discovery – identify other routers on the network

2.    Route management – keep track of all the possible destinations (for network messages) along with some data describing the pathway of each

3.    Path determination – make dynamic decisions for where to send each network message

A few routing protocols (called link-state protocols) enable a router to build and track a full map of all network links in a region while others (called distance-vector protocols) allow routers to work with less information about the network area.

Most Popular and useful Routing Protocols are defined below with a brief explanation.

• RIP
• OSPF
• EIGRP and IGRP
• IS-IS
• BGP and EGP

RIP:

Routing Information Protocol introduced in the 1980s for use on small like internal networks that connected to the early Internet. RIP is capable of routing messages across networks up to a maximum of 15 hops.

Routing Information Protocol (RIP) is the oldest routing protocol for the Internet. The two versions of RIP differ primarily by the inclusion of security measures. RIPv1 is the original protocol, and RIPv2 is the same but supports classless addresses and includes some security. Devices that use RIP will periodically (approximately every 30 seconds) broadcast routing information to neighboring hosts.

Routing Information Protocol introduced in the 1980s for use on small like internal networks that connected to the early Internet. RIP is capable of routing messages across networks up to a maximum of 15 hops.

RIP enabled routers discover the network by first sending a message requesting router tables from neighboring devices. Neighbor routers running RIP respond by sending the full routing tables back to the requestor, whereupon the requestor follows an algorithm to merge these updates into its own table. At scheduled intervals, RIP routers then periodically send out their router tables to their neighbors so that any changes can be propagated across the network.

The information sent by a host describes the devices they are directly connected to and the cost. RIP is not very scalable so is primarily used for small networks. RIP uses UDP to broadcast messages; port 520 is used by servers, whereas clients use a port above 1023.

Traditional RIP supported only IPv4 networks but the newer RIPng standard also supports IPv6. RIP utilizes either UDP ports 520 or 521 (RIPng) for its communication. 

OSPF:

Open Short Path First (OSPF), which was developed after RIP. As such OSPF is considered an improvement because it converges faster and it incorporates authentication.

Interestingly, OSPF is not built on the transport layer but instead talks directly to IP. It is considered protocol 89 by the IP layer. OSPF messages are broadcast using two special multicast IP addresses: 224.0.0.5 (all SPF/link state routers) and 224.0.0.6 (all designated routers). The use of multicast addresses and setting the packet Time to Live (TTL) to one (which is done by OSPF) typically means a firewall will not pass this routing information.

Open shortest path first (OSPF) is a link-state routing protocol which is used to find the best path between the source and the destination router using its own shortest path first (SPF) algorithm. A link-state routing protocol is a protocol which uses the concept of triggered updates.

Open Shortest Path First was created to overcome some of its limitations of RIP including:

• 15 hop count restriction
• Inability to organize networks into a routing hierarchy, important for manageability and performance on large internal networks
• Significant spikes of network traffic generated by repeatedly re-sending full router tables at scheduled intervals.

As the name suggests, OSPF is an open public standard with widespread adoption across many industry vendors. OSPF enabled routers discover the network by sending identification messages to each other followed by messages that capture specific routing items rather than the entire routing table. It is the only link-state routing protocol listed in this category.

EIGRP and IGRP:

IGRP (Interior Gateway Routing Protocol) and EIGRP (Enhanced EIGRP) are the two routing protocols used in routing operations. The IGRP is a distance vector interior gateway routing protocols, but the EIGRP incorporates the features of the link state routing with the distance vector routing protocol.

Cisco developed Internet Gateway Routing Protocol as another alternative to RIP. The newer Enhanced IGRP (EIGRP) made IGRP obsolete starting in the 1990s. EIGRP supports classless IP subnets and improves the efficiency of the routing algorithms compared to older IGRP. It does not support routing hierarchies, like RIP.

Originally created as a proprietary protocol runnable only on Cisco family devices, EIGRP was designed with the goals of easier configuration and better performance than OSPF.

IS-IS:

The IS-IS (Intermediate System - Intermediate System) protocol is one of a family of IP Routing protocols, and is an Interior Gateway Protocol (IGP) for the Internet, used to distribute IP routing information throughout a single Autonomous System (AS) in an IP network.

IS-IS is a link-state routing protocol, which means that the routers exchange topology information with their nearest neighbors. The topology information is flooded throughout the AS, so that every router within the AS has a complete picture of the topology of the AS. This picture is then used to calculate end-to-end paths through the AS, normally using a variant of the Dijkstra algorithm. Therefore, in a link-state routing protocol, the next hop address to which data is forwarded is determined by choosing the best end-to-end path to the eventual destination.

The main advantage of a link state routing protocol is that the complete knowledge of topology allows routers to calculate routes that satisfy particular criteria. This can be useful for traffic engineering purposes, where routes can be constrained to meet particular quality of service requirements.

The main disadvantage of a link state routing protocol is that it does not scale well as more routers are added to the routing domain. Increasing the number of routers increases the size and frequency of the topology updates, and also the length of time it takes to calculate end-to-end routes. This lack of scalability means that a link state routing protocol is unsuitable for routing across the Internet at large, which is the reason why IGPs only route traffic within a single AS.

Every IS-IS router distributes details of its local state to other routers using a Link State PDU (LSP) message. Each router uses the received messages to build up an identical database that describes the topology of the AS.

From this database, each router calculates its own routing table using a Shortest Path First (SPF) or Dijkstra algorithm. This routing table contains all the destinations the routing protocol knows about, associated with a next hop IP address and outgoing interface.

• The protocol recalculates routes when network topology changes, using the Dijkstra algorithm, and minimizes the routing protocol traffic that it generates.
• It provides support for multiple paths of equal cost.
• It provides a multi-level hierarchy (two-level for IS-IS) called "area routing," so that information about the topology within a defined area of the AS is hidden from routers outside this area. This enables an additional level of routing protection and a reduction in routing protocol traffic.
• All protocol exchanges can be authenticated so that only trusted routers can join in the routing.

BGP and EGP:

The Border Gateway Protocol (BGP) is one of a family of IP Routing protocols, and is an Exterior Gateway Protocol (EGP) designed to distribute routing information between ASs.

EGPs are all vector routing protocols. In a vectoring protocol like BGP, the routers exchange network reachability information with their nearest neighbors. In other words, the routers communicate to each other the sets of addresses ("address prefixes") that they can reach, and the next hop address to which data should be sent in order to reach those addresses. This contrasts with link-state IGPs; the EGP routers exchange routes with one another, whereas the IGP routers exchange topology information and calculate their own routes locally.

The EGP floods reachability information through the Internet, so that every EGP router has a routing table containing address prefixes and next hops that cover the entire public Internet. The EGP has little or no knowledge of the end-to-end route; it only knows about the next hop along the route. Hence, the path along which data is forwarded is chosen based on a comparison of all available next hops.

Vector routing protocols scale much better than link-state routing protocols because the time taken to determine the best next hop is not a function of the number of nodes in the network, which makes them suitable for routing traffic on the backbone of the Internet.

The routes in the BGP routing table are combined with routes learned from other routing protocols (for example, OSPF) to generate the complete routing table for the router. This routing table contains all the destinations the router knows about, associated with a next hop IP address and outgoing interface.

• BGP allows for routes to be modified before they are distributed to peers, through the use of implementation-specific policies.
• BGP uses timers to prevent a rapidly changing route from being continually advertised throughout the internet.
• BGP protocol exchanges can be authenticated so that only trusted routers can join in the routing exchanges.

BGP detects modifications to routing tables and selectively communicates those changes to other routers over TCP/IP.

Internet providers commonly use BGP to join their networks together. Additionally, larger business sometimes also use BGP to connect multiple internal networks. Professionals consider BGP the most challenging of all routing protocols to master due to its configuration complexity.