Routing protocols explained and how to choose and use them (2023)

Routing protocols explained and how to choose and use them (1)

Network routing protocols: IGRP, EIGRP, OSPF, ISIS, BGP

Overview of routing protocols

The purpose of routing protocols is to learn the routes available on the corporate network, create routing tables, and make routing decisions. Some of the more common routing protocols are RIP, IGRP, EIGRP, OSPF, IS-IS, and BGP.

There are two types of primary routing protocols, although many different routing protocols have been defined using these two types. The link state and distance vector protocols comprise the primary types.

distance vector logsthey advertise their routing table to all directly connected neighbors at regular, frequent, high-bandwidth intervals and are slow to converge. When a route becomes unavailable, all router tables must be updated with this new information. The problem is that each router must communicate this new information to its neighbors. It takes a long time for all routers to have an up-to-date and accurate view of the network. Distance vector protocols use fixed-length subnet masks that are not scalable.

Link state protocolsannounce routing updates only when they occur, making more efficient use of bandwidth. Routers do not advertise the routing table, which speeds up convergence. The routing protocol floods the network with link-state advertisements to all neighboring routers per domain in an attempt to converge the network with new route information. The incremental change is all that is advertised to all routers as a multicast LSA update. They use variable-length subnet masks that are scalable and use addressing more efficiently.

Interior Gateway Routing Protocol (IGRP)

Interior Gateway Routing Protocol is a distance vector routing protocol developed by Cisco Systems to route multiple protocols through small to medium Cisco networks.

It is proprietary, which requires you to use Cisco routers. This is in contrast to IP RIP and IPX RIP, which are designed for multi-vendor networks.

IGRP routes IP, IPX, Decnet, and AppleTalk, making it very versatile for clients with many different protocols. It is slightly more scalable than RIP, as it supports a hop count of 100, only announces every 90 seconds, and uses a combination of five different metrics to choose the best route target.

Note that because IGRP advertises less frequently, it uses less bandwidth than RIP, but it converges much more slowly, as it takes 90 seconds for IGRP routers to detect changes in the network topology. IGRP recognizes the allocation of different autonomous systems and automatically combines them at network class boundaries. There is also the option of balancing traffic across equal or unequal cost paths.


  • distanciavektor
  • Forward IP, IPX, Decnet, Appletalk
  • Table ads routing every 90 seconds
  • Metrics: bandwidth, delay, reliability, load, MTU size
  • Number of jumps: 100
  • Fixed Length Subnet Masks
  • Network Class Address Summary
  • Load balancing on 6 equal or unequal cost paths (IOS 11.0)
  • Metric Calculation = Min Target Path BW * Delay (usec) .
  • divided horizon
  • Timers: Invalid Timer (270s), Dump Timer (630s), Standby Timer (280s)

Enhanced Interior Gateway Routing Protocol (EIGRP)

Enhanced Interior Gateway Routing Protocol is a hybrid routing protocol developed by Cisco Systems to route many protocols through a Cisco enterprise network.

It exhibits characteristics of both distance vector routing protocols and link state routing protocols. It is proprietary, which requires you to use Cisco routers. EIGRP routes the same protocols as IGRP (IP, IPX, Decnet, and Appletalk) and uses the same composite metrics as IGRP to choose the best route destination.

There is also the option of balancing traffic across equal or unequal cost paths. Aggregation is done automatically at a network class address, but can also be configured to aggregate at subnet boundaries. Redistribution between IGRP and EIGRP is also automatic. A hop count of 255 and variable-length subnet masks are supported.


Convergence with EIGRP is faster because it uses an algorithm called the Dual Update Algorithm, or DUAL, which is executed when a router detects that a particular route is unavailable. The router queries its neighbors for a usable successor. This is a neighbor with a least-cost path to a given destination that does not introduce routing loops. EIGRP updates its routing table with the new route and the associated metric. Route changes are only communicated to affected routers when changes occur. This uses bandwidth more efficiently than distance vector routing protocols.

autonomous systems

EIGRP recognizes the assignment of different autonomous systems, which are processes running under the same administrative routing domain. Assigning different numbers of autonomous systems does not serve to define a backbone as it does with OSPF. In IGRP and EIGRP it is used to change route redistribution, filtering, and aggregation points.


  • extended distance vector
  • Forward IP, IPX, Decnet, Appletalk
  • Route announcements: partial when route changes occur
  • Metrics: bandwidth, delay, reliability, load, MTU size
  • Number of jumps: 255
  • Variable Length Subnet Masks
  • Summary about network class address or subnet boundary
  • Load balancing on 6 equal or unequal cost paths (IOS 11.0)
  • Timer: Active time (180 sec)
  • Metric Calculation = Destination Path Minimum BV * Delay (ms) * 256
  • divided horizon
  • LSA-Multicast-Address:

Open Shortest Path First (OSPF)

Open shortest path First is atrue link state protocoldeveloped as an open standard for IP routing in large multi-vendor networks. A link state protocol sends link state announcements to all connected neighbors in the same area to convey route information. Each OSPF-enabled router sends Hello packets to all connected OSPF routers directly at startup.

Hello packets contain information such as the router timer, router ID, and subnet mask. When the routers agree to the information, they become OSPF neighbors. When routers become neighbors, they establish adjacencies by exchanging link-state databases. Routers on point-to-point and point-to-multipoint links (as specified in the OSPF interface type configuration) automatically establish adjacencies. Routers with OSPF interfaces configured as Broadcast (Ethernet) and NBMA (Frame Relay) use a Designated Router that establishes these adjacencies.


OSPF uses a hierarchy with assigned areas connected to a central backbone of routers. EachAreait is defined by one or more routers that have established adjacencies. OSPF has defined backbone area 0, stub areas, less blunt areas, and fully blunt areas. Area 0 consists of a group of routers connected at a specific office or through WAN links between multiple offices. It is preferable to connect all routers in Area 0 to a full mesh over an Ethernet segment at a main office. This ensures high performance and prevents the area from being partitioned if a router connection fails. Area 0 is a transit area for all traffic from the connected areas. All traffic between areas must first be routed through area 0. Auxiliary areas use a default route to forward traffic destined for an external network such as EIGRP because the area border router does not send or receive external routes. Routing between areas and within areas is done as usual. Totally Stubby Areas is a Cisco specification that uses a default route for external destinations and between areas. The ABR does not send or receive external or cross-area LSAs. The soft area ABR will advertise external routes with LSA Type 7. External routes are not received in this type of area. Routing between areas and within areas is done as usual. OSPF defines internal routers, backbone routers, area border routers (ABRs), and autonomous system boundary routers (ASBRs). Internal routers are area specific. Area border routers have interfaces dedicated to more than one area, such as area 0 and area 10. An autonomous system border router has interfaces dedicated to OSPF and another routing protocol such as EIGRP or BGP. A virtual connection is used when a realm does not have a direct connection to realm 0. A virtual connection is established between an area boundary router for an area not connected to area 0 and an area boundary router for an area that is connected to area 0. The design of the area should take into account the geographical location of the offices and the traffic flows throughout the organization. It is important to be able to add addresses for many offices per area and minimize broadcast traffic.


Fast convergence is achieved by the SPF (Dijkstra) algorithm, which determines the shortest path from source to destination. The routing table is created from running SPF, which discovers all routes from neighboring routers. Because each OSPF router has a copy of the topology database and routing table for its area, route changes are detected faster than distance vector protocols and alternative routes are determined.

designated router

Broadcast networks like Ethernet and non-broadcast multi-access networks like Frame Relay have a Designated Router (DR) and a Backup Designated Router (BDR) that are elected. Designated routers establish adjacencies with all routers on that network segment. This is to reduce broadcasts from all routers sending regular Hello packets to their neighbors. The DR sends multicast packets to all routers with which it has established neighborhoods. If the DR fails, it is the BDR that multicasts to specific routers. Each router is assigned a router ID, which is the highest assigned IP address on a working interface. OSPF uses the Router Identifier (RID) for all routing processes.


  • connection status
  • carry out the IP
  • Route announcements: partial when route changes occur
  • Metric: Composite cost of each router to destination (100,000,000/interface speed)
  • Number of hops: none (limited by network)
  • Variable Length Subnet Masks
  • Summary about network class address or subnet boundary
  • Load balancing on 4 equal cost paths
  • Router Type: Internal, Backbone, ABR, ASBR
  • Bereichstypen: spine, stocky, not so stocky, totally stocky
  • LSA types: Intra-area (1.2) Inter-area (3.4), External (5.7)
  • Timer: Hello Interval and Dead Interval (varies by network type)
  • LSA multicast address: and (DR/BDR) Do not filter!
  • Schnittstellentypen: point-to-point, transmit, non-transmit, point-to-multipoint, loopback

IS-IS integrado

Integrated Intermediate System – Intersystem Routing Protocol is a link state protocol similar to OSPF used by large enterprises and ISP customers. An intermediate system is a router, and IS-IS is the routing protocol that forwards packets between intermediate systems. IS-IS uses a link state database and runs the SPF Dijkstra algorithm to select the shortest route paths. Neighboring routers on point-to-point and point-to-multipoint links establish adjacencies by sending Hello packets and exchanging link-state databases. IS-IS routers in broadcast and NBMA networks elect a Designated Router that establishes adjacency to all neighboring routers in that network. The Designated Router and each neighboring router establish adjacency with all neighboring routers by multicasting link-state advertisements to their own network. This differs from OSPF, which only establishes adjacencies between the DR and each neighboring router. IS-IS uses a hierarchical area structure with Tier 1 and Tier 2 router types. Tier 1 routers are similar to routers within the OSPF area that do not have direct connections outside of their area. Layer 2 routers comprise the backbone area that connects different areas similar to OSPF area 0. With IS-IS, a router can be an L1/L2 router, which is like an OSPF Area Border Router (ABR) that has connections to its domain and the backbone domain. It differs from IS-IS in that the connections between routers span the boundaries of the area and not the router. Each IS-IS router must be assigned an address that is unique to that routing domain. An address format consisting of an Area ID and a System ID is used. The Area ID is the assigned area number, and the System ID is a MAC address of one of the router's interfaces. There is support for variable-length subnet masks, which is standard with all link-state protocols. Note that IS-IS assigns the routing process to an interface and not to a network.


  • connection status
  • Forward IP, CLNS
  • Routing Advertisements: Partial when routing changes occur
  • Metric: Variable cost (default cost 10 assigned to each interface)
  • Number of hops: none (limited by network)
  • Variable Length Subnet Masks
  • Summary about network class address or subnet boundary
  • Load balancing on 6 equal cost paths
  • Timer: Hello Interval, Hello Multiplier
  • Area types: hierarchical topology similar to OSPF
  • Router Type: Level 1 and Level 2
  • LSP Types: Internal L1 and L2, External L2
  • Designated Router Election, No BDR

Border Gateway Protocol (BGP)

The Border Gateway Protocol is aExternal Gateway Protocol, which differs from the interior gateway protocols discussed so far. The distinction is important because the term autonomous system is used slightly differently in protocols like EIGRP than it is in BGP. External gateway protocols such as the BGP route between autonomous systems that are assigned a specific AS number. AS numbers can be assigned to an office with one or more BGP routers. The BGP routing table consists of destination IP addresses, an associated AS route to reach that destination, and a next-hop router address. The AS route is a collection of AS numbers that represent each office involved in forwarding packets. Compare that to EIGRP, which also uses autonomous systems. The difference is that their autonomous systems refer to a logical grouping of routers within a single management system.

An EIGRP network can configure many autonomous systems. All of them are managed by the company to define the aggregation, redistribution and filtering of routes. BGP is commonly used by Internet Service Providers (ISPs) and large enterprises that have dual-hosted Internet connections with single or dual routers hosted by the same or different Internet Service Providers. BGP routes packets through an ISP network, which is a separate routing domain managed by them.

The ISP has its own assigned AS number, which is assigned by the InterNIC. New customers can request an AS assignment for their office from the ISP or InterNIC. Assignment of a unique AS number is required for clients connecting via BGP. There are 10 attributes defined with a specific order or order that BGP uses as a metric to determine the best route to a destination.

Businesses with a single line connection to an ISP implement a default route in their router, which forwards all packets destined for an external network. BGP routers share routing information (peering) with all IGP routers in the network (EIGRP, RIP, OSPF, etc.), which implies the exchange of complete routing tables. Once this is complete, incremental updates with topology changes will be sent. Any BGP router can be configured to filter routing broadcasts using route maps instead of sending/receiving the entire Internet routing table.

Components of the BGP routing table

  • Destination IP Address / Subnet Mask
  • as route
  • IP address of the next hop

Shaun Hummel is the author ofNetworking Quick Start Guide: Fundamentals of Routing, Switching, Wireless, and Application Services.

Article Source:—IGRP,-EIGRP,-OSPF,-ISIS,-BGP&id=2891289

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