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Routing/Routed Protocols - PowerPoint PPT Presentation

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Routing/Routed Protocols. Part I. Routed Protocol Definition:. Routed Protocol – used to transmit user data (packets) through an internetwork. Routed protocols are assigned to an interface and determine the method of packet delivery. Examples: IP, IPX, AppleTalk, DECNet, Banyan Vines.

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Routed protocol definition
Routed Protocol Definition:

  • Routed Protocol – used to transmit user data (packets) through an internetwork. Routed protocols are assigned to an interface and determine the method of packet delivery.

  • Examples: IP, IPX, AppleTalk, DECNet, Banyan Vines

Routing protocol definition
Routing Protocol Definition:

  • Routing protocol – any protocol that defines algorithms to be used for updating routing tables between routers. Basically, a routing protocol determines the path of a packet through an internetwork.

  • Examples: RIP, RIPv2, IGRP, EIGRP, OSPF, IS-IS, BGP


  • A Routed Protocol – defines logical addressing. Most notable example on the test – IP

  • A Routing Protocol – fills the routing table with routing information. Examples on the test – RIP, IGRP, EIGRP, OSPF, IS-IS

Ccna exam tips routing
CCNA Exam Tips -- Routing

  • Since IP routing is basically what Cisco routers do, this protocol is the backdrop for the whole CCNA exam. See prior chapter notes on “IP Addressing” for more info.

  • Next, some routing protocols that are strongly rumored to be on the CCNA 801 will be outlined.

  • But first, some generalities about routing protocols…

Main goals of routing protocols
Main Goals of Routing Protocols

  • To fill the routing table with current best, loop-free routes

  • To notice when routes in the table are no longer valid and remove them from the routing table

  • To add new routes or replace lost routes

    • The time for finding a working route is called convergence.

Two categories of routing protocols
Two Categories of Routing Protocols

  • Exterior Routing Protocols – used for use between different organizations such as ISPs or ISPs and their customers.

    • Ex: Border Gateway Protocol (BGP)

  • Interior Routing Protocols – used to distribute routing information inside a single organization.


Border gateway protocol bgp
Border Gateway Protocol (BGP)

  • The most popular exterior routing protocol & the only one on the CCNA 801 exam

  • ISPs use BGP to exchange routing info between themselves and other ISPs and customers.

  • BGP advertises only routing info to specifically defined peers using TCP.

  • BGP does not use a metric like internal routing protocols

Terminology of interior routing protocols

Terminology of Interior Routing Protocols

This is not as painful as it sounds. There are only 6 basic concepts.

Type of routing protocol
TYPE of routing protocol

  • Each interior routing protocol can be characterized based on the underlying logic used by the routing protocol.

  • The underlying logic is referred to as the TYPE of routing protocol.

  • The three types are:

    • Distance vector

    • Link-state

    • Hybrid

Full partial update
Full/partial Update

  • Full routing updates – entire routing tables are sent regularly

  • Partial routing updates – only a subset of the routing table is sent, typically just information about changed routes.

  • Partial routing updates require less overhead than full routing updates.


  • Convergence refers to the time required for routers to react to changes in the network.


  • The metric refers to the numeric value that describes how good a particular route is.

  • The lower the value, the better the route.

Support for vlsm
Support for VLSM

  • Variable-length subnet masking (VLSM) means that, in a single Class A, B, or C network, multiple subnet masks can be used.

  • The advantage of VLSM is that it enables you to vary the size of each subnet, based on the needs of that subnet.

  • Some routing protocols support VLSM, and some do not

Classless or classful
Classless or Classful

  • Classless routing protocols transmit the subnet mask along with each route in the routing updates sent by that protocol.

  • Classful routing protocols do not transmit mask information.

  • Only classless routing protocols support VLSM. To say that a routing protocol is classless is to say that it supports VLSM.

Distance vector protocols rip and igrp
Distance Vector Protocols: RIP and IGRP

  • Distance vector protocols advertise routing information by sending messages, called routing updates, out the interfaces on a router.

  • These updates contain a series of entries, with each entry representing a subnet and a metric.

  • Failure to receive updates from a neighbor in a timely manner results in the removal of the routes previously learned from that neighbor.

Distance vector protocols rip and igrp1
Distance Vector Protocols: RIP and IGRP

  • Routers send periodic full updates and expect to receive periodic updates from neighboring routers.

  • When possible, routers use broadcasts or multicasts to send routing updates. This way, all neighbors on a LAN can receive the same routing information in a single update.

  • If a router learns multiple routes to the same subnet, the router chooses the best route based on the metric.

Routing information protocol rip
Routing Information Protocol (RIP)

  • Been around 15+ years for use with IP networks.

  • Easier to use than some newer routing protocols, but severely limited in comparison.

Basic rip summary
Basic RIP Summary

  • Based on distance vector logic

  • Uses hop count for the metric

    • Hop count = number of routers between two points

  • Sends periodic full routing updates every 30 seconds

  • Converges slowly, often taking 3 to 5 minutes

  • Does not support VLSM, also making it a classful routing protocol

Interior gateway routing protocol igrp
Interior Gateway Routing Protocol (IGRP)

  • IGRP is a Cisco-proprietary IP routing protocol created to provide a better distance vector protocol.

  • The most obvious difference between RIP-1and IGRP is the metric.

  • IGRP advertises up to five parameters that describe the metric for each route, although, by default only two are used – bandwidth and delay.


  • Other three possible parameters used to describe IGRP metric can include: reliability, load, and MTU (maximum transmission unit).

  • IGRP calculates the metric based on a mathematical formula that “you do not really need to know for the exam.” (Wendell Odom, CCNA INTRO, p.415)

Link state protocols ospf and integrated is is
Link-State Protocols: OSPF and Integrated IS-IS

  • The goal of link-state protocols is to fill the routing tables with the current best routes.

  • Link-state advertises a large amount of topological info about the network

  • Discovers neighbor routers before exchanging routing information.

  • A router running a link-state protocol uses more memory and more processing cycles than do distance vector protocols.

Link state protocols ospf and integrated is is1
Link-State Protocols: OSPF and Integrated IS-IS

  • To figure out the current best routes, a router processes the link-state topology database using an algorithm called the Dijkstra Shortest Path First (SPF) algorithm.

  • This info helps link-state protocols avoid loops & converge quickly.

  • Quick convergence – often less than 10 seconds.

Open shortest path first ospf
Open Shortest Path First (OSPF)

  • OSPF is the most popular link-state IP routing protocol today.

  • Because OSPF does not send full updates on a regular short interval (like RIP), the overall number of bytes sent for routing information is typically smaller.

Open shortest path first ospf1
Open Shortest Path First (OSPF)

  • OSPF uses a concept called cost for the metric. Each link is considered to have a cost; a route’s cost is the sum of the cost for each link.

  • By default, Cisco derives the cost value for a link from the bandwidth.

  • OSPF supports VLSM.

Integrated is is
Integrated IS-IS

  • OSI defines a network layer protocol called the Connectionless Network Protocol (CLNP). It also defines a routing protocol – a routing protocol used to advertise CLNP routes, called Intermediate System-to-Intermediate System (IS-IS). IS-IS advertises CLNP routes between “intermediate systems,” which is what OSI calls routers.

Integrated is is1
Integrated IS-IS

  • Integrated IS-IS has the capability to advertise IP routes as well as CLNP routes.

  • “…most installations could care less about CLNP.” (Wendell Odom, CCNA INTRO, p.419)

  • Supports VLSM

Balanced hybrid protocols enhanced igrp
Balanced Hybrid Protocols: Enhanced IGRP

  • EIGRP uses features similar to link-state protocols, and others similar to distance vector protocols, and yet others unlike either of the two.

  • The internal workings of EIGRP depend on an algorithm called the Diffusing Update Algorithm (DUAL).

    • Requires less processing than the Dijkstra SPF algorithm.

Eigrp summary
EIGRP Summary

  • A balanced hybrid protocol

  • Converges in less than 3 seconds

  • Discovers neighbors (via Hello packets) before sending them information.

  • Requires little design effort

  • Supports VLSM

  • Cisco proprietary

  • Metric based on bandwidth & delay, scaled by multiples of 256.

That s it
That’s IT

Next week, May 3, to be discussed:

-- Routing Protocol Configuration Commands

-- Routing Protocol Logic