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

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

Remember:

• 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

• 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

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• To notice when routes in the table are no longer valid and remove them from the routing table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

• 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. – Ex: RIP, IGRP, EIGRP, OSPF, IS-IS

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

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

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:1) Distance vector2) Link-state3) Hybrid

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

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

Metric

• 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

• 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 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.

Now we apply those terms to some interior routing protocols.

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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 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)

• 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

• 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)

• 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.

IGRP

• 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)

Distance Vector Protocols

Feature RIP (v1) IGRP

Update timer for full routing updates

30 seconds 90 seconds

Metric Hop Count Function of bandwidth and delay (default)

Supports VLSM No No

Infinite-metric value

16 4,294,967,295

Convergence Slow Slow

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-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)

• 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 (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

• 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-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

• 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

• 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

Next week, May 3, to be discussed:

-- Routing Protocol Configuration Commands

-- Routing Protocol Logic

-- VLSM