Routing Logic

8/13/2019 Routing Logic

http://slidepdf.com/reader/full/routing-logic 1/28

Routing/Routed Protocols

Part 2



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



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



Key Points of Interior Routing

Protocols:

• Each interior routing protocol can becharacterized based on the underlyinglogic used by the routing protocol.

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

• The three types are:

1)Distance vector2)Link-state

3)Hybrid



Support for VLSM

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

used.• The advantage of VLSM is that it enables

you to vary the size of each subnet, basedon 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



No VLSM with RIPv1 or IGRP

• Neither RIPv1 nor IGRP routing protocolshas a field for subnet information.

• If a router running RIPv1 has a subnet

mask of a certain value, it assumes that allinterfaces interfaces within the classfuladdress space have the same have thesame subnet mask.

• This is called classful routing, and RIP andIGRP are both considered classful routingprotocols.



VLSM with RIPv2, EIGRP, & OSPF

• Classless routing protocols do support the

advertisement of subnet information.

• Use VLSM with routing protocols such as

RIPv1, EIGRP, or OSPF.

• The advantage with this type of network is

that bunches of IP addresses are saved.



More Routing Protocol

Basics



Static vs. Dynamic Routing

• Static routing occurs when you manually

add routes in each router’s routing table. It

will be covered in subsequent routing

command procedures since it is mainlydemonstrated through routing commands.

• Dynamic routing is when protocols are

used to find networks and update routingtables on routers.



Interior vs. Exterior Gateway

Protocols

• IGPs are used to exchange routinginformation with routers in the sameautonomous system (AS).

• An AS is a collection of networks under acommon administrative domain, whichbasically means that all routers sharing thesame routing table information are in thesame AS.

• EGPs are used to communicate between Ass such as in WAN links.



Administrative Distances

• The administrative distance (AD) is used

to rate the trustworthiness of routing

information received on a router from a

neighbor router.

• An administrative distance is an integer

from 0 to 255, where 0 is the most trusted

and 255 means NO traffic will be passedvia this route.



The Three Classes of Routing

Protocols

• Distance Vector – finds the best path to a

remote network using hop count. (RIP, IGRP)

• Link State – (also called shortest-path-first

protocols) – the routers each create threeseparate tables. 1) keeps track of directly

attached neighbors, 2) topology of network, 3)

the routing table. (OSPF, IS-IS)

• Hybrid – uses aspects of both distance vector

and link state. (EIGRP)



Distance Vector Terms• Route Poisoning – occurs when an

“unreachable,” or “infinite,” message isdistributed when a link is down.

• Holddown Timer – A holddown prevents

regular update messages from reinstatinga route that is going up and down (called

“flapping”). Typically, this happens on a

serial link that is losing connectivity andthen coming back up. Network might never

converge otherwise.



Routing Information Protocol (RIP)

• A true distance-vector routing protocol.

• Sends the complete routing table to allactive interfaces every 30 seconds.

• RIP only uses hop count to determine thebest way to a remote network – max hopcount = 15, with 16 being “infinite,” or

unreachable.• Works well in small networks.

• RIPv1 uses only classful routing.



Open Shortest Path First (OSPF)

Basics

• An open standards routing protocol (non-

Cisco proprietary

• Good for large networks (over 15 routers)

• Uses the Dijkstra algorithm

• Only supports IP routing

• Classless



OSPF Logic

1) Each router discovers its neighbors on

each interface. The list of neighbors is

kept in a neighbor table.

2) Each router uses a reliable protocol to

exchange topology information with its

neighbors.

3) Each router places the learned topology

information in its topology database.



OSPF Logic (cont.)

4) Each router runs the SPF algorithm

against its own topology database to

calculate the best routes to each subnet in

the database.

5) Each router places the best route to each

subnet in the IP routing table.



OSPF

• A link-state protocol uses a neighbor table

and a topology database in addition to

adding routes to the routing table.

• OSPF converges more quickly than do

distance-vector protocols.

• Uses “Hello” packets to keep in touch with

neighbors.

• Defined in RFC 2328



Enhanced Interior Gateway

Routing Protocol (EIGRP)

• Cisco proprietary

• Classless

• Uses autonomous system numbers

– A number assigned to a group of routersunder mutual administration.

• Referred to as a “hybrid routing protocol”

• Provides support for IP, IPX, and Appletalk• Best path selection using the Diffusing

Update Algorithm (DUAL)



EIGRP Feature Comparison

Link-state Features

• Converges quickly

• Discovers neighbors

via “Hello” packets • Builds topology table

• After learning itsneighbor’s routes,

only changes to therouting table arepropagated.

Distance-vector Features

• Uses autonomoussystem number (like

IGRP)• Uses metric based onbandwidth & delay

• Advertises entire

routing table to newneighbors.



Try a

Quiz!

Routing Logic

Documents

Transcript of Routing Logic