ccna routing fundamentals - eigrp, ospf and rip

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CCNA: ROUTING

By Sushmil Garde

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ROUTING

Router IOS

Boot Process

Command Line Interface (CLI)

Routing protocols and static routing


ROUTER IOS

Carries Network Protocols and Functions

Connects high speed traffic between network devices

Adds security to network access

Provides scalability for growth

Supplies reliability


ROUTER BOOTUP PROCESS

Three stage process

Perform POST (Power On Self Test) and load Bootstrap Program

Locate and load the Cisco IOS software

Bootstrap Program performs this task

By default IOS is stored in flash memory

Locate Startup Configuration file or enter in Setup Mode.

Bootstrap searches for startup config (present in NVRAM)

Or else goes in Setup Mode.


ROUTERS- MODES OF OPERATION

User Mode

Privileged Mode

Global Configuration Mode


EDITING COMMANDS FOR NETWORK GEEKS

Commands Explanation

Ctrl+A Moves the cursor to the begining of the line

Ctrl+E Moves the cursor to the end of the line

Esc+B Moves back one Word

Esc+F Moves forward one Word

Ctrl+R Redisplays a line

Ctrl+U Erases a line

Ctrl+W Erases a word

Ctrl+Z Ends configuration mode

TAB Finishes command for you


EDITING COMMANDS FOR NETWORK GEEKS

Commands Explanation

Ctrl+P Shows Last entered command

Ctrl+N Shows Previous commands entered

Show history Show last 10 commands entered by default

Show terminal Shows terminal configurations and history buffer size

Terminal history size Changes buffer size (max 256)


BASIC CONFIGURATION AND IMPORTANT COMMANDS

Configuring Hostname: Router(config)# hostname <NAME>

Configuring Banner: Router(config)#banner motd ^c <TYPE THE BANNER>

Configuring Password: Router(config)#enable password Router(config)#enable secret

Interface configuration Router(config)#

…and many more


ROUTING

WHAT DOES A ROUTER KNOW?

Router knows about all the networks it can reach

All paths to reach all networks

Best path to reach all network

Neighbor router


ROUTING

IMPORTANT STATEMENTS:

For end to end communication, it is not enough to have only forwarding path, but having reverse path is also important.

If you can reach one host on a network that does not mean you can reach all hosts on that network.

Initially router knows only about its directly connected neighbors, hence can only communicate with them.

Router must know the route to reach the destination network


STATIC vs DYNAMIC

There are two ways to provide routes to the router

Static Routing

Provide the route manually

Not very helpful in larger network

Better when there is a need to provide a specific route

Dynamic Routing

Configuring Routing Protocols on routers

Very useful in larger network

Calculates the best path automatically


LEARNING A ROUTE THROUGH MULTIPLE ROUTING PROTOCOLS

PROBLEM:

If a route is learnt from static entry/any routing protocol and same is learnt from other routing protocol and both paths are different, then which path to prefer?

SOLUTION:

Administrative Distance (AD)

This is a value associated with each routing protocol and also with static route.

Lower the AD value better the path.

So, the router will prefer the path dictated by the one with LOWER AD value.


ONE ROUTING PROTOCOL MULTIPLE PATHS

PROBLEM:

If there is only one Routing Protocol running on the router and it receives two different routes to reach same network, then which one to prefer?

SOLUTION:

Metric:

This is a value associated with the path. Every routing protocol has a way to calculate its own metric. Lower the Metric Better the path and hence preferred.

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DYNAMIC ROUTING- ROUTING PROTOCOLS

Routing protocols can be categorized as Interior Gateway Protocol

Exterior Gateway Protocol

Routing Protocols can also be categorized as Classful Protocols

Classless Protocols

…and also as Link state Routing

Distance Vector Routing

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DYNAMIC ROUTING- ROUTING PROTOCOLS

Interior Gateway Protocols (IGP):

– RIPv1 (CF/DV)

– RIPv2 (CL/DV)

– IGRP (CF/DV)

– IBGP (CL/DV)

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– EIGRP (CL/Ad. DV)

– OSPF (CL/LS)

– ISIS (CL/LS)

Exterior Gateway Protocols (EGP):

– EBGP (CL/DV)

ROUTING PROTOCOL


CLASSFUL ROUTING

Classful Routing Protocols do not send subnet mask with its route information

Classful Routing automatically summarizes the mask at Classful boundaries.

May lead to Sub-optimal paths.

Examples of Classful Routing Protocols: • RIP Version 1 (RIPv1)

• IGRP

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CLASSLESS ROUTING

Classless Routing Protocol send subnet mask with IP route information

Classless routing protocols support variable-length subnet masking (VLSM).

Examples of classless routing protocols: i. RIP Version 2 (RIPv2)

ii. EIGRP

iii. OSPF

iv. IS-IS

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DISTANCE VECTOR ROUTING PROTOCOL

These are the protocols in which… Updates are sent periodically (every 30/90 seconds)

Entire Routing Table is sent as an update

Updates are broadcasted

Updates are sent to directly connected neighbors only and not to the entire group

Routers don’t have end-to-end visibility of the entire network, Directly connected neighbors are the worlds

Convergence is slow.

RIP, IGRP and BGP are DISTANCE VECTOR Routing Protocols

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ROUTING INFORMATION PROTOCOL

Used for smaller networks

Metric: Hop Count

Maximum Hop Count: 15

Administrative Distance: 120

Load Balancing: over equal metric path (4 by default)

RIP versions: RIPv1 and RIPv2

RIPv1 sends updates on Broadcast address

RIPv2 sends updates on Multicast address 224.0.0.9

Encrypted Authentication between 2 RIPv2 routers

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RIP TIMERS

• Update Timer: (default: 30 sec): Indicates how often the router will send out a routing update

• Invalid Timer: (default: 180 sec): Indicates how long the route will remain in the routing table before it gets invalid, if no new updates are received

• Hold-Down Timer: (default: 180 sec): Indicate how long RIP will suppress the route that it has placed in HOLD state.

– HOLD state: • Invalid timer has expired.

• Update received for a router marking that route with Metric 16 (unreachable)

• An update received from a router, with a higher metric than what is currently in the routing table. (to prevent loops)

• Flush Timer: (default: 240 sec): Indicates how long a route can stay in the routing table before its been flushed, if updates are not received

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LINK STATE ROUTING PROTOCOL

These are the protocols in which Updates are triggered and not periodic

Updates are incremental, entire routing table is not sent as a part of routing update

Updates are sent as a multicast and not as broadcast

Convergence is fast as updates are triggered

Routers have end to end visibility of entire network through topology table.

OSPF and ISIS are Link State Routing Protocols

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ADVANCED DISTANCE VECTOR ROUTING PROTOCOL

Combination of both Distance vector and Link State routing protocols

Best features of both are used in this

Best feature of D.V Simple Configuration

Best feature of L.S triggered updates, convergence is fast etc.

Used only in EIGRP

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ENHANCED INTERIOR GATEWAY ROUTING PROTOCOL (EIGRP)

Classless Routing Protocol

Cisco Proprietary Protocol

Advanced Distance Vector Protocol

Uses DUAL (Diffusion Update Algorithm) to determine Best Path to reach destinations

Neighbors are formed with only directly connected routers

Mostly the EIGRP traffic is sent on Multicast Address 224.0.0.10

Updates are incremental and triggered

Summarization can be done on any router interface manually

Administrative Distance:

Internal EIGRP : Routes originating within local AS- 90

External EIGRP: Routes coming from outside AS- 170

Summary EIGRP: 5

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EIGRP

Maintains 3 tables Neighbor Table: List of all neighbor routers

Topology Table: List of all routes in the AS

Routing Table: Best routes to reach all networks

Load Balancing: Load balancing on unequal metric path is possible.

Default value: 4; Max Value: 6

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EIGRP OPERATION

When routers are up, they exchange Multicast Hellos.

Neighbor relation is formed after this exchange. Only directly connected routers can become neighbors

Topology table will be exchanged, till every router reaches converged state

After every router reaches converged state…

No more exchange of Topology table, only updates will be sent after modification in the network topology.

EIGRP DUAL is run to find the best path to reach all networks

Multicast echoes are exchanged to check the connectivity (Keep Alive)

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EIGRP METRIC

EIGRP is called COMPOSIT METRIC

Bandwidth (K1) and Delay of the line (K3) is used by default to calculate the distance Metric

Reliability (K4/K5), MTU and Load (K2) are the other 3 parameters that can be considered

By default:

EIGRP METRIC= 10000000

𝐵𝑊 (𝐾𝑏𝑝𝑠) + delay * 256

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EIGRP- EXTRA

EIGRP elects Next Best path at the same time, when it elects BEST PATH

Best Path is called as SUCCESSOR

Next Best Path is called as FEASIBLE SUCCESSOR

RULES:

For a path to be considered as a feasible successor, its Advertised Distance should be less than current Feasible Distance.

For a path to be considered as a valid path, its AD should be less than 2 times current FD

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DUAL

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(a)

(1)

(2)

(1)

(1)

(1)

(2)

A

D

E C

B

FOR ROUTER C:

FD AD Topology

Via B 3 1 Successor

Via D 4 2 Feasible Successor

Via E 4 3

AD: Advertised Distance FD: Feasible Distance


OPEN SHORTEST PATH FIRST- OSPF

Linked State Routing Protocol

Metric: COST

Higher the Bandwidth Lower the Cost, Lower the Cost Better the Path.

𝐶𝑂𝑆𝑇 =108

𝐵𝑊

Determine the Best Path to reach all networks: Dijkstra Shortest Path Algorithm

AD Value: 110

Sends Mask as a part of updates hence supports VLSM

Updates are triggered

Load Balancing on EQUAL costs path

Multicast Address: 224.0.0.5 and 224.0.0.6

Every OSPF router carries the visibility of the entire network with the help of Link State Database

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TABLES

Neighbor Table

Topology Table- Link State Database EIGRP and other DV protocols: Info that just have been passed on

by neighbors

They know the roadmap to their entire AREA

Routing Table

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THE PROBLEM

OSPF is a highly scalable protocol and can scale up to 1000s of routers.

Has end to end visibility every router knows about all routes to reach entire networks

Updates are triggered after each Network Modification. This includes addition/deletion of the route from routing table

High Process Utilization

High Bandwidth Utilization

High Memory Utilization

Solution: Summarization

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CONCEPT OF AREA

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AREA 2 AREA 1

AREA 0- BACKBONE Internal

Router

Area Border Router (ABR)

Autonomous System Boundary

Router (ASBR)


CONCEPT OF AREA

All areas must connect to Area 0- One interface of ABR must be connected to AREA 0

All routers in an area have the same Topology Table

Purpose: Localizing updates within a area

Automatic Summarization is done at the ABR/ASBR only

Hierarchical design is required

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OSPF ROUTER ID

In OSPF neighbors are identified by their RIDs

RID is the highest IP of LOOPBACK interface.

In absence of LOOPBACK interface, it’s the IP of highest ACTIVE INTERFACE when router starts

RID is also Hardcoded in the configuration using router id command- Highest Preference

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OSPF NEIGHBOR RELATIONSHIP

Determining own Router ID

Hardcoded

Loopback

Physical

Add interfaces to Link State Database dictated by Network Command

Send Hello on those interfaces

This is DOWN state: Sending Hello but still waiting to hear back

Receives Hello

Goes in INIT state Checks: Hello/Dead timer, Authentication password, Area ID, Net mask

must be identical Other parameters: DR/BDR IP address, Router ID and Router Priority Reaches TWO WAY state.

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OSPF NEIGHBOR RELATIONSHIP

Determines Master-Slave Relationship Determines who will send the routing first

This is an EX_START ST

Higher priority Masters

Master sends Database Description Packets (DBD) like cliff notes

Slave sends DBDs

DBDs are Acknowledged and Reviewed Router Goes in LOADING state

First slave reviews the DBD and asks for missing info through LINK STATE REQUEST to Master

Master sends response in terms of LINK STATE UPDATES

Master sends LSR and Slave responds back with LSU

Neighbors are Synchronized This is the FULL STATE

Now, Dijkstra’s SPF algorithm is applied to chose the Best Path and put it in the routing table

Keep Alive are sent

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DR/BDR

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DR/BDR listen on 224.0.0.6 DROTHERS listen on 224.0.0.5 DR/BDR is only for shared segments DR/BDR election is based on Router

Priority (Default: 1) If Router Priority is equal then the

Tie Breaker is the Router ID DROTHERS DO NOT need to

exchange their entire Routing info with each other. Send it to only DR and BDR

Hence can stuck in TWO WAY state


OSPF PACKETS

Hello: Sent every 10/30 sec Contains- Router ID, DR/BDR, Subnet Mask, Neighbors, Hello/Dead Intervals,

Auth Password, Router Priority, Area ID Neighbor relationship formation and keep alive

Data-Base Description: Cliff notes sent in Ex-start state

Link State Request: Sent in Loading state for requesting detailed information about a Network

Link State Update: Response LSR Contain Multiple LSA

Link State Advertisement: Response of each sub request

Link State Acknowledgement: Reliability Mechanism OSPF itself is a Reliability Protocol

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LSA TYPES

LSA 1 (Router LSA): contains a list of links local to the router and generated by all the routers in the area

LSA 2 (Network LSA): Generated by DR, contains list of all routers attached to it

LSA 3 (Network Summary LSA): Generated by ABRs; Used for inter-area communication

LSA 4 (ASBR Summary LSA): Route to reach ASBR

LSA 5 (External LSA): Generated by ASBR and contains routes to reach destinations outside local AS.

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ccna routing fundamentals - eigrp, ospf and rip

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