Interconnecting Cisco Networking

Devices Part 2 (ICND2)

Course 01 - Implementing

Scalable Medium-Sized Networks

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Slide 1

Lesson 1

Troubleshooting VLAN Connectivity

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Slide 2

VLAN Review

A VLAN represents:

A separate broadcast domain

A subnet (logical network)

An independent LAN

VLANs help with the following:

Segmentation of traffic

Security Boundaries

Flexibility in designing a network

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Slide 3

How to Create a VLAN

Adds VLAN 2 and names it “switchlab99”

Assigns interface FastEthernet 0/2 to VLAN 2

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Slide 4

How to Create a VLAN (Cont.)

Displaying VLAN information

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Slide 5

What is a Trunk

A Trunk can carry traffic for multiple VLANs

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Slide 6

How to Configure a Trunk

Create a trunk from interface configuration mode

Configure the Fa0/11 interface as a VLAN trunk

The native VLAN is changed to VLAN 99

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Slide 7

How to Configure a Trunk (Cont.)

Verifies switchport settings on FastEthernet0/11

Verifies that FastEthernet0/11 is trunking

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Slide 8

DTP

Avoid using DTP (manual configuration recommended

Configure both sides as trunks

The command nonegotiate turns off the negotiation

Dynamic

Auto

Dynamic

Desirable

Trunk Access

Dynamic

Auto

Access Trunk Trunk Access

Dynamic

Desirable

Trunk Trunk Trunk Access

Trunk Trunk Trunk Trunk Limited

Connectivity

Access Access Access Limited

Connectivity

Access

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Slide 9

VLAN Troubleshooting

Show vlan

Show mac address-table

Show vlan

Show interfaces

Show interfaces switchport

Yes Yes

No No

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Slide 10

VLAN Troubleshooting (Cont.)

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Slide 11

VLAN Troubleshooting (Cont.)

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Slide 12

Troubleshooting Trunks

Show interfaces trunk

Show interfaces trunk

Yes Yes

No No

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Slide 13

Troubleshooting Trunks (Cont.)

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Slide 14

Lesson 2

Building Redundant Switched Topologies

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Slide 15

Problems with Redundant Topologies

Redundancy avoids a single point of failure.

A redundant switch topology causes broadcast storms, multiple

frame copies, and MAC address table instability problems.

A loop-avoidance mechanism is required.

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Slide 16

Problems with Redundant Topologies (Cont.)

Solution: STP (Spanning Tree Protocol)

Provides a loop-free, yet redundant topology by dynamically

closing ports.

Published in IEEE 802.1D specification

X

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Slide 17

How Spanning-Tree Works

The spanning-tree algorithm follows these steps:

1. Elects a root bridge

2. Elects a root port for each non-root switch

3. Elects a designation port for each segment

4. Ports transition to forwarding or blocking state

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Slide 18

How Spanning-Tree Works (Cont.)

Switch A

Priority 28672

MAC 0000.0cab.3274

Switch D

Priority 32768

MAC 0000.0c39.f28a

Switch B

Priority 28672

MAC 0000.0c9f.3127

Root Bridge

Switch C

Priority 32768

MAC 0000.0cf6.9370

10Gbs

1Gbs1Gbs

1Gbs1Gbs

Step 1: Elect a Root Bridge

Decision based on lowest BID

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Slide 19

Switch A

Priority 28672

MAC 0000.0cab.3274

Switch D

Priority 32768

MAC 0000.0c39.f28a

Switch C

Priority 32768

MAC 0000.0cf6.9370

How Spanning-Tree Works (Cont.)

Cost 2

Cost 4Cost 4

Cost 4Cost 4

Step 2: Elect a root port for each non-root switch

Decision based on lowest root path cost.

If necessary, ties are broke by upstream BID and port ID values

RP

RP

RP

DP DP

Switch B

Priority 28672

MAC 0000.0c9f.3127

Root Bridge

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Slide 20

How Spanning-Tree Works (Cont.)

Switch A

Priority 28672

MAC 0000.0cab.3274

Switch D

Priority 32768

MAC 0000.0c39.f28a

Switch C

Priority 32768

MAC 0000.0cf6.9370

Cost 2

Cost 4Cost 4

Cost 4Cost 4

F

F

F

FF

Switch B

Priority 28672

MAC 0000.0c9f.3127

Root Bridge

Step 3: Elect a designated port for each segment.

Decision is based on the lowest root path cost.

IF necessary, ties are broken by upstream BID and port ID.

B

B

F

F

F

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Slide 21

How Spanning-Tree Works (Cont.)

Switch A

Priority 28672

MAC 0000.0cab.3274

Switch D

Priority 32768

MAC 0000.0c39.f28a

Switch C

Priority 32768

MAC 0000.0cf6.9370

Cost 2

Cost 4Cost 4

Cost 4Cost 4

RP

RP

RP

DP DP

Switch B

Priority 28672

MAC 0000.0c9f.3127

Root Bridge

Step 4: The ports transition to the forwarding or blocking state.

Root ports and designated ports transition to the forwarding state

Other ports stay in the blocking state

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Slide 22

Versions of Spanning-Tree

IEEE 802.1D: Legacy Spanning-Tree

CST: Assumes one spanning-tree instance for the entire bridged

network, regardless of the number of VLANs

PVST+: A Cisco enhancement of STP that provides a serparate

802.1D spanning-tree instance for each VLAN configured in the

network

802.1w (RSTP): Improves convergence over 1998 STP by adding

roles to ports and enhancing BPDU exchanges

Rapid PVST+: A Cisco enhancement of RSTP using PVST+

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Slide 23

Versions of Spanning-Tree (Cont.)

Protocol Standard Resources

Needed

Convergence Number of

Trees

STP 802.1D Low Slow One

PVST+ Cisco High Slow On for every

VLAN

RSTP 802.1w Medium Fast One

Rapid PVST+ Cisco Very high Fast One for every

VLAN

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Slide 24

Versions of Spanning-Tree (Cont.)

Cisco Catalyst switches have the following

defaults:

PVST

Enabled on all ports

Slower convergence than with RSTP

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Slide 25

PVST+

Forwarding Port for VLAN 1

Blocking Port for VLAN 2

Forwarding Port for VLAN 1

Blocking Port for VLAN 2

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Slide 26

PVST+ (Cont.)

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Slide 27

Changing the Bridge ID

SW1 is not the root bridge for VLAN1. This is the switch that is

connected to FastEthernet0/3 on SW1.

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Slide 28

Changing the Bridge ID

Configures SW1 as the root bridge for VLAN 1

After modification, SW1 is the root bridge for VLAN 1

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Slide 29

Reviewing the STP Topology

Show cdp neighbors

Use STP knowledge

Show spanning-tree vlan

Show spanning-tree vlan

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Slide 30

Reviewing the STP Topology (cont.)

Switch A Switch B:

Root Bridge

Verify the topology

F

F

F

FF

B

B

F

F

F

Switch C Switch D

X

X

VLAN 100

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Slide 31

Reviewing the STP Topology (cont.)

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Slide 32

What if STP Fails?

Switch A

Priority 28672

MAC 0000.0cab.3274

Switch D

Priority 32768

MAC 0000.0c39.f28a

Switch C

Priority 32768

MAC 0000.0cf6.9370

Cost 2

Cost 4Cost 4

Cost 4Cost 4

F

F

F

FF

Switch B

Priority 28672

MAC 0000.0c9f.3127

Root Bridge

B

B

F

F

F

If STP fails for any reason, it fails catastrophically.

X

X

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Slide 33

What if STP Fails? (Cont.)

What if Switch D changed it’s blocking port to forwarding?

Now we have a loop.

Switch A Switch B:

Root Bridge

F

F

F

FF

F

B

F

F

F

Switch C Switch D

X

VLAN 100

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Slide 34

What if STP Fails? (Cont.)

A Bridging Loop is sever

The load on each link will increase, and quickly become overwhelmed

The Switch CPU will max out and become unreachable

Switch A Switch B:

Root Bridge

F

F

F

FF

F

B

F

F

F

Switch C Switch D

X

VLAN 100

Frame enters

here

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Slide 35

PortFast and BPDU Guard

PortFast Characteristics:

moves an Access port to forwarding immediately

Configured only on access ports

BPDU guard characteristics:

If BPDU is received, it will shut down port

Used in a combination with PortFast

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Slide 36

PortFast and BPDU Guard (Cont.)

Configures BPDU guard and PortFast interface FastEthernet0/1

Enables PortFast on all nontrunking interfaces and enables BPDU guard globally for

all PortFast-enabled ports

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Slide 37

PortFast and BPDU Guard (Cont.)

Verifies that PortFast and BPDU guard have been configured on

interface FastEthernet0/1

Verifies that PortFast is enabled on FastEthernet0/1

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Slide 38

Lesson 3

Improving Redundant Switched Topologies with

EtherChannel

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Slide 39

What is EtherChannel

When Multiple links aggregate on a switch, congestion occurs.

One solution is to increase uplink speed, but that solution cannot

scale indefinitely.

Another solution is to multiple uplinks, but loop prevention

mechanisms disable some ports.

xxx

x

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Slide 40

EtherChannel Protocols

Logical aggregation of links between switches

High Bandwidth

Load sharing across links

Viewed as one logical port to STP

Redundancy

xEtherChannel

EtherChannel

EtherChannel

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Slide 41

EtherChannel Protocols (Cont.)

Two protocols exist to negotiate EtherChannel creation

and maintenance:

PAgP is a Cisco proprietary protocol.

LACP is an IEEE 802.3ad standard.

Static EtherChannel can be configured without PAgP or LACP

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Slide 42

EtherChannel Protocols (Cont.)

PAgP negotiates EtherChannel formation and maintenance

On: Channel member without negotiation

PAgP Modes:

Desirable: Actively asking if the other side can or will participate

Auto: Passively waiting for other side

PAgP

Channel establishment On Desirable Auto

On YES NO NO

Desirable NO YES YES

Auto NO YES NO

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Slide 43

EtherChannel Protocols (Cont.)

LACP negotiates EtherChannel formation and maintenance

On: Channel member without negotiation (no protocol).

LACP Modes:

Active: Actively asking if the other side can or will participate

Passive: Passively waiting for other side

LACP

Channel establishment On Desirable Auto

On YES NO NO

Active NO YES YES

Passive NO YES NO

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Slide 44

How to Configure EtherChannel

Interfaces within an EtherChannel must have the same

configuration

Speed/duplex

Trunk or access mode

Same native VLAN

Same allowed VLANs for Trunks

Same Access VLANs for access ports

LACP

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Slide 45

How to Configure EtherChannel (Cont.)

SW1 SW2

Fa0/1

Fa0/2

Fa0/4

Fa0/5

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Slide 46

Verify EtherChannel

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Slide 47

Verify EtherChannel (Cont.)

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Slide 48

Verify EtherChannel (Cont.)

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Slide 49

Lesson 4

Understanding Layer 3 Redundancy

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Slide 50

Why have Default Gateway Redundancy

Server 10.9.1.50

Router B 10.1.10.3Router A 10.1.10.2

No Default GatewayI can’t get to my

Gateway

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Slide 51

Default Gateway Redundancy

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Slide 52

Default Gateway Redundancy (Cont.)

Forwarding

Router

Standby

Router

Link or device failure: The roles

of standby and forwarding router

are reversed

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Slide 53

FHRP

The idea of FHRP is to provide redundancy at

Layer 3

The common protocols that are used are:

HSRP – Cisco proprietary

VRRP – Similar, but an open standard

GLB – Also Cisco proprietary

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Slide 54

HSRP

Hot Standby Routing Protocol

This solution is designed for two routers to represent

themselves as a single “virtual” router

For those end systems on a segment, they see a

single default gateway to access the network

The routers then elect an active router to forward

traffic

If that active router fails, then the one that is in standby mode

will take over

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Slide 55

HSRP (Cont.)

Routers are configured in a “standby” group for

each segment

Routers can belong to more than one group to provide

redundancy

The groups are configured per interface

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Slide 56

HSRP (Cont.)

Each router will still have a unique IP address

for the interface

The routers will share a common Virtual IP

address that end systems use as their gateway

The election will be decided by which router has

the highest priority, or the highest IP address

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Slide 57

HSRP (Cont.)

The failover will occur based on:

If the active router completely fails

If the active router is tracking an interface that fails

If the “hello” messages are no longer received

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Slide 58

HSRP Configuration

Standby “group”

Priority

Standby ip

Preempt

Track

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Slide 59

Introducing HSRP

HSRP defines a group of routers – one active and one standby.

They share a virtual IP and MAC address for use by hosts as their

default gateway

To verify HSRP state, use the show standby command.

HSRP is Cisco proprietary, and VRRP is a standard protocol.

HSRP Group 1

StandbyActive Virtual

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Slide 60

HSRP (Cont.)

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Slide 61

HSRP (Cont.)

Active Router:

Responds to the default gateway,

ARP requests with the virtual router

MAC address

Sends hello messages

Assumes the forwarding of all

packets for the Virtual Router

Knows the virtual router IP

Standby Router:

Listens for the periodic hello’s

Assumes active forwarding of

packets if it does not hear from

active router

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Slide 62

HSRP Track Interface

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Slide 63

HSRP Load Balancing

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Slide 64

Gateway Load Balancing Protocol

Allows full use of resources on

all devices without the

administrative burden of creating

multiple groups

Provides a single virtual IP

address and multiple virtual MAC

address

Routers traffic to single gateway

distributed across routers

Automatically can re-route traffic

on a tracked failure

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Slide 65

Gateway Load Balancing Protocol (Cont.)

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Slide 66

Gateway Load Balancing Protocol (Cont.)

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Slide 67

QoS

Quality of Service

Many applications will suffer at having poor service,

such as VoIP

Latency

Jitter

Dropped packets

Congestion

QoS tries to prioritize these application’s

packets for better service

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Slide 68

QoS

Give certain traffic precedence for delivery over

less important traffic

Best used for networks that have contention or

congestion of packets being delivered

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Slide 69

QoS Configuration

Time sensitive traffic must be identified

Class-mapping

Then this traffic must be marked for importance

Policy-mapping

Then this policy is placed on the outgoing

interface

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Slide 70

QoS Options

Precedence

Shaping

Policing

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Slide 71

Marking

CoS – Layer 2 on ethernet

ToS – Layer 3 IP/IPv6 packet

DSCP – Layer 3 – more detailed than ToS

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Slide 72

Queuing Methods

Class-based WFQ

LLQ – usually called the best practice

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Slide 73

Congestion Avoidance

Tail drop

Weighted Random Early Detection

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Review Questions:

1. QoS provides prioritization for different types of traffic. One goal is to reduce

latency of traffic flows. Which of the following applications are always given a

higher priority to reduce latency?

A. VoIP

B. FTP

C. HTTP

D. Video

E. A & B

F. B & D

G. A & D

2. To configure any protocol/packet for priority, the router must first know what is to

be matched. What configuration command will tell the router what to match?

A. Policy-map

B. Class-map

C. Service-policy

D. NBAR

3. Which of the following is a packet marking?

A. FTP

B. DSCP

C. MPLS

D. HTTP

4. What configuration construct is used to set the marking and priority of packets

matched by a class-map?

A. Service-policy

B. Policy-map

C. DSCP

D. ToS

5. At what layer of the OSI is the use of FHRP going to provide redundancy?

A. Layer 1

B. Layer 2

C. Layer 3

D. Layer 4

6. For two routers to be redundant, using HSRP, the routers will need some

common configuration on each of their interfaces. What at minimum should be

configured the same on each router's interface?

A. Interface IP

B. Standby IP

C. Standby group number

D. Standby priority

E. A & B

F. B & C

G. C & D

7. What must be configured for a router to take the "ACTIVE" role in an HSRP

standby group? Specifically, what setting allows you to choose the router to be

ACTIVE?

A. Standby group number

B. Track interface

C. Standby group IP

D. Standby group priority

8. The router that was ACTIVE in an HSRP group failed, and then the role of

ACTIVE moved to what was the STANDBY router. When the original router

comes back online it will not retake the role of ACTIVE, unless you've configured

what?

A. Standby priority

B. Standby preempt

C. Standby refresh

D. Standby IP

9. What do VLANs create?

A. Broadcast domains

B. Subnets

C. Collision domains

D. WAN connections

10. What type of Switchport will carry all tagged VLAN traffic by default?

A. Access

B. Trunk

C. Routed

D. Serial

Answer Key:

1. G

Voice over IP and video will greatly suffer if they have more than 150ms latency.

Any more, and these applications will generally be unusable.

2. B

The class-map construct allows you to list those protocol/packets that are to be

matched. This does not configure the priority.

3. B

Differentiated Services Code Point is a method of marking a packet for higher

priority. For example, VoIP will be marked with "EF" or expedited forwarding.

4. B

The policy-map is used to set priority and markings for packets matching

specified class-maps.

5. C

These are a collection of redundancy protocols used by Layer 3 devices, such as

routers.

6. F

These routers will have to be in the same standby group, and must agree on

what their virtual IP (VIP) is going to use.

7. D

To specify which router will be the ACTIVE router, you must configure it with a

higher priority value when compared to the STANDBY router.

8. B

A router will not retake the ACTIVE role, unless it's been configured with

preempt.

9. A

A VLAN will create a new broadcast domain, which may or may not be a part of a

new subnet.

10. B

Trunk ports connect switches to each other and by default will carry all tagged

and untagged traffic.