ccna - switching concepts made easy

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CCNA: Switching

By Sushmil Garde

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SWITCH

Laptop

Desktop Desktop

Laptop

Switch is an Intelligent Device

Switch and bridge are similar devices but used on different medium. Switches are used on twisted pairs/fibers and Bridge on Thin/Thick Wires


SWITCH

When switch receives a frames on its port it will open L2 info. Read MAC Address, refer to MAC table to take forwarding decisions.

Switch takes forwarding decisions on basis on MAC address. MAC address is a layer 2 address and hence the switch is a layer 2 device

As soon as switch receives a frame on its port, it will read MAC address, refer it to the MAC table and forward the packet to the APPROPRIATE Port

When switch receives the frame on its port, it Opens L2 information, reads source MAC address and makes an entry in its MAC table against the port number on which it was received

Switch SEGMENTS network


SWITCH

MAC table size should be precise and concise Performance purpose For quick reference and fast forwarding To conserve resources

To keep table size precise an concise, entries in MAC table should be controlled and stale entries should be deleted

Any entry in MAC table that is not referred for a stipulated time is considered stale and removed from MAC table. Stipulated time can be 5-20 minutes depending upon switch


SWITCH OPERATION

If switch receives a Broadcast frame on a port, it creates multiple copies and forwards it to all OTHER ports. (Switch Does not Broadcast)

If the sender and receivers are connected on the same port then, switch will block/discard the frame

Switch SEGMENTS network whereas Hub EXTENDS the network

Switch allows simultaneous communication between multiple communication pairs connected on different ports

Every port of the switch has its own dedicated bandwidth. Switch is a dedicated bandwidth device.

If Receiver is not known then switch forwards information to all other ports

Every port of the switch is a member of different collision domain. Switch is a MULTIPLE COLLISION DOMAIN device

Every port of the switch is a member of same Broadcast domain. Switch is a SINGLE BROADCAST DOMAIN device.

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VLANS

Definition: A logical grouping of network users and resources connected to administratively defined ports on a switch

By default, all ports of a switch belong to VLAN 1

Smaller Broadcast domains

Organized by…

Location

Function

Department

Application of Protocol

Advantages:

Simplifies Network Management

Eliminates unnecessary broadcast, hence improves network performance

Logically separates the user/departments restricting access between them

Removes physical boundaries

6


VLAN

7

HR

HR MARKETING

MARKETING FINANCE

FINANCE

Access Link: A link that is a part of only one VLAN

Trunk Link: Carries multiple VLANs


VLAN MEMBERSHIP

Two ways to configure it

Statically: Manually assign one or more ports to a VLAN

Dynamically: Based on the MAC address of the host. User can connect to any physical port and still it will get same VLAN every time. It requires MAC address to VLAN mapping. Cisco developed VLAN MEMBERSHIP POLICY SERVER (VMPS) for this

8


FRAME TAGGING

It’s used to mark a packet with a VLAN number

When the packet is sent over a trunk port, it is marked (tagged) with the VLAN ID

When a packet is going out of an Access port, VLAN tag is removed, hence VLAN ID is transparent to the end devices

9


FRAME TAGGING METHODS

Inter-Switch Link (ISL) Cisco Proprietary

ISL encapsulates frames with 26 byte header and 4 byte trailer increasing the overhead

Supports Maximum of 1000 VLANs on a trunk port

IEEE 802.1Q (dot1Q) Industry standard

It embeds 4 byte VLAN tag into layer 2 header

Supports maximum of 4096 VLANs on a trunk port

10


CONFIGURING VLANs

Creating VLANs: SW1(config)# vlan 100 VLAN created

SW1(config)# name <name> Assigns a Name

Assigning switch port to VLAN – Access Port: SW1(config)# interface Gi0/1

SW1(config-if)# switchport mode access Making the port Access

SW1(config-if)# switchport access vlan 100 Assigning VLAN to a port

SW1# show vlan

Configuring a Trunk Link SW1(config)# interface Gi0/1

SW1(config-if)# switchport mode trunk

SW1(config-if)# switchport trunk encapsulation <dot1q/isl> Adding encapsulation

11


DYNAMIC TRUNKING PROTOCOL

DTP negotiates whether a port becomes a TRUNK port at all

DTP has 2 modes

Desirable: Port will actively attempt to form a Trunk with neighbor switch

Auto: Port will passively wait for the other switch to initiate the trunk request

Configuration: SW1(config)# interface gi0/1

SW1(config-if)# switchport mode dynamic desirable

SW1(config-if)# switchport mode dynamic auto

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VLAN TRUNCKING PROTOCOL- VTP

Purpose: To maintain a database of configured VLAN for easier management in a large switching environment

Cisco Proprietary Protocol

VLAN information is shared with the switches that are the part of same VTP Domain through VTP Advertisements

Modes of Operation:

Server: Responsible for Creating, Deleting and modifying the VLAN entries

Client: Cannot Create, Delete or Modify entries. Relies on updates from other switches and forwards the advertisement on every trunk port

Transparent: Switch maintains it’s own database and does not accept VLAN information from any switch. It forwards the advertisement out on its trunk port

With every update the Revision number is incremented by 1

13


SWITCHING LOOP

All ports of a switch belong to single Broadcast Domain.

Broadcast packets are forwarded to all the ports except the one it was received on.

Switching loop causes a BROADCAST STORM, that ends up chocking network traffic

14

B

A

C

E D


SPANNING TREE PROTOCOL

Purpose: To prevent Broadcast Storms caused by switching loops

Standard RFC: IEEE 802.1D

STP builds a Topology of the network and identify whether there is loop in the network

One or more ports are blocked to avoid switching loop

Blocked port can be reactivated, when necessary

15


BUILDING STP TOPOLOGY

STP enabled switches exchange BPDUs (Bridge Protocol Data Units) every two seconds

These BPDUs are sent out on every port on a dedicated Multicast MAC address- 01:80:C2:00:00:00

Steps in Building STP Topology:

Root Bridge Election

Identifying Root Ports

Identifying Designated Ports

Placing ports in Blocking state if required

16


ROOT BRIDGE ELECTION

Root Bridge is the central reference point of an STP topology

Root bridge is elected on the basis of Bridge ID

Bridge ID= Bridge Priority (16-bits) + MAC Address (48-bits)

Default Bridge Priority- 32768

Lowest Priority Wins

In case of equal Bridge Priorities, MAC address is the Tie-breaker and Lowest MAC Address Wins

17

B

A

C

E D

Priority 4096

Priority 32768 Priority 32768

Priority 32768 Priority 32768


IDNTIFYING ROOT PORT

The port with lowest path cost to reach Root Bridge is the Root Port

Path cost is the cumulative cost to reach the Root Bridge

Path cost info is mentioned in the BPDUs and the BPDU with lowest path cost is considered as Superior BPDU and others are considered as inferior BPDUs

If path cost is equal, then select the port connected to neighbor switch with lowest Bridge ID

If all the paths go through the same neighboring switch then local port that receives the lowest Port ID on it will become the Root Port

Each switch can have only one root port

Root Bridge does not have a Root Port

18


IDENTIFYING DESIGNATED PORT

Only one designated port for each network segment

Forwards BPDU frames to that network segment

If two ports are eligible to become Designated port, then there is a loop and one needs to be placed in the Blocking state

Designated ports are determined by the lowest cumulative path cost

If path cost is equal, then the Bridge-ID is the Tie-Breaker

19


STP PORT STATES

BLOCKING:

Initially all ports are placed in the BLOCKING State

Will not learn MAC address.

Listen to BPDUs but will not send

LISTENING:

Port will listen and send BPDUs to participate in Root Bridge, Root/Designated port election

Port will neither learn MAC nor will it forward the Frame

LEARNING:

Port continues to send and receive BPDUs

Learns MAC addresses but doesn’t forward the frame yet

FORWARDING:

Port will send/receive the BPDUs, Learn MAC Addresses and Forward the frames

DISABLED:

Port Administratively Shut Down

20


STP TIMERS

HELLO TIMER:

Determines how often switch sends BPDUs

BPDUs are sent every 2 seconds, by default

MAX AGED TIMER:

Determines how long BPDU info is retained

Default duration: 20 Sec

FORWARDING DELAY:

Introduced to ensure that STP has enough time to detect and eliminate the loop

Default Duration: 15 Sec

Occurs Twice in the convergence process

21


RAPID SPANNING TREE PROTOCOL (RSTP)

30-50 seconds of convergence delay (in STP) is not acceptable today

To eliminate this drawback, some modifications were made to the protocol, resulting in new RFC standard- RFC 802.1w

RSTP as well elects the Root Bridge and identifies Root/Designated Port

22


RSTP- PORT ROLES

Root Port:

Port with Best path cost to reach Root Bridge

Alternate Port:

Backup root port that has a less desirable path cost

Designated Port:

Non-root port that represent the best path cost to reach Root port in each Network Segment

Backup Port:

Designated port with less desirable path cost

23


RSTP- STATES

DISCARDING:

Neither will forward frames nor will learn MAC

Listens to the BPDUs

Alternate and Backup ports are in Discarding state

LEARNING:

Learn MAC Addresses

Doesn’t forward Frames

FORWARDING:

Send/receive BPDUs, Learn MAC address and Forward frames

Root and Designated Ports are placed in Forwarding state

24


ETHERCHANNEL- WHY?

Multiple switches are connected to each other in a network

They are connected through a trunk port to extend the VLANs to other switch

Only one Trunk port is used hence there is no redundancy and a hinder that all other (access) ports will send the traffic on only one port to send the traffic across to the other switch

If multiple ports are used,

Either spanning tree will block one or more ports to avoid loops

Or If STP is disabled, then there will be a Switching loop

25


PORT AGGREGATION

Port aggregation is bundling multiple Physical ports to form a single logical port

Provides redundancy (without placing any port in Blocking state) and provides high Bandwidth

Cisco’s implementation of Port Aggregation is called EtherChannel

EtherChannel supports Fast Ethernet, Gigabit Ethernet and 10 G Ethernet ports

Maximum of 8 ports can be bundled on one EtherChannel

EtherChannel can be configured on Access, Trunk and even on Layer 3 ports

26


ACTIVE ETHERCHANNEL

All the ports must be configured identically for an EtherChannel to become Active

Following configurations must be identical on all the ports in an EtherChannel

Speed

Duplex

VLAN configurations including allowed VLANs on ports

Trunking Encapsulation Protocol

27


CONFIGURING ETHERCHANNEL

Two ways to do it Manual- Admin

SW1(config)#interface range gi0/1-4 Select a Range of ports

SW1(config-if)# channel-group 1 mode on logical grouping

Do the same config on other switch

Make sure to have all settings equal

Channel-group number is the port channel identifier

Gi0/1-4 Port-channel 1

Dynamic- Aggregation Protocols

Port Aggregation Protocol (PAgP)- Cisco Proprietary

Link Aggregation Control Protocol (LACP)- IEEE 802.3ad

28


PORT AGGREGATION PROTOCOL

Modes of Operation:

Auto- Waits for remote switch to initiate a channel

Desirable- Actively tries to form a channel

Port Channel will Form when…

Switch A (Desirable) (Desirable) Switch B

Switch A (Desirable) (Auto) Switch B

Port channel will not form if…

Both switches are configured with Auto settings

One switch is configured with Desired and other Manually or with LACP

29


LINK AGGREGATION CONTROL PROTOCOL

Modes of Operation:

Passive- Waits for remote switch to initiate a channel

Active- Actively tries to form a channel

Port Channel will Form when…

Switch A (Active) (Active) Switch B

Switch A (Active) (Passive) Switch B

Port channel will not form if…

Both switches are configured with Passive settings

One switch is configured with Active and other Manually or with PAgP

Along with 8 active ports, adding 8 more ports in standby state is allowed in LACP

30


FIRST HOP REDUNDANCY PROTOCOL

31

Three main First Hope Redundancy Protocols

Hot Standby Redundancy Protocol

Virtual Router Redundancy Protocol

Gateway Load Balancing Protocol


HOT STANDBY REDUNDANCY PROTOCOL

It provides a layer 3 redundancy to a network where default gateway is configured

Multiple routers are deployed at the edge of the network and along with their physical address (MAC and IP) configured on the interfaces they are assigned with a Virtual IP and MAC address

Routers within same HSRP group must be assigned with same group number

It’s a Redundancy protocol and not a Load Balancing protocol

32


ROUTER ROLES

ACTIVE ROUTER: Currently serving as a gateway. Elected on the basis of Priority. Highest priority router becomes Active

STANDBY ROUTER: Backup router who will assume the services of Active router after failover. Router with second highest priority becomes Standby

LISTENING ROUTER: All other routers participating in HSRP

33

INSIDE NETWORK

R1

R2


HSRP STATES

Disabled: admin down/ HSRP not configured

Initial: Router begins in this state once its configured for HSRP

Learn: When HSRP does not know Virtual IP

Listen: Knows VIP but is not elected as Active/standby

Speak: Participating in Active/Standby election on the basis of Hello packets

Standby: Elected as Standby and exchanging hello packets with Active Router

Active: Elected as Active and exchanges hellos with Standby

34


HSRP CONFIGURATION

RTR1(config)# int fa0/0

RTR1(config-if)# standby 1 ip 10.0.0.3

RTR1(config-if)# standby 1 priority 110

RTR1(config-if)# standby 1 preempt

10.0.0.3 VIP

1 Group

35


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