Version 2

Alp ISIKAlp ISIK

Netas Enterprice NTS EngineerNetas Enterprice NTS Engineer

Ethernet Transmission

• Half-duplex transmission– Data sent in one direction at a time– Results in collisions– Uses CSMA/CD to resolve collisions– Hubs are the most common half-duplex devices

• Full-duplex transmission– Data sent in both directions at the same time– Requires point-to-point connections– No collisions– An approach to higher network efficiency – Switches are the most common full-duplex

devices

Half-Duplex Operation (CSMA/CD )

– All hosts constantly listen to the line.

– Host A transmits.

– Hosts B, C, and D listen to host A and do not transmit.

– All hosts receive host A’s message.

Hub

A B C D

Half-Duplex Operation (CSMA/CD)

– All hosts constantly listen to the line.

– Host A and host B transmit simultaneously.

– Messages collide.

– Both hosts back off for a random time interval.

Hub

A B C D

Full-Duplex Operation

– Attached to a dedicated switched port

– Requires full-duplex support on both ends

– Collision-free

Switch

A B C ED

Auto-Negotiation

• Ethernet’s negotiable operation

– Speed

• 10 Mb/s

• 100 Mb/s

• 1000 Mb/s

– Operation mode

• Half-duplex (CSMA/CD)

• Full-duplex

• If auto-negotiation is enabled, Ethernet nodes connected by a twisted pair cable negotiate their speed as well as duplex mode prior to establishing a link.

1 Collision Domain,1 Collision Domain,1 Broadcast Domain1 Broadcast Domain

Resource

2 Collision Domains, 2 Collision Domains, 1 Broadcast Domain1 Broadcast Domain

Router

3 Broadcast Domains,3 Broadcast Domains,3 Collision Domains3 Collision Domains

Bridge

Switch

Network Domains

Collision Domains

In this figure, there are 8 collision domains and 3 broadcast domains.

Hub

Hub

Hub

Hub

Hub

Hub

Switch

Switch

Router

Collision

Domain

Collision

Domain

Collision

Domain

Collision

Domain

Collision

Domain

Collision

Domain

Collision

Domain

Collision

DomainBroadcast

Domain

Broadcast

Domain

Broadcast

Domain

Switch

11 22

Host A

00 00 A2 00 00 01

Host B

00 00 A2 00 00 02

Switch Forwarding TableNode MAC Address00 00 A2 00 00 0100 00 A2 00 00 02

Interface12

Switching

1/2

1/1

1/3

1/4

1/1

1/2

1/3

1/4

Host A

0000.8c01.000A

Host B

0000.8c01.000B

Host C

0000.8c01.000C

Host D

0000.8c01.000D

Step 1: Host A sends a frame to Host B.

Step 2: The switch receives the frame on 1/1 and places source in MAC table.

Step 3: The destination is not in the MAC table so the switch forwards the frame to all ports except the source.

Step 4: Host B responds to Host A. The switch adds the source address of Host B to the MAC table.

Step 5: Host A and Host B can now send unicast frames bidirectionally.

Step 6: Similarly, Host C and Host D will send frames and populate the MAC table. Step 2

Step 4

0000.8c01.000A

0000.8c01.000B

0000.8c01.000C

0000.8c01.000D

Step 6

Building Up the MAC Forward/Filter Table

Spanning Tree Protocol 802.1

Spanning Tree Protocol — What Is It?

– Link management protocol that is part of IEEE 802.1

– Spanning tree algorithm provides path redundancy in Ethernet bridge/switch networks

– Provides 1 active path at a time between 2 bridges or switches

– Provides backup paths to the active path, should the active path fail

– Primary function is to avoid looping in redundant path Ethernet networks

Redundant Topology — Without STP

• Redundancy

– Advantages:

• Necessary for the link of a switch failover

• Load balancing

– Disadvantages:

• May cause broadcast storms

• May cause multiple frame copies to be sent

• May cause FDB table instability

• Frame looping problems

– Layer 2 has no mechanism to stop looping as layer 3 has with TTL

Receiving Multiple Copies

Segment 1

Segment 2

Host X Host Y

Switch 1 Switch 2

Database Instability

Segment 1

Segment 2

Host A

Unicast Unicast

Port 0

Port 1

Port 0

Port 1

Host B

MAC Address DB

Host A Port 0

MAC Address DB

Host A Port 0

Host A Port 0 Host A Port 1

Switch 1 Switch 2

Broadcast Storms

Segment 1

Segment 2

Host X

Broadcast

Host Y

Switch 1 Switch 2

STP and IEEE 802.1d

– STP is defined in 802.1d as a link management protocol

– Initially developed in 1990, based on the ISO/IEC 10038 standard

– Designed to provide path redundancy in Ethernet bridge/switch-based networks, while preventing loops

– STP uses a root/branch/leaf model, which determines a single path to each leaf spanning the entire L2 network

– End hosts (e.g., PCs) are oblivious to STP and instead see a single LAN segment

STP Port States

• All ports in an STP network go through the following states.

Initialization

Blocking

Listening

Learning

Forwarding

Disabled

STP Port States and Activities

STP port state Part of active topology

Learning of MAC addresses

Disabled No No

Blocking

Listening

Learning No Yes

Forwarding Yes Yes

STP in Action: State 2 — Root Bridge/Switch Election

• Root bridge/switch election calculation:

• After bridges/switches have initialized and all ports are in blocking mode, root bridge election occurs.

• Each bridge/switch has a user-assigned bridge priority.

• The bridge priority ranges from 0 to 65 535 (default is 32 768).

• Each bridge/switch sends its BID to every other bridge/switch. The BID is 8 bytes: 2 for bridge priority and 6 that contain the MAC address of the bridge/switch.

• Election of the root bridge is determined using the Bridge ID, which is made up of the Priority and MAC address

– the switch with lowest Bridge ID value is selected

• Any subsequent physical change in the network after election of the root bridge will cause an STP recalculation.

STP in Action: State 2

Host A

Host B

Boot UpBoot Up

Boot UpBoot Up

Boot UpBoot Up

Boot UpBoot Up

State 2 — Blocking

BPDU

BP

DU

BP

DU

BPDU

BPDU

BPDU

BP

DU

BP

DU

RootBridge/Switch

RootBridge/Switch

RootBridge/Switch

RootBridge/Switch

B

A

C

D

STP in Action: State 2 — Root Bridge/Switch Election

Host A

Host B

State 2 — Blocking

RootBridge/Switch

RootBridge/Switch

RootBridge/Switch

RootBridge/Switch

Priority - 32MAC - 00-80-21-00-00-10

Priority - 16MAC - 00-80-21-00-00-30

Priority - 48MAC - 00-80-21-00-00-20

Priority - 16MAC - 00-80-21-00-00-40

B

A

C

D

STP in Action: State 2 — Root Bridge/Switch Election

Host A

Host B

RootBridge/Switch

LeafBridge/Switch

LeafBridge/Switch

LeafBridge/Switch

Priority - 32MAC - 00-80-21-00-00-10

Priority - 16MAC - 00-80-21-00-00-30

Priority - 48MAC - 00-80-21-00-00-20

Priority - 16MAC - 00-80-21-00-00-40

BP

DU

BP

DU

BPDU

BPDU BPDU

BP

DU

BP

DU

BPDU

B

A

C

D

STP in Action: State 2 — Path Calculation

• Each port on a bridge/switch has a path cost value assigned, depending on bandwidth.

• The accumulated path cost determines the total cost to reach the root bridge/switch.

• Path cost values can be found in the IEEE 802.1d standard.

Link bandwidth

STP cost value

4 Mb/s 250

10 Mb/s 100

16 Mb/s 62

45 Mb/s 39

100 Mb/s 19

155 Mb/s 14

622 Mb/s 6

1 Gb/s 4

10 Gb/s 2

STP in Action: State 2 — Path Calculation

Host A

Host B

Root

Leaf

Leaf

Leaf

BP

DU

BP

DU

BPDU

BPDU BPDU

BP

DU

BP

DU

BPDU

Path Cost 2

Path Cost 10

Path Cost 10

Path Cost 10

B

A

C

D

STP in Action: State 2 — Calculating Forwarding Paths

Host A

Host B

Root

LeafLeaf

Leaf

Path Cost 2

Path Cost 10

Path Cost 10

Path Cost 10

Root Port

Designated Port

Designated Port

Designated Ports

Root Port

Root Port

B

A

C

D

STP in Action: State 3 — Listening State

FDB

Forwarded Traffic

BPDUs

NM Messages

Bridge/Switch

STP in Action: State 4 — Learning State

FDB

Forwarded Traffic

BPDUs

NM Messages

Bridge/Switch

STP in Action: State 5 — Final Forwarding Paths

Host A

Host B

Root

Leaf

Leaf

Leaf

Path Cost 2

Path Cost 10

Path Cost 10

Path Cost 10

B

A

C

D

STP in Action — Topology Change (Deleting a Link)

Host A

Host B

Root

Leaf

Leaf

Leaf

Path Cost 2

Path Cost 10

Path Cost 10

Path Cost 10 A

B C

D

Wait 20 seconds (Max

age time)

BPDU

BP

DU

Designated Ports

Root Port

Listen (15 seconds)Learn (15 seconds)

STP in Action — Topology Change (Path Cost Change)

Host A

Host B

Root

Leaf

Leaf

Leaf

Path Cost

Path Cost 10

2Path Cost

Path Cost 10

Path Cost Change

1 — TCN BPDU sent to Root

2 — Reply w/TCA BPDU set

3 — Topology changed

10 BPDU

BP

DU

TBPDU TBPDU

BPDU

BP

DU

BPDUT

BP

DU

TB

PD

U

TB

PD

U

TB

PD

U Listen (15 seconds)

Learn (15 seconds)

B

A

D

C

STP in Action — Topology Change (Adding a Switch)

Host A

Host B

Root

Leaf

Leaf

Leaf

Path Cost 2

Path Cost 10

Path Cost 10

Path Cost 10

Pa

th C

os

t 10

Path Cost 10

Priority - 16MAC - 00-80-21-00-00-30

Priority - 16MAC - 00-80-21-

00-00-10

BP

DU

BPDU

BPDU

B

D

C

EA

New Root

BPDU

BPDU

BP

DU

BP

DU

Designated Ports

Root Port

Leaf

New switch E added

All ports in listening state

New BPDUs sent

New root switch elected

Final topology

STP and BPDU• The root bridge/switch sends

STP messages via BPDUs to the branches/leaves.

• On individual branches and leaves, the user can specify IDs and path costs.

• The root bridge/switch sets the forwarding delay, hello time, and maximum age.

• BPDU is sent in Ethernet frame with the port’s address as source and the STP Multicast address 01:80:C2:00:00:00 as destination

Protocol ID (2 bytes)Version (1 byte)

Message type (1 byte)Flags (1 byte)

Root ID ( 8 bytes)

Path cost (4 bytes)

Bridge ID (8 bytes)

Port ID (2 bytes)

Message age (2 bytes)

Maximum age (2 bytes)

Hello time (2 bytes)

Forwarding delay (2 bytes)

BPDU Packet

BPDU Packet Details

Protocol ID Always set to 0

Version Always set to 0

Message type Determines which of two BPDU types; configuration or TCN

Flags Handle changes in the active topology

Root ID Contains the bridge ID of root bridge (after convergence, all BPDUs should contain the same value)

Root path cost Cumulative path cost of all links to the root bridge

Bridge ID Identifies the bridge that is transmitting the current configuration message

Port ID Contains a unique value for each port

Message age Time stamp since the root bridge created this BPDU

Maximum age Maximum amount of time this BPDU is saved

Hello time Time between configuration BPDUs

Forwarding delay Time spent in the listening and learning states

Configurable on each bridgeConfigurable on root bridge

Spanning Tree Exercise

• Highlight the steps that will ensure that Switch D is added to the existing Bridge topology using STP

Priority - 16

MAC - 00-80-21-00-00-10

Priority - 16

MAC - 00-80-21-00-00-20

Priority - 16

MAC - 00-80-21-00-00-30

Priority - 16

MAC - 00-80-21-00-00-40

A

B C

D

10

10

10

10

10

Rapid Spanning Tree

What is RSTP?

• What is RSTP?

– Stands for rapid spanning tree protocol

– An evolution to the loop prevention algorithm (STP) from 802.1d

– New IEEE specification is 802.1w

– Achieves rapid failover and convergence times

– Unlike STP, RSTP is not timer-based

– Allows backward compatibility with 802.1d STP

• Why do we need RSTP?

– Network topology convergence is significantly faster than STP

STP port state

RSTP port state

Part of active

topology

Learning of MAC

addresses

Disabled Discard No No

Blocking

Listening

Learning Learning No Yes

Forwarding

Forwarding Yes Yes

STP vs. RSTP — Port States

Port states STP port role (assigned by STP algorithm)

RSTP port role

(configurable)

Forwarding Root Root

Designated Designated

Blocking Blocked Backup

Blocked Alternate

• Role — A new variable assigned to a bridge port

STP vs. RSTP — Port Roles

Alternate Port

Root

Root Port Root Port

Designated PortDesignated Port

Designated PortAlternate Port

BPDU

Backup Port

Root

Root PortRoot Port

Designated PortDesignated Port

Designated PortAlternate Port Backup Port

BPDUBPDU

RSTP BPDU Format

Protocol ID (2 bytes)

Version (1 byte)

Message type (1 byte)

Flags (1 byte)

Root ID ( 8 bytes)

Path cost (4 bytes)

Bridge ID (8 bytes)

Port ID (2 bytes)

Message age (2 bytes)

Maximum age (2 bytes)

Hello time (2 bytes)

Forwarding delay (2 bytes)

Version 1 length (2 bytes)

Configurable

Configurableon root bridge

Bit 0 – Topology change

Bit 1 – Proposal

Bit 2, 3 – Port role0 0 Unknown

0 1 Alternate/backup

1 0 Root

1 1 Designated

Bit 4 – Learning

Bit 5 – Forwarding

Bit 6 – Agreement

Bit 7 – Topology change ACK

STP RSTP

BPDU handling

Non-root bridge only transmits BPDUs when it receives one on the root port

Bridge sends BPDU at hello time intervals

Aging BPDU is aged after the max-age timer expires (and no BPDU is received on the port)

BPDUs are used like keepalive messages (after 3 BPDUs in a row are missed it ages it out)

Accepting inferior BPDUs

— Inferior BPDU is accepted and previously stored information is replaced

Transition to forwarding state

Based on timers (Forward Delay and Max-Age)

Uses a feedback mechanism (no timers involved)

STP vs. RSTP — BPDUs

STP RSTP

Topology change notification

Sends TCN BPDUs toward root

Sends BPDUs (with TC bit set) on all designated and root ports

Topology ACKs

Replies with BPDU with TCA bit set

No acknowledgement (clears MAC addresses on all ports)

Topology change

First sent to root bridge/switch, then relayed from root all the way to the leaf bridge/switch

1-step process (topology change flooded quickly across the network)

STP vs. RSTP — Topology

Virtual LAN

Switches and VLANs

– A VLAN permits a group of ports to share a common broadcast domain regardless of physical location.

– A VLAN can reside on 1 switch or on many switches.– A port that is not in a specific VLAN is in a default VLAN, and

thus in a different broadcast domain.– Each VLAN is identified by a VLAN ID.– Devices in different VLANs can only communicate with each

other if the frame is first sent to a layer 3 device (a router).

Why VLANs?

There are two main reasons for the development of VLANs:

The amount of broadcast traffic and increased security.

Broadcast traffic increased in direct proportion to the number of stations in the

LAN. The goal of the VLAN is the isolation of groups of users so that one group

is not interrupted by the broadcast traffic of another.

VLANs also have the benefit of added security by separating the network into

distinct logical networks. Traffic in one VLAN is separated from another VLAN

as if they were physically separate networks. If traffic is to pass from one VLAN

to another, it must be routed.

VLAN 101VLAN 102VLAN 103

Ethernet switch

Internal switchVLAN 101

Internal switchVLAN 102

Internal switchVLAN 103

Port 1

Port 2

Port 3

Port 5

Port 6

Port 7

How Do VLANs Work?

VLAN 101

Host 1 sends out a broadcast. Which hosts will receive the broadcast?

VLAN Exercise

Switch 1

VLAN 102

VLAN 102

VLAN 101

Host 1

Host 2

Host 3

Host 4

BPDU

BPDU

Switch 1

Switch 2 Switch 3

VLAN 101VLAN 102VLAN 103

VLANs across Multiple Switches

VLANs over Multiple Switches

Switch 1

Switch 2

MAC FDB VLAN 101

MAC FDB VLAN 102

MAC FDB VLAN 103

MAC FDB VLAN 101

MAC FDB VLAN 102

MAC FDB VLAN 103

VLAN 101VLAN 102VLAN 103

Separate Physical

Interfaces

VLAN Trunking

Switch 1

Switch 2

VLAN 101VLAN 102VLAN 103

MAC FDB VLAN 101

MAC FDB VLAN 102

MAC FDB VLAN 103

MAC FDB VLAN 101

MAC FDB VLAN 102

MAC FDB VLAN 103

SFDPre-amble DA SA Length

/Type P a y l o a d (46 to 1500 bytes) FCS

802.1q tag type (value 81 00) Tag control information

2 bytes2 bytes 2 bytes2 bytes

CFICFI (Canonical format: bit ordering can be different)(Canonical format: bit ordering can be different) CFICFI (Canonical format: bit ordering can be different)(Canonical format: bit ordering can be different)

User_priorityUser_priorityUser_priorityUser_priority VLAN_IDVLAN_IDVLAN_IDVLAN_ID

3 bits3 bits3 bits3 bits 1 bit1 bit1 bit1 bit 12 bits12 bits12 bits12 bits

Length of the Length of the MAC frame + 4 bytesMAC frame + 4 bytes

Length of the Length of the MAC frame + 4 bytesMAC frame + 4 bytes

VLANtag

802.1q Ethernet FrameVLAN Tagging

Multiple Spanning Tree Protocol

Multiple Spanning Tree Protocol (MSTP)

– What is MSTP?– Why do we need MSTP?– Differences: MSTP vs. STP– Where to use MSTP

• Example

Multiple Spanning Tree Protocol• What is MSTP?

– An IEEE standard that allows more than one instance of STP– A natural progression from RSTP, introduced in 2003 as part of

802.1s• Why do we need MSTP?

– Allows load balancing of network between different sets of VLANs

– Allows a set of VLANs to run a single instance of the spanning tree while another set runs another instance of the spanning tree

– Some early versions of MSTP, before 802.1s, used a single STP instance per VLAN, which was very CPU-intensive. MSTP lowers CPU usage in these instances.

– Reduce overhead of BPDUs as otherwise they're sent for every VLAN

– Interoperability– Scalabitility

Switch A

Switch CSwitch B

R

D

A

VLAN 1-500

VLAN 501-1000

Root

LeafLeaf

D

R

D

D - DesignatedR - RootA - Alternate

Port States

Standard STP

D - DesignatedR - RootA - Alternate

Port StatesSwitch A

Switch CSwitch B

R

D

A

VLAN 1-500VLAN 501-1000

D

R

D

D

R D

D

A R

MSTP

Spanning Tree Protocol Group (STG)

Multiple STGs provide multiple data paths, which can be used for

load-sharing and redundancy. Enable load sharing between two

switches using multiple STGs by configuring each path with a

different VLAN and then assigning each VLAN to a separate STG.

Each STG is independent. Each STG sends its own Bridge Protocol

Data Units (BPDU), and you must independently configure each

STG. The tagging for the BPDUs from STG1, or the default STG, is

user-configurable (as are tagging settings for all STGs). However,

by default STG1 sends only untagged BPDUs to operate with all

devices that support only one instance of STP. (By default, STG2

through STG8 are tagged.) The tagging setting for each STG is

user-configurable.

Spanning Tree Groups and VLANs

• VLANs are a subset of the STG– With the setup below connectivity to VLAN-4 across the switches

is lost.– Solution is to create a trunk links between the switches

STG-1 STG-1

VLAN-3 VLAN-3

VLAN-4 VLAN-4Blocked

Spanning Tree Groups and VLANs

STG-1 STG-1

STG-1 STG-1

SW1 SW2

SW3 SW4

VLAN-3 VLAN-3

VLAN-3

VLAN-4

VLAN-4

VLAN-4

VLAN-4

TrunkLink

Failure

Port Blocked

Root

Spanning Tree Groups and VLANs

STG-1 STG-1

STG-1 STG-1

SW1 SW2

SW3 SW4

VLAN-3 VLAN-3

VLAN-3

VLAN-4

VLAN-4

VLAN-4

VLAN-4

TrunkLink

Failure

CreateVLAN-3Without

Access Ports

Root

Why have multiple STG

STG-1 STG-1

STG-1 STG-1

SW1 SW2

SW3 SW4

VLAN-3 VLAN-3

VLAN-3

Root

Gigabit Link

Gigabit Link

Gigabit LinkGigabit Link

With a single STG configured a Gig port is not utilised as it is in a blocking state

VLAN-3

VLAN-4

VLAN-4

VLAN-4

VLAN-4

VLAN-3

Why have multiple STG

STG-1 STG-1

STG-1 STG-1VLAN-3 VLAN-3

VLAN-3

VLAN-4

VLAN-4

VLAN-4

VLAN-4

Gigabit Link

Gigabit Link

Gigabit LinkGigabit Link

With VLAN-3 in STG1 and VLAN-4 in STG-2 all links in the network are now being utilised

STG-2 STG-2

STG-2STG-2

blocking In STG-2blocking in STG-1

Tagged BPDUs

• In the previous slide BPDU’s were being passed across a tagged link. On the Passport switch :– STG–1 BPDU are always untagged. This is

necessary inorder for the 8600 to be compatiable with other vendor switches.

– All other STG BPDU’s when passed across tagged links are tagged

Vlan 10

Vlan 20

Vlan 10

Vlan 10

Vlan 20

Vlan 10

Vlan 10

Vlan 20

STG 1

STG 2

Spanning Tree Fast Learning

• Enhanced port mode supported by theNortel.

If you enable Spanning Tree Fast Learning on a port with no other bridges, the port starts more quickly after a switch initialization or a spanning tree change. The port passes through the normal blocking and learning states before the forwarding state, but the hold times for these states is the bridge hello timer (2 seconds by default) instead of the bridge forward delay timer (15 seconds by default). The port configured with Fast Learning can forward data immediately, as soon as the switch learns that the port is enabled.

• Fast Learning is intended for access ports in which only one device is connected to the switch (as in workstations with no other spanning tree devices). For these ports, it is not desirable to wait the usual 30 to 35 seconds for spanning tree initialization and bridge learning.

ATTENTION

If trunk ports are STP-enabled, ensure that all

potential trunk members are connected to their

corresponding members; otherwise, STP cannot

converge correctly, and traffic loss can result.

Troubleshooting• Verifiying the STG BPDU’s

- show port stat stg

• Verifying the Vlan settings– show vlan info …

• Displaying the Forwarding DataBase– show vlan info fdb-entry or fdb-static…

• Verifying the STG– show stg info config [<sid>]– show stg status config [<sid>]

• Verifying the port status– show ports error … or stats …– monitor ports error … or stats …