CSEE W4140 Networking Laboratory Lecture 8: LAN Switching Jong Yul Kim 03.25.2009.

CSEE W4140Networking Laboratory

Lecture 8: LAN Switching

Jong Yul Kim03.25.2009

Announcements

Reminder of lab rules

Labs are mandatory.

Don’t connect rack machines to Internet.

Don’t bring food / drinks to the lab.

Announcements Grades

Will be uploaded today

Field Trip Two separate groups / dates

Projects Projects in place of finals? Please come see me after class

Short review of midterm

Today’s lecture

Hubs

Switches Learning algorithm Spanning Tree Protocol

Various equipments are used to interconnect networks

X.25 Network

EthernetHub

EthernetHub

Hosts Hosts

RouterBridge

Token-ring

Gateway

We already know routers Routers operate at the Network Layer (Layer 3) Interconnect different subnetworks

Subnet-work

Router

Subnet-work

Router

Subnet-work

Application

TCP

IP

NetworkAccess

Application

TCP

IP

NetworkAccess

IP protocol IP protocol

DataLink

NetworkAccess

IP

NetworkAccess

NetworkAccess

IP

NetworkAccess

DataLink

DataLink

IP protocol

RouterRouter HostHost

We’ve heard about gateways The term Gateway is used with different meanings in

different contexts

Gateway is a generic term for routers (Level 3) “Default gateway”

Gateway is also used for a device that interconnects different Layer 3 networks and which performs translation of protocols (“Multi-protocol router”)

SNANetwork

Gateway

IP Network

Gateway

X.25Network

Ethernet Hub A simple repeater (extends the physical cable) Frame collisions are propagated Good for sniffing traffic in a network we want to monitor

IP

LLC

802.3 MAC

IP

LLC

802.3 MACHubHub

EthernetHub

EthernetHub

HostHost

Bridges/LAN switches A bridge or LAN switch is a device that interconnects two or more Local

Area Networks (LANs) and forwards packets between these networks.

Bridges/LAN switches operate at the Data Link Layer (Layer 2)

BridgeIP

LLC

802.3 MAC 802.3 MAC 802.5 MAC

LLC

IP

LLC

802.5 MACLAN LAN

Token-ring

Bridge

Terminology: Bridge, LAN switch, Ethernet switch

There are different terms to refer to a data-link layer interconnection device:

The term bridge was coined in the early 1980s. Today, the terms LAN switch or (in the context of

Ethernet) Ethernet switch are used.

Convention: Since many of the concepts, configuration

commands, and protocols for LAN switches were developed in the 1980s, and commonly use the old term `bridge’, we will, with few exceptions, refer to LAN switches as bridges.

Internet

A Switched Enterprise Network

Router

Switch

Bridges versus RoutersRouters

Each host’s IP address must be configured

If network is reconfigured, IP addresses may need to be reassigned

Routing done via RIP or OSPF

Each router manipulates packet header (e.g., reduces TTL field)

Bridges

MAC addresses are hardwired

No network configuration needed plug-and-play!

No routing protocol needed (sort of) learning bridge algorithm spanning tree algorithm

Bridges do not manipulate frames

Frame Forwarding Each bridge maintains a forwarding table

with entries< MAC address, port, age> MAC address: host name or group address

port: port number of bridgeage: aging time of entry

with interpretation: a machine with MAC address lies in direction of

the port number from the bridge. The entry is age time units old.

Assume a MAC frame arrives on port x.

Frame Forwarding

Bridge 2Port A Port C

Port x

Port B

Is MAC address of destination in forwardingtable for ports A, B, or C ?

Forward the frame on theappropriate port

Flood the frame, i.e.,

send the frame on all ports except port x.

Found?Notfound ?

Routing tables entries are set automatically with a simple heuristic:

The source field of a frame that arrives on a port tells which hosts are reachable from this port.

Learning Algorithm

Port 1

Port 2

Port 3

Port 4

Port 5

Port 6

Src=x, Dest=ySrc=x, Dest=y

Src=x, Dest=y

Src=x, Dest=y

Src=x, Dest=y

Src=x, Dest=y

x is at Port 3

Src=y, Dest=x

Src=y, Dest=xSrc=x, Dest=y

y is at Port 4

Src=x, Dest=y

Algorithm: For each frame received, the source

stores the source field in the forwarding database together with the port where the frame was received.

All entries are deleted after some time (default is 15 seconds).

Learning Algorithm

Learning Algorithm Example

Bridge 2

Port1

LAN 1

A

LAN 2

CB D

LAN 3

E F

Port2

Bridge 2

Port1 Port2

•Consider the following packets: (Src=A, Dest=F), (Src=C, Dest=A), (Src=E, Dest=C)

•What have the bridges learned?

Consider the two LANs that are connected by two bridges.

Assume host n is transmitting a frame F with unknown destination.

What is happening? Bridges A and B flood the frame

to LAN 2. Bridge B sees F on LAN 2 (with

unknown destination), and copies the frame back to LAN 1

Bridge A does the same. The copying continues

Where’s the problem? What’s the solution ?

Danger of Loops

LAN 2

LAN 1

Bridge BBridge A

host n

F

F F

FF

F F

A solution is to prevent loops in the topology

IEEE 802.1d has an algorithm that builds and maintains a spanning tree in a dynamic environment

Bridges that run 802.1d are called transparent bridges

Bridges exchange messages to configure the bridge (Configuration Bridge Protocol Data Unit, Configuration BPDUs) to build the tree.

Using the BPDUs, each bridges makes a local decision which of its ports are part of the spanning tree

Spanning Tree Protocol (STP)

Diagram of a spanning tree

Disabled ports

Forwarding ports

Concepts Each bridge as a unique identifier:

Bridge ID = <priority number + MAC address>Note that a bridge has several MAC addresses

(one for each port), but only one ID * lower priority number has higher priority (The lower the better!!)

Each port within a bridge has a unique identifier (port ID).

Root Bridge The bridge with the lowest identifier is the root of

the spanning tree. Root Port

Each bridge has a root port which identifies the next hop from a bridge to the root.

Concepts Root Path Cost

For each bridge, the cost of the min-cost path to the root.

The lower the better!!

Designated Bridge, Designated Port Lowest cost bridge on the segment is the designated

bridge. On the designated bridge, the port that is attached to

the segment is the designated port.

if two bridges have the same cost, select the one with highest priority

if the min-cost bridge has two or more ports on the LAN, select the port with the lowest identifier

Configuration BPDUs

time since root sent a

message on

which this message is based

DestinationMAC address

Source MACaddress

ConfigurationMessage

protocol identifier

version

message type

flags

root ID

Cost

bridge ID

port ID

message age

maximum age

hello time

forward delay

Set to 0 Set to 0Set to 0

lowest bit is "topology change bit (TC bit)

ID of root Cost of the path from the

bridge sending this

message

priority of configurable interface

(used for loop detection)

ID of bridge sending this message

Time between

recalculations of the

spanning tree

(default: 15 secs)

Time between

BPDUs from the root

(default: 1sec)

Steps of Spanning Tree Algorithm

1. Determine the root bridge2. Determine the root port on all other

bridges3. Determine the designated port on each LAN

Each bridge is sending out BPDUs that contain the following information:

root bridge (what the sender thinks it is) root path cost for sending bridge

Identifies sending bridge

root IDroot ID costcost bridge ID/port IDbridge ID/port ID

Initially, all bridges assume they are the root bridge.

Each bridge B sends BPDUs of this form on its LANs:

Each bridge looks at the BPDUs received on all its ports and its own transmitted BPDUs.

Root bridge is the smallest received root ID that has been received so far (Whenever a smaller ID arrives, the root is updated)

Determine the Root Bridge

BB 00 BB

At this time: A bridge B has a belief of who the root is, say R.

Bridge B determines the Root Path Cost (Cost) as follows:

If B = R : Cost = 0. If B R: Cost = {Smallest Cost in any of BPDUs that were

received} + cost of B’s interface where this BPDU was received

B’s root port is the port from which B received the lowest cost path to R

Knowing R and Cost, B can generate its BPDU (but will not necessarily send it out)

Calculate the Root Path CostDetermine the Root Port

RR CostCost BB

Default Cost

Ethernet Speed Cost

10 Mbps 100

100 Mbps 19

1 Gbps 4

10 Gbps 2

At this time: B has generated its BPDU

B will send this BPDU on one of its ports, say port x, only if its BPDU is lower than any BPDU that B received from port x.

In this case, B also assumes that it is the designated bridge for the LAN to which the port connects.

Calculate the Root Path CostDetermine the Root Port

RR CostCost BB

Selecting the Ports for Spanning Tree

Each bridges makes a local decision which of its ports are part of the spanning tree

B will decide which ports are in the spanning tree: B’s root port is part of the spanning tree All designated ports are part of the spanning

tree All other ports are not part of the spanning tree

B’s ports that are in the spanning tree will forward packets (=forwarding state)

B’s ports that are not in the spanning tree will not forward packets (=blocking state)

Homework

Prelab 6 due this Friday

Lab reports due this week

Main Points of Lab 6

Hubs vs. switches

Switches Learning algorithm Spanning Tree Protocol