Communication Networks

Recitation 3Bridges & Spanning trees

Bridges

Link layer device stores and forwards Ethernet frames examines frame header and selectively

forwards frame based on MAC dest address when frame is to be forwarded on segment,

uses CSMA/CD to access segment transparent

hosts are unaware of presence of bridges plug-and-play, self-learning

bridges do not need to be configured

Some bridge features Isolates collision domains resulting in higher

total max throughput limitless number of nodes and geographical

coverage Can connect different Ethernet types Transparent (“plug-and-play”): no

configuration necessary

Bridges: traffic isolation

Bridge installation breaks LAN into LAN segments

bridges filter packets: same-LAN-segment frames not usually

forwarded onto other LAN segments segments become separate collision

domains

bridge collision domain

collision domain

= hub

= host

LAN (IP network)

LAN segment LAN segment

Forwarding

How do determine to which LAN segment to forward frame?• Looks like a routing problem...

Self learning

A bridge has a bridge table entry in bridge table:

(Node LAN Address, Bridge Interface, Time Stamp)

stale entries in table dropped (TTL can be 60 min)

bridges learn which hosts can be reached through which interfaces when frame received, bridge “learns” location

of sender: incoming LAN segment records sender/location pair in bridge table

Filtering/ForwardingWhen bridge receives a frame:

index bridge table using MAC dest addressif entry found for destination

then{ if dest on segment from which frame arrived

then drop the frame else forward the frame on interface

indicated } else flood

forward on all but the interface on which the frame arrived

Question

Bridge B1 = {1,a};{1,f};{2,b}a b c

1

2

Bridge B2 = {1,f};{1,c};{2,b}d e

1

2

f g

Find all errors in the table and explain why?

Bridge name Error in table Explain

All Errors

Bridge B1 = {1,a};{1,f};{2,b}a b c

1

2

Bridge B2 = {1,f};{1,c};{2,b}d e

1

2

f g

Does a message reaches destination?

From C to GFrom A to FFrom F to A

What will happen to the tables?

From C to G

Bridge B1 = {1,a};{1,f};{2,b}{1,c}a b c

1

2

Bridge B2 = {1,f};{1,c};{2,b}d e

1

2

f g

From A to F

Bridge B1 = {1,a};{1,f};{2,b}a b c

1

2

Bridge B2 = {1,f};{1,c};{2,b}d e

1

2

f g

From F to A

Bridge B1 = {1,a};{ ,f};{2,b}a b c

1

2

Bridge B2 = { ,f};{1,c};{2,b}d e

1

2

f g2

1

12

12

Loop Resolving

The simple learning mechanism described fails in presence of loops in the LAN

Loops may be present by mistake, or deliberately provided for redundency

This problem is resolved by running a distributed spanning tree algorithm

Spanning Tree Algorithm Creates a logical, or “active” topology

that behaves like a spanning tree Makes alternate bridges redundant Is run periodically, so will discover failures

and use alternate bridges if necessary

Spanning tree

Think of the LAN as a graph that possibly has loops (LAN segments as nodes, bridges as edges)

The spanning tree is a sub graph of this graph that covers all vertices (LAN segments), but contains no cycles.

(a) (b)

Spanning tree algorithm

Spanning tree algorithm is a protocol used by a set of bridges to agree upon a spanning tree for a particular extended LAN.

Essentially, this means that each bridge decides the ports over which it is and is not willing to forward packets.

Some ports (or even entire bridges) may not participate in a spanning tree

How does the bridge select the ports to include (/exclude)?

Spanning Tree Algorithm Working: Bridges regularly exchange frames known as

Bridge Protocol Data Units (BPDUs). This exchange does the following:

1. Each bridge has a unique Identifier2. Bridge with highest priority and smallest ID is selected

as root bridge.3. Each bridge determines for each port, the least cost

path from root bridge to this port. This is the Root Path Cost (RPC) for this port.a) Select the port which has the least RPC and

designate it as the Root Port (RP). This is the port which will be used for communicating with the root.

•Algorithm...

1. Once root port is determined, one bridge port is selected for each LAN segment as the designated bridge port (DP) over which frames will be sent for that LAN segment.

a) This is a port (which is NOT a root port) which has the least path cost to the root

b) The ports of the root bridge are always DPs for the LAN segments connected to the root bridge

2. The state of the bridge ports can be set either to forwarding or blocking.

a) All ports that are either RPs or DPs are forwarding, the rest are blocking.

Example:

A

C

E

D

B

K

F

H

J

G

I

B3

B7

B4

B2

B5

B1

B6

A

C

E

D

B

K

F

H

J

G

I

B5

B2

B3

B7

B4

B1

B6

B1 is the root bridge B3 and B5 are both

connected to LAN A, but B5 is the designated port since it's closer to root

B5 and B7 are both connected to LAN B, but B5 is the designated port due to smaller ID (equal distance).

Topology Initialization BPDUs are sent to a broadcast MAC address of all bridges

on the LAN All bridges initially assume they are the root bridge Each BPDU contains (self ID, root ID, transmitting port ID,

RPC of this port) A bridge updates its own info if it receives an update which

identifies a root with smaller id or identifies a root with equal id but with shorter distance the root id and distance are equal, but the sending

bridge has a smaller id The bridge adds 1 to the received RPC in the above update

and saves this info.

Designated port / Root PortA

C

E

D

B

K

F

H

J

G

I

B5

B2

B3

B7

B4

B1

B6

What are these

And theseAnd this one

And

STP Run – Find RootA

C

E

D

B

K

F

H

J

G

I

B3

B7

B4

B2

B5

B1

B6

B8

L

M

B9

3

57

46

1

2

9

8

B3 sends BPDU

3

B2 sends BPDU

2

2B1 sends BPDU

21

11

1

B4, B2 sends BPDU

B8 sends BPDU

1

1

1

1

Proof sketch

if there is a bridge which has a different value THEN There is a segment on which one bridge has the correct minimum and the other a larger value.

When the minimum will broadcast, the other bridge would update, and we have one more correct bridge

STP Run – Block PortsA

C

E

D

B

K

F

H

J

G

I

B3

B7

B4

B2

B5

B1

B6

B8

L

M

B9

B5: 5, 0, 1B2: 2, 1, 1B3: BLOCK

B7: 7, 0, 1B5: 5, 0, 1B7: BLOCK

DataA

C

E

D

B

K

F

H

J

G

I

B3

B7

B4

B2

B5

B1

B6

B8

L

M

B9

Laptop A

Laptop B

Message A to BMessage B to A