1CMPE 150- Introduction to Computer Networks

CMPE 150

Fall 2005Lecture 19

Introduction to Computer Networks

2CMPE 150- Introduction to Computer Networks

Announcements

• Homework 3 due today.• Lab this week: – Layer 2.

– Ethernet.

• Holiday this Friday (11.11).• Monday, 11.14 class meets in E2 room 506.

3CMPE 150- Introduction to Computer Networks

Today

• Data layer switching.

4CMPE 150- Introduction to Computer Networks

LAN Interconnection

• Extend range of LAN.• Support more users.• Security and robustness.• Heterogeneity.

Hub

Switch

5CMPE 150- Introduction to Computer Networks

LAN Interconnection: Example

• Multiple LANs connected by a backbone.

6CMPE 150- Introduction to Computer Networks

Bridges

• Connect LANs.• Operate at the DLL.– Route based on DLL addresses.

• Routers route based on layer 3 addresses.

• Distinction between bridges, switches, and routers gets fuzzier as technology advances.

7CMPE 150- Introduction to Computer Networks

Repeaters and Bridges

• Repeaters: – Extend scope of LANs.

– Serve as amplifiers.

– No storage/routing capabilities.

• Bridges:– Also extend scope of LANs.

– Routing/storage capabilities.

8CMPE 150- Introduction to Computer Networks

More on Bridges

• Have multiple interfaces, 1 per LAN.

• Use destination address to forward unicast frames; if destination is on the same LAN, drops frame; otherwise forwards it.

• Forward all broadcast frames.

• Have storage and routing capability.

9CMPE 150- Introduction to Computer Networks

More on Bridges

• No additional encapsulation.

• But they may have to do header conversion if interconnecting different LANs (e.g., 802.3 to 802.4 frame).

• May interconnect more than 2 LANs.

• LANs may be interconnected by more than 1 bridge.

10CMPE 150- Introduction to Computer Networks

Interconnecting Different LANs

11CMPE 150- Introduction to Computer Networks

Bridges from 802.x to 802.y

• Operation of a LAN bridge from 802.11 to 802.3.

12CMPE 150- Introduction to Computer Networks

Interconnecting Different LANs

• Conversion between different frame formats.– E.g., different frame lengths.

• Speed mismatch.– Buffering.

• Security.– Different security services provided by

different DLLs/MACs.

• QoS?

13CMPE 150- Introduction to Computer Networks

Routing

• Determining where to send frame so that it reaches the destination.

• Routing by learning: adaptive or backward learning.

14CMPE 150- Introduction to Computer Networks

Routing with Bridges

• Bridge decides to relay frame based on destination MAC address.

• If only 2 LANs, decision is simple.

• If more complex topologies, routing is needed, i.e., frame may traverse more than 1 bridge.

15CMPE 150- Introduction to Computer Networks

Routing with Bridges

B1 4

5 8

Frames for5->8.

Frames for1->4

LAN A

LAN B

16CMPE 150- Introduction to Computer Networks

Routing with Bridges (Cont’d)

• Listens to all frames on LAN A and accepts those addressed to stations on LAN B.

• Retransmits frames onto B.• Does the same for B-to-A traffic.

17CMPE 150- Introduction to Computer Networks

Transparent Bridges

• Plug them in and they work!• How do they work?– Promiscuous mode operation.

– Upon receiving frame, decide whether to forward it or not.

– Routing table mapping destination addresses to outgoing interface.

18CMPE 150- Introduction to Computer Networks

Local Internetworking

Example: 4 LANs and 2 bridges.

. In promiscuous mode, bridge accepts every incoming frame.

. Decides to forward or discard frame.

.

A LAN1

B LAN1C LAN2

…

19CMPE 150- Introduction to Computer Networks

Backward Learning• Routing algorithm used by transparent

bridges.• When a bridge starts, its routing table is

empty.– So, it floods: every incoming frame is

forwarded in all outgoing interfaces, except the one the frame was received.

– In promiscuous mode, bridge sees all frames.– They look at the frame’s source and

“remember” which LAN it came from. – Entries are timestamped and expire after a

certain interval.• Allows for topology changes.

20CMPE 150- Introduction to Computer Networks

Backward Learning (Cont’d)

• If source LAN = destination LAN, discard frame.

• If source LAN <>destination LAN, forward frame.– If destination LAN unknown, flood frame.

21CMPE 150- Introduction to Computer Networks

Loops

• Sometimes, more than 1 bridge interconnect 2 LANs.

• Topology loops are introduced.

22CMPE 150- Introduction to Computer Networks

Loops: Example

• Alternate routes: loops.

• Example:– LAN A, bridge 101,

– LAN B, bridge 104,

– LAN E, bridge 107,

– LAN A.

LAN A

LAN B

E

2

4 5

101

103104

1

107

23CMPE 150- Introduction to Computer Networks

Loop: Problems

A

B

LAN 1

LAN 2

B1 B2

1. Station A sends frame to B; bridges B1 and B2 don’t know B.2. B1 copies frame onto LAN1; B2 does the same.3. B2 sees B1’s frame to unknown destination and copies it onto LAN 2.4. B1 sees B2’s frame and does the same.5. This can go on forever.

24CMPE 150- Introduction to Computer Networks

Loop Resolution

• Goal: remove “extra” paths by removing “extra” bridges.

• Spanning tree:– Given graph G(V,E), there exists a tree that spans all

nodes where there is only one path between any pair of nodes, i.e., NO loops.

– LANs are represented by nodes and bridges by edges.

25CMPE 150- Introduction to Computer Networks

Definitions (1)• Bridge ID: unique number (e.g., MAC address + integer) assigned to each bridge.• Root: bridge with smallest ID.• Cost: associated with each interface; specifies cost of transmitting frame through that interface.• Root port: interface to minimum-cost path to root.

26CMPE 150- Introduction to Computer Networks

Definitions (2)• Root path cost: cost of path to root bridge.• Designated bridge: on any LAN, bridge closest to root, i.e., the one with minimum root path cost.

27CMPE 150- Introduction to Computer Networks

Spanning Tree Algorithm (1) 1. Determine root bridge.

2. Determine root port on all bridges.

3. Determine designated bridges.

28CMPE 150- Introduction to Computer Networks

Spanning Tree Algorithm (2)

• Initially all bridges assume they are the root and broadcast message with its ID, root path cost.

• Eventually, lowest-ID bridge will be known to everyone and will become root.

• Root bridge periodically broadcasts it’s the root.

29CMPE 150- Introduction to Computer Networks

Spanning Tree Algorithm (3)• Directly connected bridges update their cost to

root and broadcast message on other LANs they are attached.

• This is propagated throughout network.

• On any (non-directly connected) LAN, bridge closest to root becomes designated bridge.

30CMPE 150- Introduction to Computer Networks

Spanning Tree: Example

B3

LAN 2

LAN 1

LAN 3 LAN 4

LAN 5

B5

B4B1

B2

10

10

10

10

5

5

5

5

1055

B3

LAN 2

LAN 1

LAN 3 LAN 4

LAN 5

B5

B4B1

B2

10

10

10

10

5

5

5

5

1055

31CMPE 150- Introduction to Computer Networks

Spanning Tree: Example

B1

LAN 1 LAN 2

B2

LAN 3 LAN 4

LAN 5

B4

B5B3

. Only designated bridgeson each LAN allowed toforward frames.

. Bridges continue exchanging info to react to topology changes.

32CMPE 150- Introduction to Computer Networks

Remote Bridges

• Remote bridges can be used to interconnect distant LANs.