Introduction to Computer
Networks
CMPE 150
Fall 2005
Lecture 19
CMPE 150- Introduction to Computer Networks
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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.
CMPE 150- Introduction to Computer Networks
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Today
• Data layer switching.
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LAN Interconnection
•
•
•
•
Extend range of LAN.
Support more users.
Security and robustness.
Heterogeneity.
Switch
Hub
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LAN Interconnection: Example
• Multiple LANs connected by a backbone.
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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.
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Repeaters and Bridges
• Repeaters:
– Extend scope of LANs.
– Serve as amplifiers.
– No storage/routing capabilities.
• Bridges:
– Also extend scope of LANs.
– Routing/storage capabilities.
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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.
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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.
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Interconnecting Different LANs
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Bridges from 802.x to 802.y
• Operation of a LAN bridge from 802.11 to 802.3.
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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?
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Routing
• Determining where to send frame so that it
reaches the destination.
• Routing by learning: adaptive or backward
learning.
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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.
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Routing with Bridges
LAN A
1
LAN B
4
5
8
CMPE 150- Introduction to Computer Networks
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Frames for
5->8.
Frames for
1->4
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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.
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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.
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Local Internetworking
Example: 4 LANs and 2 bridges.
A LAN1
B LAN1
C LAN2
…
. In promiscuous mode, bridge accepts every incoming frame.
. Decides to forward or discard frame.
.
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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.
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Backward Learning (Cont’d)
• If source LAN = destination LAN, discard
frame.
• If source LAN <>destination LAN, forward
frame.
– If destination LAN unknown, flood frame.
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Loops
• Sometimes, more than 1 bridge interconnect
2 LANs.
• Topology loops are introduced.
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Loops: Example
• Alternate routes: loops.
• Example:
–
–
–
–
1
2
LAN A
LAN A, bridge 101,
LAN B, bridge 104,
LAN E, bridge 107,
LAN A.
101
LAN B
107
103
104
E
4
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Loop: Problems
B
LAN 1
B1
B2
LAN 2
A
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.
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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.
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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.
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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.
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Spanning Tree Algorithm (1)
1. Determine root bridge.
2. Determine root port on all bridges.
3. Determine designated bridges.
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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.
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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.
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Spanning Tree: Example
LAN 2
LAN 2
10
10
B3
10
B1
10
5
LAN 3
5
B4
5
LAN 5
5
B5
5
LAN 1
10
5
B2
LAN 4
CMPE 150- Introduction to Computer Networks
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10
B3
10
B1
10
5
LAN 3
5
B4
5
LAN 5
5
B5
5
LAN 1
10
5
B2
LAN 4
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Spanning Tree: Example
B1
. Only designated bridges
on each LAN allowed to
forward frames.
LAN 2
LAN 1
B4
B3
. Bridges continue
exchanging info to react
to topology changes.
B5
LAN 5
B2
LAN 3
LAN 4
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Remote Bridges
• Remote bridges can be used to interconnect distant
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LANs.
CMPE 150- Introduction to Computer Networks