LAN switching and Bridges
Relates to Lab 6.
Covers interconnection devices (at different layers) and the difference
between LAN switching (bridging) and routing. Then discusses LAN
switching, including learning bridge algorithm, transparent bridging, and
the spanning tree protocol.
1
Outline
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Interconnection devices – Repeaters, Bridges, Routers
Bridges/LAN switches vs. Routers
Bridges
Learning Bridges
Transparent bridges
2
Introduction
• There are many different devices for interconnecting
networks.
Ethernet
Ethernet
Ethernet
Repeater
Bridge
Router
X.25
Network
Tokenring
Gateway
3
Repeaters
• Used to interconnect multiple Ethernet segments
• Merely extends the baseband cable
• Amplifies all signals including collisions/errors
Repeater
IP
IP
LLC
LLC
802.3 MAC
Repeater
802.3 MAC
4
Bridges/LAN switches
• Interconnect multiple LAN, possibly different types
• Bridges operate at the Data Link Layer (Layer 2) and
only switch link layer frames
Token-ring
Bridge
IP
IP
Bridge
LLC
802.3 MAC
LLC
LAN
802.3 MAC
LLC
802.5 MAC
LAN
802.5 MAC
5
Routers
• Routers operate at the Network Layer (Layer 3)
• Interconnect different subnetworks
Subnetwork
Subnetwork
Subnetwork
Router
Router
Application
Application
TCP
TCP
IP
Network
Access
Host
IP
IP protocol
Data
Link
Network
Access
IP
IP protocol
Network
Access
Router
Data
Link
Network
Access
IP protocol
Network
Access
Router
Data
Link
IP
Network
Access
Host
6
Bridges/Switches versus Routers
• An enterprise network (e.g., university network) with a large
number of local area networks (LANs) can use routers or
bridges/switches
• Until early 1990s: most LANs were interconnected by routers
• Since mid1990s: LAN switches replace most routers
8
A Routed Enterprise Network
Router
Internet
Hub
FDDI
FDDI
9
A Switched Enterprise Network
Internet
Router
Switch
10
Bridges/Switches versus Routers
Routers
Bridges
• Each host’s IP address must be
configured
• MAC addresses are hardwired
• If network is reconfigured, IP
addresses may need to be
reassigned
• No network configuration needed
• Routing done via RIP or OSPF
• Each router manipulates packet
header (e.g., reduces TTL field)
• No routing protocol needed (sort of)
– learning bridge algorithm
– spanning tree algorithm
• Bridges do not manipulate frames
12
Need for Routing
• What do bridges do if
some LANs are
reachable only in
multiple hops ?
• What do bridges do if the
path between two LANs
is not unique ?
LAN 2
d
Bridge 4
Bridge 3
Bridge 1
LAN 5
Bridge 5
 Intelligent briges
LAN 1
Bridge 2
LAN 3
LAN 4
13
Transparent Bridges/Switches
• Three principal approaches can be found to forward frames
from a source to a destination via bridges:
– Fixed Routing
– Source Routing
– Spanning Tree Routing (IEEE 802.1d)
• We only discuss the last one in detail.
• Bridges that execute the spanning tree algorithm are called
transparent bridges
14
Transparent Bridges
Overall design goal: Complete transparency
• “Plug-and-play”
• Self-configuring without hardware or software
changes to end devices
• Bridges should not impact operation of existing LANs
Three parts to transparent bridges:
(1) Forwarding of Frames
(2) Learning of Addresses
(3) Spanning Tree Algorithm
15
(1) Frame Forwarding
• Each bridge maintains a forwarding database with entries
< MAC address, port, age>
MAC address:
host name or group address
port:
age:
port number of bridge
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.
16
(1) Frame Forwarding
• Assume a MAC frame arrives on port X.
Port x
Is MAC address of
destination in forwarding
database for ports A, B, or C ?
Bridge 2
Port A
Port C
Port B
Found?
Not
found ?
Flood the frame,
Forward the frame on the
appropriate port
i.e.,
send the frame on all
ports except port X.
17
(2) Address Learning (Backwards Learning
Bridges)
• 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.
Src=x, Dest=y
Src=x, Dest=y
Src=x,
Src=y, Dest=x
Dest=y
Port 1
Port 4
x is at Port 3
y is at Port 4
Port 2
Port 3
Port 5
Port 6
Src=x,
Src=y, Dest=x
Dest=y
Src=x, Dest=y
Src=x, Dest=y
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(2) Summary: Learning Bridges
Algorithm:
• For each frame received, the bridge stores the source field
in the forwarding database together with the port on which
the frame was received.
• All entries are deleted after some time (default is 15
seconds).
Port 1
Port 4
x is at Port 3
y is at Port 4
Port 2
Port 5
Port 3
Port 6
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Example
•Consider the following packets:
(Src=A, Dest=F),
(Src=C, Dest=A), (Src=E, Dest=C)
•What have the bridges learned?
Bridge
Bridge
2
Port1
1
Bridge 2
Port2
LAN 1
A
B
Port2
Port1
LAN 2
C
LAN 3
D
E
F
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Danger of Loops
• Consider the two LANs that are
connected by two bridges.
LAN 2
• Assume host n is transmitting a
frame F with unknown
destination.
F
F
What happens?
Bridge B
• Bridges A and B flood the frame Bridge A
to LAN 2 and enter LAN1 as
F
F
source for host n.
LAN 1
• Bridge B sees F on LAN 2, and
copies the frame back to LAN 1
F
as it has no destination for F.
• Bridge A does the same.
host n
• The copying continues
Where’s the problem? What’s the
solution ?
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Spanning Trees / Transparent Bridges
• 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
BPDU) to build the tree.
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What do the BPDUs do?
With the help of the BPDUs, bridges can:
• Elect a single bridge as the root bridge.
• Calculate the distance of the shortest path to the root bridge
• Each LAN can determine a designated bridge, which is the
bridge closest to the root on a shared medium (e.g.,
Ethernet). The designated bridge will forward packets towards
the root bridge.
• Each bridge can determine a root port, the port that gives the
best path to the root.
• Select ports to be included in the spanning tree (Active and
Passive/Blocked).
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Configuration BPDUs
Destination
MAC address
Source MAC
address
Set to 0
Set to 0
version
message type
lowest bit is "topology change bit (TC bit)
flags
root ID
Configuration
Message
Set to 0
protocol identifier
Cost
bridge ID
port ID
ID of root
Cost of the path from the
bridge sending this
message
ID of bridge sending this message
message age
maximum age
Time between
BPDUs from the root
(default: 1sec)
priority of configurable interface
(used for loop detection)
hello time
forward delay
Time between
recalculations of the
spanning tree
(default: 15 secs)
time since root sent a
message on
which this message is based
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Concepts
• Each bridge as a unique identifier:
Bridge ID = <MAC address + priority level>
– Note that a bridge has several MAC addresses (one for each port), but only
one ID. It picks the lowest MAC address
• 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 that bridge to the root. This port
is identified during the spanning tree process
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Concepts behind Spanning Tree
• Root Path Cost:
For each bridge, the cost of the min-cost path to the root.
Usually it is measured in #hops to the root
• Designated Bridge, Designated Port:
– Single bridge on a LAN that provides the minimal cost path
to the root bridge for this LAN:
• if two bridges on a LAN have the same cost, select the
one with highest priority (lowest ID)
• if the min-cost bridge has two or more ports on the LAN,
select the port with the lowest identifier
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Steps of Spanning Tree Algorithm
1. Determine the root bridge
2. Determine the root port on all other bridges
3. Determine the designated port on each LAN
• Each bridge is sending out BPDUs that contain the following
information:
root ID cost bridge ID/port ID
root bridge (what the sender thinks it is)
root path cost for sending bridge
Identifies sending bridge
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Ordering of Messages
• We can order BPDU messages with the following ordering
relation “<<“:
M1
ID R1
C1
ID B1
<
ID R2
C2
ID B2
If (R1 < R2)
M1<< M2
elseif ((R1 == R2) and (C1 < C2))
M1 << M2
elseif ((R1 == R2) and (C1 == C2) and (B1 < B2))
M1 << M2
• If above holds, M1 is dominant and M2 will change its
information to match M1,
• Else M2 is dominant and M1 changes to follow M2
M2
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Determine the Root Bridge
• Initially, all bridges assume they are the root bridge.
• Each bridge B sends BPDUs of this form on its LANs:
B
0
B
• 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 field is updated in
a bridges BPDU
– Otherwise bridge maintains the current value
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Calculate the Root Path Cost
Determine the Root Port
• At some point 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 in direction from R} + 1
• B’s root port is the port from which B received the lowest
cost path to R (in terms of relation “<<“).
• Knowing R and Cost, B can generate its BPDU with its root
port A.
Cost
B
A
R
• Bridge B will only send its BPDU to a neighbor if it receives
“worse news” on any of its ports from its neighbors.
• It updates its BPDU it if receives better news from a neighbor
and broadcasts new BPDU on all its non (updated) root ports.30
Calculate the Root Path Cost
Determine the Root Port
• Now that B has generated its BPDU
R
Cost
B
A
• B will send this BPDU on one of its ports, e.g., port X, only if
its BPDU is lower (via relation “<<“) than any BPDU that B
received from port X (i.e., carries better news).
• In this case, B also assumes that it
Port x
is the designated bridge for the
LAN to which port X connects.
Bridge B
Port A
Port C
Port B
31
Example
• Bridge B (ID 8) receives on port “X” the following BPDU from a
neighboring bridge (ID 12):
3
12
C
2
2
8
A
2
• And B’s current BPDU is:
• Because Cost 2 << Cost 3, B will broadcast on its port “X” its
current BPDU to let bridge (ID 12) know it has a shorter path to
the root bridge (ID 20) via port A (its current root port).
• If instead B’s current BPDU is:
5
8
A
2
• Then B will broadcast on all its other ports (A,B,C) the
following BPDU:
4
8
2
X
• Where “4” (3+1) is new cost from Bridge B on port X to the root
bridge (ID 20) (via bridge ID 12). And port X is now its root port
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Selecting the Ports for the Spanning Tree
• Now that Bridge B has calculated the root, the root path cost,
and the designated bridge for each LAN.
• B can decide which ports are in the spanning tree:
• B’s root port is part of the spanning tree (every bridge
has to have a root port)
• The ports on all the LANs for which B is the designated
bridge are part of the spanning tree (designated ports).
• 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)
• Note that a bridge may not be the designated bridge on any
LAN. As such, all its ports, other than the root port will be
blocked.
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Building the Spanning Tree
• Consider the network on the
right.
• Assume that the bridges
have calculated the
designated ports (D), the
blocked ports (B) and the
root ports (R) as indicated.
LAN 2
B
Bridge 3
d
D
Bridge 2
D
R
Bridge 1
R
LAN 5
R
B
Bridge 4
D
• What is the spanning tree?
LAN 1
R
D
LAN 3
Bridge 5
D
LAN 4
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