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LAN switching and Bridges
Relates to Lab 6.
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Outline
• Interconnection devices• Bridges/LAN switches vs. Routers• Bridges• Learning Bridges• Transparent bridges
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Introduction
• There are many different devices for interconnecting networks
X.25Network
EthernetHub
EthernetHub
Hosts Hosts
RouterBridge
Token-ring
Gateway
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Ethernet Hub
• Used to connect hosts to Ethernet LAN and to connect multiple Ethernet LANs
• Collisions are propagated
IP
LLC
802.3 MAC
IP
LLC
802.3 MACHubHub
EthernetHub
EthernetHub
HostHost
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Bridges/LAN switches
• We will use the terms bridge and LAN switch (or Ethernet switch in the context of Ethernet) interchangeably. Interconnect multiple LAN, possibly with different type
• Bridges 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
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Ethernet Hubs vs. Ethernet Switches
• An Ethernet switch is a packet switch for Ethernet frames • Buffering of frames prevents collisions. • Each port is isolated and builds its own collision domain
• An Ethernet Hub does not perform buffering:• Collisions occur if two frames arrive at the same time.
HighSpeedBackplane
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
OutputBuffers
InputBuffers
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
Hub Switch
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Dual Speed Ethernet hub
• Dual-speed hubs operate at 10 Mbps and 100 Mbps per second
• Conceptually these hubs operate like two Ethernet hubs separated by a bridge
100 Mbps
100 Mbps
100 Mbps
100 Mbps
10 Mbps
10 Mbps
10 Mbps
10 Mbps
Dual-Speed Ethernet Hub
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Routers
• Routers operate at the Network Layer (Layer 3) • Interconnect IP networks
IP networkIP network IP networkRouter RouterHost Host
Application
TCP
IP
NetworkAccess
Application
TCP
IP
NetworkAccess
IP protocol
DataLink
NetworkAccess
IP
NetworkAccess
NetworkAccess
IP
NetworkAccess
DataLink
DataLink
IP protocol
RouterRouter HostHost
IP protocol
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Gateways
• The term “Gateway” is used with different meanings in different contexts• “Gateway” is a generic term for routers (Level 3)• “Gateway” is also used for a device that interconnects different Layer 3 networks and which performs translation of protocols
(“Multi-protocol router”)
SNANetworkIP Network X.25
NetworkGateway Gateway
Host Host
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Bridges versus Routers
• An enterprise network (e.g., university network) with a large number of local area networks (LANs) can use routers or bridges– 1980s: LANs interconnection via bridges– Late 1980s and early 1990s: increasingly use of routers– Since mid1990s: LAN switches replace most routers
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Internet
A Routed Enterprise Network
Router
Hub
FDDI
FDDI
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Internet
A Switched Enterprise Network
Router
Switch
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Example: Univ. of Virginia CS Department Network
• Design of the network architecture (Spring 2000)• There is no router !
350T 350T 350T 350T350T
350T
100/GigaEthernet Switch
Gigabit EthernetSwitch
100 MbpsEthernet Switch
350T350T 350T 350T 350T
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Bridges versus Routers
Routers
• 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
• No routing protocol needed (sort of)
– learning bridge algorithm– spanning tree algorithm
• Bridges do not manipulate frames
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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
Bridge 2
LAN 5
LAN 3
LAN 1
LAN 4
Bridge 5
Bridge 4Bridge 3
d
Bridge 1
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Transparent Bridges
• Three principal approaches can be found:– 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
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Transparent Bridges
Overall design goal: Complete transparency
“Plug-and-play”Self-configuring without hardware or software changesBridges should not impact operation of existing LANs
Three parts to transparent bridges:(1) Forwarding of Frames(2) Learning of Addresses(3) Spanning Tree Algorithm
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(1) Frame Forwarding
• Each bridge maintains a forwarding database 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.
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• Assume a MAC frame arrives on port x.
(1) Frame Forwarding
Bridge 2Port A Port C
Port x
Port B
Is MAC address of destination in forwarding
database 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 ?
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• Routing table 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.
(2) Address Learning (Learning Bridges)
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
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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).
(2) Address Learning (Learning Bridges)
Port 1
Port 2
Port 3
Port 4
Port 5
Port 6
x is at Port 3
Src=y, Dest=x
Src=y, Dest=x
y is at Port 4
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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 1
Port1
LAN 1
A
LAN 2
CB D
LAN 3
E F
Port2
Bridge 2
Port1 Port2
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• 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 continuesWhere’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
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Spanning Trees / Transparent Bridges
• A solution is to prevent loops in the topology
• IEEE 802.1d has an algorithm that organizes the bridges as spanning tree in a dynamic environment– Note: Trees don’t have loops
• 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.
LAN 2
Bridge 2
LAN 5
LAN 3
LAN 1
LAN 4
Bridge 5
Bridge 4Bridge 3
d
Bridge 1
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Configuration BPDUs
time since root sent amessage 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 thebridge sending this
message
ID of port from whichmessage is sent
ID of bridge sending this message
Time betweenrecalculations of the
spanning tree(default: 15 secs)
Time betweenBPDUs from the root
(default: 1sec)
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What do the BPDUs do?
With the help of the BPDUs, bridges can compute a spanning tree:
• 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. • 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.
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LAN 2
Bridge1
LAN 5
LAN 3
LAN 1
LAN 4
Bridge2
Bridge5Bridge4
•d
Bridge3
•D•D
•D
•R
•D•R •R
•R
•D
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Concepts
• Each bridge has a unique identifier: Bridge ID Bridge ID = { Priority : 2 bytes;
Bridge MAC address: 6 bytes}– Priority is configured– Bridge MAC address is lowest MAC addresses of all ports
• 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.
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Concepts
• Root Path Cost: For each bridge, the cost of the min-cost path to the root.
Assume 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 for this LAN:
- 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
• Note: We assume that “cost” of a path is the number of “hops”.
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Steps of Spanning Tree Algorithm
• Each bridge is sending out BPDUs that contain the following information:
• The transmission of BPDUs results in the distributed computation of a spanning tree
• The convergence of the algorithm is very quick
root bridge (what the sender thinks it is) root path cost for sending bridge
Identifies sending bridgeIdentifies the sending port
root ID cost bridge ID port ID
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Ordering of Messages
• We define an ordering of BPDU messages
We say M1 advertises a better path than M2 (“M1<<M2”) if (R1 < R2),
Or (R1 == R2) and (C1 < C2), Or (R1 == R2) and (C1 == C2) and (B1 < B2), Or (R1 == R2) and (C1 == C2) and (B1 == B2) and (P1 < P2)
ID R1 C1 ID B1
M1 M2ID P1 ID R2 C2 ID B2 ID P2
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• Initially, all bridges assume they are the root bridge. • Each bridge B sends BPDUs of this form on its LANs from
each port P:
• 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)
Initializing the Spanning Tree Protocol
B 0 B P
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• Each bridge B looks on all its ports for BPDUs that are better than its own BPDUs
• Suppose a bridge with BPDU:
receives a “better” BPDU:
Then it will update the BPDU to:
• However, the new BPDU is not necessarily sent out• On each bridge, the port where the “best BPDU” (via relation “<<“) was
received is the root port of the bridge.
Operations of Spanning Tree Protocol
R1 C1 B1 P1M1
R2 C2 B2 P2M2
R2 C2+1 B1 P1
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• Say, B has generated a BPDU for each port x
• B will send this BPDU on port x only if its BPDU is better (via relation “<<“) 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
• And port x is the designated port of that LAN
When to send a BPDU
R Cost B
Bridge BPort A Port C
Port x
Port B
x
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Selecting the Ports for the Spanning Tree
• Each bridge makes a local decision which of its ports are part of the spanning tree
• Now B can 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)
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LAN 2
Bridge1
LAN 5
LAN 3
LAN 1
LAN 4
Bridge2
Bridge5Bridge4
•d
Bridge3
•D•D
•D
•R
•D•R •R
•R
•D
Building the Spanning Tree
• Consider the network on the right.• Assume that the bridges have
calculated the designated ports (D) and the root ports (P) as indicated.
• What is the spanning tree?– On each LAN, connect D ports
to the R ports on this LAN
• Suppose a packet is originated in LAN 5. How is the packet flooded?
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Example
• Assume that all bridges send out their BPDU’s once per second, and assume that all bridges send their BPDUs at the same time
• Assume that all bridges are turned on simultaneously at time T=0 sec.
Brige2
LAN 1
LAN 2
LAN 3
LAN 4
Brige1
Brige5
Brige3 Brige4AB
A B
AB A
BA
B
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Example: BPDU’s sent by the bridges
Bridge 1 Bridge 2 Bridge 3 Bridge 4 Bridge 5
T=1sec
T=2sec
T=3sec
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Example: Settings after convergence
Bridge 1 Bridge 2 Bridge 3 Bridge 4 Bridge 5
Root Port
DesignatedbridgeDesignatedports
Brige2
LAN 1
LAN 2
LAN 3
LAN 4
Brige1
Brige5
Brige3 Brige4AB
A B
A B AB A
B
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Routing protocols versus the spanning tree protocol
• Routing protocols build all pair shortest paths– From any source to any destination, the data packet takes
the shortest path.– Every node is the root of a shortest path tree to all
destinations.• The spanning tree protocol
– A single spanning tree for broadcasting when the forwarding state of a MAC address is unknown