1. Chapter 3 Hubs, Bridges and Switches
Lecture 3

2. Interconnecting LANs Q: Why not just one big LAN?
Limited amount of supportable traffic: on single LAN, all stations must share bandwidth
Ethernet: limited length: specifies maximum cable length
Ethernet: large “collision domain” (can collide with many stations)
collision domain: set of stations such that simultaneous transmission of any two of them will generate a collision
Token Ring: token passing delay per station: limits number of stations per LAN:
Lecture 3

3. Hubs
Physical Layer devices: essentially multi-leg repeaters operating at bit levels: repeat bits received on one interface to all other interfaces
Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top
Lecture 3

4. Hubs (more) Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains: node may collide with any node residing at any segment in LAN
Hub Advantages:
simple, inexpensive device
Multi-tier provides graceful degradation: portions of the LAN continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
Lecture 3

5. Hub limitations
single collision domain results in no increase in max throughput
multi-tier throughput capacity same as single segment throughput
individual LAN restrictions pose limits on number of nodes in same collision domain and on total allowed geographical coverage
cannot connect different Ethernet types (e.g., 10BaseT and 100baseT) Qn: Why?
Lecture 3

6. Bridges Link Layer devices: forward Ethernet frames selectively:
learn where each station is located
examine the header of each frame
forward on the proper link (if known)
if dest. and source on same link, drop frame WHY?
if not known where dest. is, broadcast frame
except on originating link, of course
also called Layer 2 switches
Lecture 3

7. Bridges Bridge isolates collision domains
buffers frame
then forwards it, if needed, using CSMA/CD
A broadcast frame is forwarded on all interfaces (except the incoming one)
thus broadcast frames propagate across bridges
A set of segments connected by bridges and hubs is called a broadcast domain
Lecture 3

8. Bridges (more) Bridge advantages:
Isolates collision domains resulting in higher total throughput capacity, and does not limit the number of nodes nor geographical coverage
Can connect different type Ethernet since it is a store and forward device (e.g. 10 & 100BaseT)
Transparent: no need for any change to hosts LAN adapters (invisible to them)
Lecture 3

9. Backbone Bridge
Lecture 3

10. Interconnection Without Backbone
Not recommended for two reasons:
- single point of failure at Computer Science hub
- all traffic between EE and SE must path over CS segment
Lecture 3

11. Bridges: frame filtering, forwarding
bridges filter packets
same-LAN -segment frames not forwarded onto other LAN segments
forwarding:
how to know on which LAN segment to forward frame?
Lecture 3

12. Bridge Filtering
bridges learn which hosts can be reached through which interfaces: maintain filtering tables
when frame received, bridge “learns” location of sender: incoming LAN segment
records sender location in filtering table
filtering table entry:
(Node MAC Address, Bridge Interface, Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
Lecture 3

13. Bridge Operation pseudocode
Init: set filtering table to void
Case: frame arrives on port P, src MAC , dest MAC 
/* Table Update stage */
if  not listed, or mapped to port not equal P then add mapping   P with expiration time
else update expiration time /* if listing fits */
/* Frame Forwarding stage */
look up  in filtering table: listing “  Q” /* if listed */
if  not listed, forward on all ports except P /* “flood */
else,if Q= P , drop the frame /* WHY ? */
otherwise, forward the frame on port Q only
Lecture 3

14. Bridge Learning: example
Suppose C sends frame to D and D replies back with frame to C
C sends frame, bridge has no info about D, so floods to both LANs 2 and 3
bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
Lecture 3

15. Bridge Learning: exampleC
D generates reply to C, sends it
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1, so selectively forwards frame out via interface 1 only
Lecture 3

16. What will happen with loops? Incorrect learning
Frame sent from A to B A B 1 2
A , 1
Problems: (1) frame loops infinitely
(2) unstable filtering tables
Lecture 3

17. Loop-free: tree C B A message from A will mark A’s location A
Lecture 3

18. Loop-free: tree C B A:  A message from A will mark A’s location A
Lecture 3

19. Loop-free: tree A:  C B A:  A message from A will mark A’s location
Lecture 3

20. Loop-free: tree A:  A:  A:  C B A:  A: 
A message from A will mark A’s location A
Lecture 3

21. Loop-free: tree A:  A:  A:  C B A:  A: 
A message from A will mark A’s location A
Lecture 3

22. Loop-free: tree A:  A:  A:  C B A:  A: 
A message from A will mark A’s location
So a message to A will go by marks… A
Lecture 3

23. Bridges-Spanning Tree
for increased reliability, it is desirable to have redundant, alternative paths from source to dest
this causes cycles - bridges may multiply and forward frame forever
solution: organize bridges in a spanning tree and disable all ports not belonging to the tree
Disabled
Lecture 3

24. Introducing Spanning Tree
Objective: Find tree spanning all LAN segments
each bridge transmits on a single port
each LAN transmits on a single bridge
Bridges run the Spanning Tree Protocol
Use a distributed algorithm
Objective: select what ports should actively forward frames, and which ports should accept frames
Bridges communicate using special configuration messages (BPDUs) to perform this selection
BPDU = Bridge Protocol Data Unit
STP standardized in IEEE 802.1D
Lecture 3

25. Method Each bridge sends periodically a BPDU to all its neighbors
BPDU contains:
ID of bridge the sender views as root (my_root_ID)
known distance to that root
senders own bridge ID
port ID of the port from which BPDU sent
Lecture 3

26. Introductory STP
In order to help understanding STP we first present it as 3 separate algorithms
How to agree on a root bridge?
How to compute a ST for bridges?
How to compute a ST for LAN segments?
Actual STP does all 3 functions in the same iterative process
Note: we assume throughout that the network is connected
Lecture 3

27. 1. Choosing a root bridge Assume
each bridge has a unique identifier (ID)
within a bridge each port has a unique ID
Each bridge remembers smallest bridge ID seen so far (= my_root_ID)
including own ID
Periodically, send my_root_ID to all neighbors (“flooding”) (included in BPDU)
When receiving ID, update if necessary
Qn: Is that enough for universal agreement?
Lecture 3

28. 2. Compute ST given a root
Idea: each bridge finds its shortest path to the root  generate shortest paths tree
Output: At each node, parent pointer and distance to root (parent=bridge leading to root along shortest path)
Spanning tree T: A link belongs to T iff it connects some bridge to its parent
Qn: Does this idea fully specify an algorithm producing a spanning tree?
How: Bellman-Ford algorithm
Lecture 3

29. Distributed Bellman-Ford
Assumption: There is a unique root node s
this was done in Step 1
Idea: Each node, periodically, tells all its neighbors what is its distance from s
But how can they tell?
s: easy. dists = 0 always!
Another node v:
Bridge calls the neighbor with least distance to root - its “parent”
If bridges tie: choose bridge with lowest ID
Lecture 3

30. Why does this work?
Suppose all nodes start with distance , and suppose that updates are sent every time unit. 2 1
ID=3 E
ID=21  1 1 D C 1
ID=17 A 1 2 G
ID=7 3 B 1 F
Means: BPDU
Means: link admitted to bridge spanning tree
B sees same distance from A and E; A chosen since has smaller ID
Lecture 3

31. Bellman-Ford: properties
Works for any positive link weights w(u,v):
Works also when the system operates asynchronously.
Works regardless of the initial distances
Lecture 3

32. Actual STP What is missing so far?:
Can’t discard redundant links, since we need to connect host, not just bridges
Instead can disable redundant bridge ports leading to them
Graph model too simple, since there can be many bridges on one LAN (see next slide)
We need to look at forwarding paths and not just graph paths
STP protocol does all the “steps” together:
Selection of root bridge
Evaluation of distance to root and parent bridge
Selection of the active ports and blocked ports
Lecture 3

33. Example of a network L6 L2 L5 B A D C E L1 L3 F L7 L4
Note: LAN L2 connects three bridges, 4 ports
Lecture 3

34. STP plan
Objective: prune given network to render a forwarding tree, i.e:
between any two hosts there is a single forwarding path through the network, no loops possible
Method: Classify all ports into three types:
Root ports: one for each bridge
Designated ports: one for each LAN
All other ports are blocked
Root and designated ports transfer data frames in both directions.
Blocked ports don’t transfer data
Lecture 3

35. BPDU’s (1) Each bridge sends BPDUs on all its ports.
Based on received BPDUs, bridge determines:
determines Root
finds own distance/cost to root
classifies of own ports: root/designated/blocked
The BPDU contains bridge’s current view of:
the root bridge of the network
own distance to this root
own ID number
the sending port’s ID number
Lecture 3

36. BPDUs (2)
A BPDU is computed by a bridge for each of its ports and sent out on that port
it will reach all ports attached to port’s LAN
STP prerequisites
each bridge is given a bridge ID number
The ID number is unique in the network
Each port is given a port ID number
The port ID is unique within its bridge
ID numbers assigned manually or automatically
Each link (LAN) has a positive cost
Lecture 3

37. BPDU Processing in a bridge (1)
Determine current view of root: this is lowest root ID received, including own bridge ID.
Only BPDUs reporting this root are considered in sequel
Compare all reported distances to root.
own distance to root= lowest received distance + + cost of the link to the reporting bridge
Lecture 3

38. Designated Ports
all BPDUs received on a port are compared, including own message sent on it;
the best message has:
smallest root ID and
smallest distance to that root
if tied, choose the one with lowest bridge ID
if tied, choose lowest port ID
Qn: When does the last tie happen?
If the message sent by the bridge on that port is best, label it a designated port
there is exactly one designated port on each LAN
Lecture 3

39. Root Ports
now compare the best messages received on all the ports of the bridge, according to the same criteria as above
the port on which best message was received is labeled root port
root bridge has no root port
there is exactly one root port per bridge
only root and designated ports receive and send data.
BPDU’s are sent periodically
even after convergence of algorithm
indicate bridge is active / discover failures
Lecture 3

40. Summary after convergence: all bridges agree which bridge is the root
each LAN has exactly one designated port
frames from LAN enter the bridge on that port on the way to root (upstream)
frames coming from root exit the bridge on that port on the way to remote LANs (downstream)
all bridges on LAN agree who is the designated port
a LAN may have any ≥ 0 number of root ports on it
each bridge has exactly one root port
the port leads through a LAN to the parent bridge
this is the next bridge on a shortest path to root
a bridge may have any ≥ 0 number of designated ports
a bridge with no designated ports blocks also the root port, and so becomes inactive
Lecture 3

41. Notes only bridges make decisions, LANs are passive
More discussion of the validity of STP will be given in homework and recitation
Lecture 3

42. Example Spanning Tree Protocol operation: B8 Pick a root
Each bridge picks a root port B8 B3 B5 B7 B2 B1 B6 B4
Lecture 3

43. Example Spanning Tree Spanning Tree: Root B8 B3 B5 B7 B2 B1 B6
port
Spanning Tree: B3 B5 B1
root
port B7 B2 B2 B4 B5 B6 B7 B1
Root B3 B8 B6 B4
LANs not connecting bridges omitted here
Lecture 3

44. Spanning Tree Protocol: Execution
(B8,root=B8, dist=0) B8 B3
ignore msg B5 B7 B2 B1
(B1,root=B1,dist=0)
(B1,root=B1, dist=0) B6 B4
WHY?
(B4, root=B1, dist=1)
(B6, Root=B1, dist=1)
Lecture 3

45. Bridges vs. Routers both are store-and-forward devices
routers: network layer devices (examine network layer headers)
bridges are link layer devices
routers have routing tables, use routing algorithms, designed for Wide Area addressing
bridges have filtering tables, use filtering, learning & spanning tree algorithms, designed for local area
Lecture 3

46. Routers vs. Bridges Bridges + and -
+ bridge operation is simpler, requiring less processing
- topologies are restricted with bridges: a spanning tree must be built to avoid cycles (with routers cycles are avoided by the Layer 3 routing algorithm)
- bridges do not offer protection from broadcast storms (endless broadcasting by a faulty host will be forwarded by a bridge)
Lecture 3

47. Routers vs. Bridges Routers + and -
+ arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols)
+ provide barrier protection against broadcast storms
- require IP address configuration (not plug and play)
- require higher processing
bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts) and in Internet core
Lecture 3

48. Ethernet Switches = a powerful bridge
layer 2 (frame) forwarding, filtering using LAN addresses
Switching: A-to-B and A’-to-B’ with no collisions
large number of interfaces
often: individual hosts, star-connected into switch
Ethernet w. no collisions!
= Switched Ethernet
often: includes L3 function
Lecture 3

49. Ethernet Switches
cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame
slight reduction in latency
allow combinations of shared/dedicated, 10/100/1000 Mbps interfaces
Lecture 3

50. Ethernet Switches (more)
Dedicated
Shared
Lecture 3

51. Data Link: Summary Chapter 5 Kurose and Ross
principles behind data link layer services:
error detection, optional: error correction
sharing a broadcast channel: multiple access
link layer addressing, ARP
various link layer technologies
Ethernet
hubs,
bridges, switches (require STP)
IEEE LANs
PPP
Chapter 5 Kurose and Ross
Lecture 3

52. Optional: Wireless LAN and PPP
Lecture 3

53. IEEE 802.11 Wireless LAN wireless LANs: mobile networking
IEEE standard:
MAC protocol
unlicensed frequency spectrum: 900Mhz, 2.4Ghz
Basic Service Set (BSS) (a.k.a. “cell”) contains:
wireless hosts
access point (AP): base station
BSS’s combined to form distribution system (DS)
Lecture 3

54. Ad Hoc Networks
Ad hoc network: IEEE stations can dynamically form network without AP
Applications:
“laptop” meeting in conference room, car
interconnection of “personal” devices
battlefield
IETF MANET (Mobile Ad hoc Networks) working group
Lecture 3

55. IEEE 802.11 MAC Protocol: CSMA/CA
CSMA: sender
- if sense channel idle for DIFS sec.
then transmit entire frame (no collision detection)
-if sense channel busy then binary backoff
CSMA receiver:
if received OK
return ACK after SIFS
Why Needed?
Sifs= Short Interframe Space
DIFS = DCF Interframe Space
DCF = Distributed Coordination Function (requires listening before sending)
Lecture 3

56. IEEE 802.11 MAC Protocol 802.11 CSMA Protocol: others
NAV: Network Allocation Vector
frame has transmission time field
others (hearing data) defer access for NAV time units
Lecture 3

57. Hidden Terminal effect
hidden terminals: A, C cannot hear each other
obstacles, signal attenuation
collisions at B
goal: avoid collisions at B
CSMA/CA: CSMA with Collision Avoidance
Lecture 3

58. Collision Avoidance: RTS-CTS exchange
CSMA/CA: explicit channel reservation
sender: send short RTS: Request To Send
receiver: reply with short CTS: Clear To Send
CTS reserves channel for sender, notifying (possibly hidden) stations
avoid hidden station collisions
Lecture 3

59. Collision Avoidance: RTS-CTS exchange
RTS and CTS short:
collisions less likely, of shorter duration
end result similar to collision detection
IEEE allows:
CSMA
CSMA/CA: reservations
polling from AP
Lecture 3

60. Point to Point Data Link Control
one sender, one receiver, one link: easier than broadcast link:
no Media Access Control
no need for explicit MAC addressing
e.g., dialup link, ISDN line
popular point-to-point DLC protocols:
PPP (point-to-point protocol)
HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!)
Lecture 3

61. PPP Design Requirements [RFC 1557]
packet framing: encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards
bit transparency: must be able to carry any bit pattern in the data field
error detection (no correction)
connection livenes: detect, signal link failure to network layer
network layer address negotiation: endpoint can learn/configure each other’s network address
Lecture 3

62. PPP non-requirements no error correction/recovery no flow control
out of order delivery OK
no need to support multipoint links (e.g., polling)
Error recovery, flow control, data re-ordering
all relegated to Transport layer!!!
Lecture 3

63. PPP Data Frame (1) Flag: delimiter (framing)
Address: does nothing (only one option)
Control: does nothing; in the future possible multiple control fields
Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, PPP-NCP, IP, IPCP, etc)
Lecture 3

64. PPP Data Frame (2) info: upper layer data being carried
check: cyclic redundancy check (CRC) for error detection
Lecture 3

65. Byte Stuffing
“data transparency” requirement: data field must be allowed to include flag pattern < >
Q: is received < > data or flag?
Byte Stuffing procedure
Sender: adds (“stuffs”) extra < > byte before each < > or < > data byte
Receiver:
when receive
discard the byte,
Next byte is data, regardless of value
Receive : flag byte
Lecture 3

66. Byte Stuffing flag byte pattern in data to send flag byte pattern plus
stuffed byte in transmitted data
Lecture 3

67. PPP Data Control Protocols
Before exchanging network-layer data, data link peers must
PPP-LCP: configure PPP link (max. frame length, authentication)
learn/configure network
layer information
for IP: carry IP Control Protocol (PPP-IPCP) msgs (protocol field: 8021) to configure/learn IP address
Lecture 3

68. EXTRA SLIDES
Lecture 3

69. Spanning Tree Concepts: Path Cost
A cost is associated with each segment
= “weight” of the segment
= cost associated with transmission on the LAN segment connected to the port
bridge associates the weight with relevant port
default segment weight is 1
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
Path cost is the sum of the component segment weights
Lecture 3

70. Spanning Tree Concepts: Root Port
Each non-root bridge has a Root port: The port on the path towards the root bridge
= parent pointer
The root port is part of the lowest cost path towards the root bridge
If port costs are equal on a bridge, the port leading to parent port with lowest ID becomes root port
Finally if several ports lead to the same parent port, choose lowest own port ID
Lecture 3

71. ST Concepts: Designated Port
Each LAN has a single designated bridge
all other bridges on LAN know which one it is
all tfc of LAN towards root goes thru that bridge
This is the bridge reporting minimum cost path to the root bridge for the LAN
ties broken by choosing lowest ID
Only designated & root ports remain active in a bridge.
designated ports connect to downstream bridges/LANs
root ports connect to upstream bridges/LANs (toward the tree root)
Bridge with no designated port becomes inactive
network objective: connecting hosts, bridges are a tool
Lecture 3

72. STP Requirements Each bridge has a unique identifier
A multicast address for bridges on a LAN
A unique port identifier for all ports on all bridges
Bridge id + port number
Lecture 3

73. Forwarding/Blocking State
Only root and designated ports are active for data forwarding
Other ports are in the blocking state: no forwarding!
If bridge has no designated port, no forwarding at all  block root port too.
All ports send BPDU messages
including blocked ones
some presentations don’t send BPDU on blocked ports
To adjust to changes
Lecture 3