Communication Networks

1
Communication Networks

• Recitation 3
• Bridges Spanning trees

2
Bridges

• Link layer device
• stores and forwards Ethernet frames
• examines frame header and selectively forwards
  frame based on MAC dest address
• when frame is to be forwarded on segment, uses
  CSMA/CD to access segment
• transparent
• hosts are unaware of presence of bridges
• plug-and-play, self-learning
• bridges do not need to be configured

3
Some bridge features

• Isolates collision domains resulting in higher
  total max throughput
• limitless number of nodes and geographical
  coverage
• Can connect different Ethernet types
• Transparent (plug-and-play) no configuration
  necessary

4
Bridges traffic isolation

• Bridge installation breaks LAN into LAN segments
• bridges filter packets
• same-LAN-segment frames not usually forwarded
  onto other LAN segments
• segments become separate collision domains

LAN (IP network)
5
Forwarding

• How do determine to which LAN segment to forward
  frame?
• Looks like a routing problem...

6
Self learning

• A bridge has a bridge table
• entry in bridge table
• (Node LAN Address, Bridge Interface, Time Stamp)
• stale entries in table dropped (TTL can be 60
  min)
• bridges learn which hosts can be reached through
  which interfaces
• when frame received, bridge learns location of
  sender incoming LAN segment
• records sender/location pair in bridge table

7
Filtering/Forwarding

• When bridge receives a frame
• index bridge table using MAC dest address
• if entry found for destinationthen
• if dest on segment from which frame arrived
  then drop the frame
• else forward the frame on interface
  indicated
• else flood

forward on all but the interface on which the
frame arrived
8
Question
9
Find all errors in the table and explain why?
Bridge name Error in table Explain




10
All Errors
1
Bridge B1 1,a1,f2,b
a
b
c
2
1
Bridge B2 1,f1,c2,b
d
e
2
f
g
11
Does a message reaches destination?

• From C to G
• From A to F
• From F to A
• What will happen to the tables?

12
From C to G
1
Bridge B1 1,a1,f2,b1,c
a
b
c
2
1
Bridge B2 1,f1,c2,b
d
e
2
f
g
13
From A to F
14
From F to A
1
2
2
1
1
2
15
Loop Resolving

• The simple learning mechanism described fails in
  presence of loops in the LAN
• Loops may be present by mistake, or deliberately
  provided for redundency
• This problem is resolved by running a distributed
  spanning tree algorithm

16
Spanning Tree Algorithm

• Creates a logical, or active topology that
  behaves like a spanning tree
• Makes alternate bridges redundant
• Is run periodically, so will discover failures
  and use alternate bridges if necessary

17
Spanning tree

• Think of the LAN as a graph that possibly has
  loops (LAN segments as nodes, bridges as edges)
• The spanning tree is a sub graph of this graph
  that covers all vertices (LAN segments), but
  contains no cycles.

18
Spanning tree algorithm

• Spanning tree algorithm is a protocol used by a
  set of bridges to agree upon a spanning tree for
  a particular extended LAN.
• Essentially, this means that each bridge decides
  the ports over which it is and is not willing to
  forward packets.
• Some ports (or even entire bridges) may not
  participate in a spanning tree
• How does the bridge select the ports to include
  (/exclude)?

19
Spanning Tree Algorithm

• Working Bridges regularly exchange frames known
  as Bridge Protocol Data Units (BPDUs). This
  exchange does the following
• Each bridge has a unique Identifier
• Bridge with highest priority and smallest ID is
  selected as root bridge.
• Each bridge determines for each port, the least
  cost path from root bridge to this port. This is
  the Root Path Cost (RPC) for this port.
• Select the port which has the least RPC and
  designate it as the Root Port (RP). This is the
  port which will be used for communicating with
  the root.

20
Algorithm...

• Once root port is determined, one bridge port is
  selected for each LAN segment as the designated
  bridge port (DP) over which frames will be sent
  for that LAN segment.
• This is a port (which is NOT a root port) which
  has the least path cost to the root
• The ports of the root bridge are always DPs for
  the LAN segments connected to the root bridge
• The state of the bridge ports can be set either
  to forwarding or blocking.
• All ports that are either RPs or DPs are
  forwarding, the rest are blocking.

21
Example

• B1 is the root bridge
• B3 and B5 are both connected to LAN A, but B5 is
  the designated port since it's closer to root
• B5 and B7 are both connected to LAN B, but B5 is
  the designated port due to smaller ID (equal
  distance).

22
Topology Initialization

• BPDUs are sent to a broadcast MAC address of all
  bridges on the LAN
• All bridges initially assume they are the root
  bridge
• Each BPDU contains (self ID, root ID,
  transmitting port ID, RPC of this port)
• A bridge updates its own info if it receives an
  update which
• identifies a root with smaller id or
• identifies a root with equal id but with shorter
  distance
• the root id and distance are equal, but the
  sending bridge has a smaller id
• The bridge adds 1 to the received RPC in the
  above update and saves this info.

23
Designated port / Root Port
And
What are these
And these
And this one
24
STP Run Find Root
B3 sends BPDU
A
B2 sends BPDU
B
3
2
1
B3
B1 sends BPDU
5
3
1
C
B5
1
7
B4, B2 sends BPDU
D
2
2
2
1
B7
K
B2
B8 sends BPDU
E
F
9
1
B9
1
B1
L
G
H
6
8
1
1
4
1
B6
B4
I
B8
M
J
25
Proof sketch

• if there is a bridge which has a different value
  THEN There is a segment on which one bridge has
  the correct minimum and the other a larger value.
• When the minimum will broadcast, the other bridge
  would update, and we have one more correct bridge

26
STP Run Block Ports
B5 5, 0, 1 B2 2, 1, 1
A
B
B3
B3 BLOCK
C
B5
B7 7, 0, 1 B5 5, 0, 1
D
B7
K
B2
B7 BLOCK
E
F
B9
B1
L
G
H
B6
B4
I
B8
M
J
27
Data
Laptop B
A
B
Message A to B
Message B to A
B3
C
B5
D
B7
K
B2
E
F
B9
B1
L
G
H
B6
B4
I
B8
M
J
Laptop A