Recitation 3
Bridges & Spanning trees

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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

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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

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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 collision domain bridge collision domain
= hub
= host
LAN segment LAN segment
LAN (IP network)

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

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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
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 when frame received, bridge “learns” location of sender: incoming LAN segment records sender/location pair in bridge table

When bridge receives a frame: index bridge table using MAC dest address if entry found for destination then{ 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

a d b e
1
Bridge B1 = {1,a};{1,f};{2,b}
2
1
Bridge B2 = {1,f};{1,c};{2,b}
2 f c g

Bridge name Error in table Explain

f a d b e
1
Bridge B1 = {1,a}; {1,f} ; {2,b}
2
1
Bridge B2 =
{1,f}
;{1,c}; {2,b}
2 c g

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

a d b e
1
Bridge B1 = {1,a};{1,f};{2,b}{1,c}
2
1
Bridge B2 = {1,f};{1,c};{2,b}
2 c f g

a d b e
1
Bridge B1 = {1,a};{1,f};{2,b}
2
1
Bridge B2 = {1,f};{1,c};{2,b}
2 f c g

a b d e f
2
1
2
1
2
1 c g

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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

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

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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.
(a) (b)

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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)?

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1.
2.
3.
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.
a) 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 .

1.
2.
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. a) This is a port (which is NOT a root port) which has the least path cost to the root b) 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.
a) All ports that are either RPs or DPs are forwarding, the rest are blocking.

I
I
B B
F F
K
K
J
J
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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).

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BPDUs are sent to a broadcast MAC address of all bridges on the LAN
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All bridges initially assume they are the root bridge
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Each BPDU contains (self ID, root ID, transmitting port ID, RPC of this port)
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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
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The bridge adds 1 to the received RPC in the above update and saves this info.

And
A What are these
B
B3
C
B5
D
B7
K
B2
E F
And these
And this one
G
B1
H
B6
B4
I
J

A
B
1
B3
C
D
B5 1
1
7
B7
2 2
E
B2
F
B3 sends BPDU
B2 sends BPDU
B1 sends BPDU
K
B4, B2 sends BPDU
B8 sends BPDU
1 9
B9
1 B1
L
G H
I
B6
B4
B8
1 8
M J

 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.
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When the minimum will broadcast, the other bridge would update, and we have one more correct bridge

A
B
B3
C
B5
D
B2
B7
K
E F
B5: 5, 0, 1
B2: 2, 1, 1
B3: BLOCK
B7: 7, 0, 1
B5: 5, 0, 1
B7: BLOCK
B9
B1
L
G H
B6
B4
B8 I
M J

A
C
E
B2
B3
D
B5
B1
G
I
B6
B4
Laptop B
H
B
B7
F
K
B9
J
Laptop A
B8
L
M