- Effect of
loops in a
computer network
- Frames (messages) will be repeated indefinitely (if a network contains loop) !!!
- When messages are
repeated
indefinitely,
the network will
become
extremely congested
- Network performance will drop dramatically and the network becomes useless !!!
- Example: a network with
a loop
- The loop problem
in a network illustrated:
- We begin with
empty
forwarding tables:
Suppose:
- Node X transmits a frame to node Y
- The frame will
first be
received by
the switches Sw0 and Sw3:
Result:
- Sw0 and Sw 3 will learn (insert) the source node X
- Sw0 and Sw 3 will then forward the frame on all other ports
- After one more round of
forwarding:
Result:
- Sw 1 will learn (insert) the source node X
Notice that:
- The entry for X in the two forwarding tables of switch Sw1 !!!!!
- After one more round of
forwarding:
Notice that:
- The frame is
forwarded on
- LAN A and LAN D
becase the destination address was found in another forwarding table !!!
- The frame is
forwarded on
- Now the network
is in trouble:
There are now:
- 2 copies of the frame on the source LAN A !!!
- Each copy will
be forwarded
through the loop
again:
Each copy will generate another 2 copies of the frame !!!
- We begin with
empty
forwarding tables:
- Conclusion:
- A loop will cause
frames (messages) to
multiply indefinitely in
the network
- Furthermore:
- The copies of a frame (message) will cycle indefinitely !!!
- A loop will cause
frames (messages) to
multiply indefinitely in
the network