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TOPIC: Network looping

Network looping 11 years 6 months ago #8559

  • apit
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can this type of design cause looping to network?
without spanning tree or MLT configuration
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Re: Network looping 11 years 6 months ago #8560

  • stefke
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Greetings Apit,

Asuming that you envision following setup:

[code:1]
[SW1] --- [SW2] --- [SW3]
| | | | | |
H1 H2 H3 H4 H5 H6
[/code:1]

I don't think a network loop would occur. If we take following example (all switches have empty MAC table):

H1 wants to sent to H5: pakket goes from H1 --->SW1: Empty MAC table so flooding = Packet goes to H2 and to SW2. SW2: Empty MAC table so flooding = Packet goes to H3, H4 and SW3: Empty MAC table so flooding = Packet goes to H5 and H6. SW3 AND SW1 have now an entry for H5 in their MAC table.

A loop would occur if e.g. SW1 was also connected to SW3. In that case SW1 would receive a MAC entry for for H5 on two ports (one connected to SW2 and one connected to SW3) and a loop is created.

Maybe some of the senior forum members can add/correct my explenation cause I'm farily new to the network world.

Greetings,

Stefan
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Re: Network looping 11 years 6 months ago #8565

  • apit
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from my understanding, looping will occur when switch is connect like below::



or




question

1-when looping occur, what is the effect to network? is it slow network performance?
2-Can software detect network looping?
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Re: Network looping 11 years 6 months ago #8569

  • stefke
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Greetings apit,

In answer to your questions:

1) When a loop occurs, your network wil saturate. Packets in a loop are muliplied exponentionaly so after certain timeframe your network will no longer be able to sent ANY traffic !

2) Spanning Tree Protocol is designed to avoid loops. The goal of STP is to make sure that there is only one way (the most efficient way) from point A to point B in your network.

check out :

www.cisco.com/univercd/cc/td/doc/product...ug2/vlan2/stpapp.htm

Greetings

Stefan
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Re: Network looping 11 years 6 months ago #8579

  • apit
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if looping occur, the network design are as below :
[SW1] --- [SW2] --- [SW3]
[SW1]
| | | | | | | |
H1 H2 H3 H4 H5 H6 H1 H2
H1 wants to sent to H5: pakket goes from H1 --->SW1: Empty MAC table so flooding = Packet goes to H2 and to SW2. SW2: Empty MAC table so flooding = Packet goes to H3, H4 and SW3: Empty MAC table so flooding = Packet goes to H5 and H6. SW3 AND SW1 have now an entry for H5 in their MAC table.

if H1 has receive H5, should the packet stop at H5 or the packet still transfer back to H1 and then H5 and again and again......[/quote]
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Looping 11 years 6 months ago #8620

  • TheBishop
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If there is one and only one route between any two points in your network then you won't have a loop. If there is more than one possible route between any two points (and I mean any) then some sort of loop will be introduced.
The network does indeed saturate when you have a loop - after a small amount of initial genuine traffic has been sent the packets circulate round and round the loop and the packet storm builds in intensity until every port on the loop path is saturated. The symptoms are than nothing works and when you look at the switches almost every light is on!. This is very instructive and everyone should try it at least once in their carreer to see what it looks and feels like (preferably on a bench, not the live network!)
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