Preventing Switch Loop

Preventing  Switch Loop

I believe that Best Practice is that Access Switches should only connect to Layer 3 devices such as  Routers or High end Switches(6500) that use VSS or  Nexus Switches.
However sometimes Access Switches connect to other Access switches in the chain before they get to High end Switch (6500).

I wonder in this case if you have Access Switches connected in chain , whether  you need to configure the closest Switch to the High end switch(6500) as a Root Switch for all VLANs ?


Thank you
jskfanAsked:
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Fred MarshallPrincipalCommented:
I'm not familiar with that language or those terms exactly.

Here are some assumptions:
There is but ONE cable going upstream (or, perhaps, aggregated links treated as one).
IF the one cable going upstream is supposed to carry all the VLANs upstream then it needs to be "trunked".
This will be the case even if the switch isn't the "closest switch to the high end".

I hope that helps.
Others with experience with the terms you're using may clarify.
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jskfanAuthor Commented:
example  of Access Switches connection:

SW1--SW2--SW3-----Distribution Switch(6500)

If someone plugs another Switch to SW1 and has a lower priority , it will become the Root ..that might cause some issues (That I am not aware of exactly)
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JustInCaseCommented:
Issue with positioning of root bridge is valid only in the case when there are redundant links in topology, otherwise root bridge placement is generally irrelevant (except in the case where there is a "too many L2 hops"), but still ... most logical place for root bridge typically is L3 device or device closest to L3 device. L2 loops are typically prevented by STP or REP protocol.

if switch is plugged into SW1 it would be bad design, but in topology
SW 4 -- SW1--SW2--SW3-----Distribution Switch(6500)
placement of root bridge would be irrelevant (but recommended root bridge location would be distribution switch).
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jskfanAuthor Commented:
Let 's say we do not have : Distribution Switch(6500) we hae a router.
if SW4 is  the Root.. will that cause any issue ?
SW 4 -- SW1--SW2--SW3-----Router
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JustInCaseCommented:
No.
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jskfanAuthor Commented:
if it can cause topology change, it might cause an issue. ?
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JustInCaseCommented:
It will just cause temporary network disruption, after network stabilizes/reconverges it will not matter where root bridge is located in such topology.

What is important to understand:
1. root bridge is base point to establish loop free topology
2. after loop free topology is created root bridge has no influence on traffic patterns (I mean - beside creating loop free topology which can cause point 4. below)
3. if there is no redundant paths in topology resulting topology always look the same regardless of root bridge position (what we are discussing in topology above)
4. wrong root bridge placement, if there are redundant links in topology, can have devastating effect on network performance
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jskfanAuthor Commented:
4. wrong root bridge placement, if there are redundant links in topology, can have devastating effect on network performance

What are things you need to watch for when you place your switches in the network, in regards to  Root Switch.?

Because if it does not matter which Switch is the Root, then we can just connect Switches  to the Network without worries.
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JustInCaseCommented:
I am not saying that root bridge placement is not important. What I am saying is that if there is no redundant links in topology - root bridge placement is not important.

No redundant links in topology (what we were discussing above - SW 4 -- SW1--SW2--SW3-----Router):

 STP - no redundant links
So, wherever root bridge is placed in topology without redundant links, resulting topology is always the same. The only difference could be where to place specific switch. Slow switches should be placed at the edge of the network, but which switch will be root bridge in such topology makes no difference.

Redundant links present in topology (root bridge placement is relevant):

 STP - redundant links
All switches are from the same manufacturer. Switch in the middle is the oldest switch supporting 100Mb speed on its interfaces (on three topologies). STP priority is default 32768. MAC addresses on oldest switches are typically lower than on new ones.
Top left corner - All links in topology between switches before STP blocks redundant links
Top right corner - Since priority is not changed on any switch oldest switch will be chosen as root bridge (oldest switch will have lowest bridge ID) . All traffic between switches will go through 100Mb links. Gigabit links between gigabit capable switches will be blocked.
Bottom left corner - on top left switch priority was lowered (for example to 4096) and that switch become root bridge. 1 Gb links will be used between 1 Gb capable switches. But, there are 3 links to forward traffic between 2 switches placed on the right side of topology which is less than ideal.
Bottom right corner - if 1 Gb capable switch is placed in the middle of topology and priority is lowered on that switch, that switch becomes root bridge and there would be no more than 2 links between any 2 switches (the best resulting topology).
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jskfanAuthor Commented:
in the Redundant Link scenarios:
the worst that can happen if a Root switch is in wrong place, will be  the traffic to take longer than if the Root is in the right place ,
that 's because of the blocked ports, and the traffic will have to take a longer path.
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JustInCaseCommented:
No, there is a lot of things that can go wrong. Above are just simple scenarios to illustrate issue.
It may mean, for example, difference between not to have traffic drops and a tons of traffic is dropped (longer path may mean a lot of congested interfaces).
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jskfanAuthor Commented:
Also..on the top Left topology where all Links are redundant. If I add a Switch and link it to one of the switches on the corners, and the switch turns out to have the lowest Bridge (it becomes Root), will this cause Topology Change, which means it will shake up the network for a moment then it will re-converge ?
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JustInCaseCommented:
On all 4 topologies links are redundant (all 4 topology are reflection of the first one (with devices rearranged in the bottom left topology), I just removed on others links that typically are to be blocked (for clearer view on resulting topology)).
When new device is added or removed TCN BPDU is always sent, even if device will not become root bridge (if portfast is not configured on port that is changing state up/down). Impact on neighboring switches that are part of same broadcast domain depends on STP type that is in use and if new switch has superior bridgeID comparing to current root bridge.
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jskfanAuthor Commented:
I will need to read more about STP later..
Thanks for the clarification
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JustInCaseCommented:
You're welcome.
You can find pretty detailed explanation - Understanding STP and RSTP Convergence
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