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Connecting switches thru uplinks

avilt
Level 3
Level 3

In my current DC I have two L2 switches connected thru one gbps uplinks with around 6 vlans. As I am running out of L2 ports so I would like to add one more L2 switch and the new setup will become like the one attached with this thread. Is this setup alright for the DC. Is it going to bring down the perforamcne as the traffic has to pass thru all three switches if the vlan spans all three L2 switches? At max how many L2 switches can be uplinked like this? Also I am keeping these three L2 switcehs in a different rack, is there any limitation on uplink cable length? I am using LAN cables for uplinks as all switches support gbps L2 ports.

Thank You

2 Accepted Solutions

Accepted Solutions

TarasKotov
Level 1
Level 1

Dear avilt,

your picture is too small to see any text. I only see 3 siwtches connected with some sort of links.

But:

1. This setup (3 switches) is alright as long as uplink and trunk ports are not fully (100%) loaded.

2. Every network device is adding some latency cause it need to store packet, analyze it, queue and transmit it. With right QoS parameters it's not a big problem having 3 switches instead of 2.

3. Theoreticaly there is no limit how many switches can be connected to each other. Just remember not to make too big broadcast domains.

4. Your cables ("twisted pair" with 4 pairs - cat.5 or cat.5e) should not be longer than 100 meters. It's a standard.

View solution in original post

Couple of points -

1) If these are 2960S switches then use Rapid PVST and not standard PVST. It is trivial to enable it and it provides much faster convergence. If you do use RPVST+ make sure your switch interconnect links are P2P links and you enable "spanning-tree portfast" on all client ports.

2) STP root bridge. It matters in terms of one vs two L2 hops eg.

STP root bridge = SW1.  Because of this the port that will block will one of the ports connecting SW2 to SW3.

If you connect your active firewall to SW1 then both SW2 and SW3 have one L2 hop to get to SW1. If you connect your active firewall to SW2 then SW1 has one L2 hop but SW3 must go via SW1 because the link between SW2 and SW3 is blocked so that is two L2 hops.

So it would make sense to have the active devices connected to your root bridge in this scenario.

Also you are in danger of overloading SW1 as it has to handle it's own traffic to the active firewall and that of SW3.

Of course this supposes that traffic between clients on SW2 and SW3 is minimal. If most of the traffic is between SW2 and SW3 then you would want to make one of those the root bridge.

Bear in mind that with Cisco it is per vlan STP so you can in theory have a different root bridge for a set of vlans. Not generally done, especially in large network because of the additional complications, but it does depend on your traffic flows.

Jon

View solution in original post

15 Replies 15

TarasKotov
Level 1
Level 1

Dear avilt,

your picture is too small to see any text. I only see 3 siwtches connected with some sort of links.

But:

1. This setup (3 switches) is alright as long as uplink and trunk ports are not fully (100%) loaded.

2. Every network device is adding some latency cause it need to store packet, analyze it, queue and transmit it. With right QoS parameters it's not a big problem having 3 switches instead of 2.

3. Theoreticaly there is no limit how many switches can be connected to each other. Just remember not to make too big broadcast domains.

4. Your cables ("twisted pair" with 4 pairs - cat.5 or cat.5e) should not be longer than 100 meters. It's a standard.

Fabio Francisco
Level 1
Level 1

Hi Avitt,

1.) what flavour of STP are you planning to use?

2.) make sure to manually assign the root bridge accordingto your network needs.

Cheers,

Fabio

All 3 are new Catalyst 2960S 48 GigE  4 x SFP LAN Base switches and I do not have core switches in the DC. Just 3 L2 switches and routers are connected to a few Vlans. It will use its default PVST+ right? Why do I need to worry bout root bridge in this setup?

Bridge switch has to be the one who is connected to a router. If not, there will be a situation where traffic from host (PC, server, etc.) to a router will traverse through all 3 switches.

For example SW2 is a root switch and router R1 is connected to R3:

PC - SW1 - SW2 - SW3 - R1

If SW3 will be a root switch, than:

PC - SW1 - SW3 - R1

Of cause, if there is no 100% trunk load between switches, you can use default STP settings. But it a poor design and can lead to a bad network habit in a future.

Please see the attached connection diagram, no core switches. We have a couple of firewalls and routers with redundency setup so I would like to distribute the cables from these devices with different switches. For example all cables from active firewall to one switch and standby firewall to another switch etc. In this scenario why do I need to concerned about root bridge? Appreciate some detailed explanation.

Are all of these switches going into the same cabinet?

If possible I would like to put them in a different rack (max distance is 5 meters) connected they Cat5e cable.

If possible I would like to put them in a different rack (max distance is 5 meters) connected they Cat5e cable.

Too bad.  You could've taken advantage by stacking them to form a logical switch.   Don't think the stacking cables are that long.

I do not have patch panels at central location so I would like to keep each switch close to the desktops, What's wrong with this setup? Any perofrmance issues? All 48 ports are gbps ports.

Couple of points -

1) If these are 2960S switches then use Rapid PVST and not standard PVST. It is trivial to enable it and it provides much faster convergence. If you do use RPVST+ make sure your switch interconnect links are P2P links and you enable "spanning-tree portfast" on all client ports.

2) STP root bridge. It matters in terms of one vs two L2 hops eg.

STP root bridge = SW1.  Because of this the port that will block will one of the ports connecting SW2 to SW3.

If you connect your active firewall to SW1 then both SW2 and SW3 have one L2 hop to get to SW1. If you connect your active firewall to SW2 then SW1 has one L2 hop but SW3 must go via SW1 because the link between SW2 and SW3 is blocked so that is two L2 hops.

So it would make sense to have the active devices connected to your root bridge in this scenario.

Also you are in danger of overloading SW1 as it has to handle it's own traffic to the active firewall and that of SW3.

Of course this supposes that traffic between clients on SW2 and SW3 is minimal. If most of the traffic is between SW2 and SW3 then you would want to make one of those the root bridge.

Bear in mind that with Cisco it is per vlan STP so you can in theory have a different root bridge for a set of vlans. Not generally done, especially in large network because of the additional complications, but it does depend on your traffic flows.

Jon

Fabio Francisco
Level 1
Level 1

IMO your version of STP is fine...

You have to choose carefully your root bridge as previously explained above.... otherwise your data will be traversing the entire network ....

I usually set the command statically specifying root primary and secondary instead of bridge ID values, that way if a device is put by mistake with lower bridge ID your root switch will lower it's own ID to mantain the level of root bridge of the network.

I usually set my access ports with: switchport host, and my redundant links with uplink fast

Cheers,

Fabio

Hi Fabio,

Fabio Francisco a écrit:

IMO your version of STP is fine...

You have to choose carefully your root bridge as previously explained above.... otherwise your data will be traversing the entire network ....

I usually set the command statically specifying root primary and secondary instead of bridge ID values, that way if a device is put by mistake with lower bridge ID your root switch will lower it's own ID to mantain the level of root bridge of the network.

I usually set my access ports with: switchport host, and my redundant links with uplink fast

Cheers,


The macro commands spanning-tree vlan x root primary/secondary are only operational at the time

they were configured.If afterwards you have a switch with lower priority it will take over as root bridge.

This command is not adaptive, it's a one-time command.

Regards.

Alain.

Don't forget to rate helpful posts.

oops gotta be careful with what I say here...

i've got some free time on my hands and I decided to be more participative in forums to help and learn..... but I believe I have been learning more than helping...

Hi fabio,

We always learn every day and helping is also learning.

I just wanted to let you know that your staement was incorrect so you can learn from it an other people reading the post also.

I also say some incorrect things because learning is a continuous neverending journey and others here gracefuly correct me.

Regards.

Alain.

Don't forget to rate helpful posts.