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Spanning tree,trunk links,vlan-s..help?

Answered Question
Sep 19th, 2010
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The picture shows part of a network consisting of Cisco 3750 L3 switch that routes between VLANs and a number of physically separate segments with 2960 switches. They are connected with trunk's and redundant optical links to the L3 switch. L3 is a VTP server and the other switches are in client mode and L3 is the root bridge. Switch addresses are in VLAN 1 and L3 is their default gateway. Spanning tree mode on the switches is pvst and ports are in switchport mode access or trunk mode.

The request is to insert the new 2960 gigabit switches which enables access to the servers with gigabit link, that would use only its trunk with VLANs 1,150 and 160, and all other VLANs to use one of its redundant links. Does topology in picture allows such a thing?

How Spanning tree selects which redundant link to use and which to block and does it have any impact if I do create topology like on picture. Will the third link be blocked regardless that carries the other VLANs, so I'll end up having only one link?

Another possibility is that the two gigabit links that go to the new 2960 4-SFPA put in the ether channel, and I have 2 gigabits, but with the change so that upper link and lower ether channel carries all VLANs and to assign QoS for prioritizing vlan 150 and 160 (if it is possible to assign a Gigabit?). How do I set the switch in the ether channel to be root port, and not the top gigabit link (I want this link in blocked state)?

Do I have to leave STP mode in pvst or to change to the rapid-pvst and do I need something to change on the access ports or trunk ports for fast convergence (faster connections on ports)-enable Portfast level Switch?

Thanks...

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Correct Answer by Scott_O'Brien about 6 years 11 months ago

i would use port priority, as you are using gig links the cost for the link is 4 so it is low. also if you ever want to place another switch into the topology (ie another link in the chain) you will need to manage the costing.

Correct Answer by Scott_O'Brien about 6 years 11 months ago

Hi Dejan,


lets split this up,


the diagram looks fine to me, if you want to use one link for vlans 1,150 and 160 you can set the priority of the port so that it is lower for these vlans with the following command:

spanning-tree vlan num port-priority


then on the same port you will need to set up so that only the vlans that you want to cross the trunk link are allowed:

switchport trunk allowed vlan num


this should make it so thoes persific vlans only go over that one link.


the other vlans will go over the other link. as pvst is via per vlan. if there is a lot of traffic going over the link that carrys vlan 1,150 and 160 then it would be best the etherchanal them i recomend using pagp but lacp is fine to ( pagp is cisco only):

config t

interface range ( number)

channel-protocol pagp

channel-group 2 mode auto or desirable ( if auto make sure other side of link is desirable)


when a port is etherchanneld then stp will not place the port into blk state for thoes vlans.


on access ports i would recomend placing portfast on them.


( if any one sees anything wrong please correct me)


hope this helps you


thanks


Scott

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Correct Answer
Scott_O'Brien Mon, 09/20/2010 - 06:50
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Hi Dejan,


lets split this up,


the diagram looks fine to me, if you want to use one link for vlans 1,150 and 160 you can set the priority of the port so that it is lower for these vlans with the following command:

spanning-tree vlan num port-priority


then on the same port you will need to set up so that only the vlans that you want to cross the trunk link are allowed:

switchport trunk allowed vlan num


this should make it so thoes persific vlans only go over that one link.


the other vlans will go over the other link. as pvst is via per vlan. if there is a lot of traffic going over the link that carrys vlan 1,150 and 160 then it would be best the etherchanal them i recomend using pagp but lacp is fine to ( pagp is cisco only):

config t

interface range ( number)

channel-protocol pagp

channel-group 2 mode auto or desirable ( if auto make sure other side of link is desirable)


when a port is etherchanneld then stp will not place the port into blk state for thoes vlans.


on access ports i would recomend placing portfast on them.


( if any one sees anything wrong please correct me)


hope this helps you


thanks


Scott

DejanMilicevic Mon, 09/20/2010 - 14:02
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At the moment the situation on one segment (others are similar) is like  in figure until we get all Gigabit switches. So, the spanning tree  blocked Gi 0/2 on third switch and that's ok. Now if I want to set  priority so that VLANs 10,20 go only through upper link, and 30,40,50,60  only through lower link, how to do that, with port priority or path  cost, or with both.
This is VLAN Load Balancing Between Trunks Using the Spanning-Tree Protocol Port Priority found on http://www.cisco.com/en/US/tech/tk389/t ... e96a.shtml.
The  example is for redundant links between two switches, so we can create  port priority for certain VLANs and there is any waste. How can use this  method in my case?
Thanks...

Correct Answer
Scott_O'Brien Mon, 09/20/2010 - 17:53
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i would use port priority, as you are using gig links the cost for the link is 4 so it is low. also if you ever want to place another switch into the topology (ie another link in the chain) you will need to manage the costing.

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