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Community Member

spanning tree

hi! I've a stack of 3x 3524 switches. The top and bottom switches will have a trunk link to the 6509 catalyst core switches. I've attached the spanning-tree config details below. How do i make it in such a way that only the bottom switch will do the forwarding whereas the top switch's trunk as a standby? Please advise. Thanks.

Bottom Switch

Gi0/2 is the one connected to the core switch 1

Interface Gi0/2 (port 48) in Spanning tree 16 is FORWARDING

Port path cost 3004, Port priority 128

Designated root has priority 100, address 00d0.0091.081f

Designated bridge has priority 100, address 00d0.0091.081f

Designated port is 8, path cost 0

Timers: message age 2, forward delay 0, hold 0

BPDU: sent 1208, received 2308085

Interface Gi0/1 (port 40) in Spanning tree 16 is FORWARDING

Port path cost 3004, Port priority 128

Designated root has priority 100, address 00d0.0091.081f

Designated bridge has priority 49152, address 0001.96bf.394b

Designated port is 40, path cost 3004

Timers: message age 0, forward delay 0, hold 0

BPDU: sent 2308981, received 1616

24-C#

Top Switch

Gi0/2 is the one connected to the core switch 2

Interface Gi0/2 (port 48) in Spanning tree 16 is FORWARDING

Port path cost 3004, Port priority 128

Designated root has priority 100, address 00d0.0091.081f

Designated bridge has priority 120, address 00d0.0091.140f

Designated port is 194, path cost 3

Timers: message age 4, forward delay 0, hold 0

BPDU: sent 275, received 2306199

Interface Gi0/1 (port 40) in Spanning tree 16 is BLOCKING

Port path cost 3004, Port priority 128

Designated root has priority 100, address 00d0.0091.081f

Designated bridge has priority 49152, address 0001.96bf.394b

Designated port is 40, path cost 3004

Timers: message age 2, forward delay 0, hold 0

BPDU: sent 724, received 2309035

12 REPLIES
Hall of Fame Super Silver

Re: spanning tree

Hello,

the stack member switches are interconnected via the stackwise cables but from the STP point of view these are just trunk ports carrying all the vlans defined on the stack and the BPDU frames.

So then it comes to elect a root port the whole stack behaves as a single switch.

See the following link

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_35_se/configuration/guide/swstp.html#wp1266920

These statements are true when the switch stack is operating in PVST+ or rapid-PVST+ mode:

•A switch stack appears as a single spanning-tree node to the rest of the network, and all stack members use the same bridge ID for a given spanning tree. The bridge ID is derived from the MAC address of the stack master.

hope to help

Giuseppe

Community Member

Re: spanning tree

hi! Thanks for the reply.....but i think that's not quite applicable to the old 3524 sw. There isn't any special stacking cable used as in the newer 3750 switches.

What i don't understand here is that, i thought there will only be one link that's active when you've multiple connection to the core switch, but now both is active. There isn't any etherchannel configured as well. Let me know if you've any other way that i can configure the switch to forward traffic though the top switch and not though the bottom one as well. Thanks.

Community Member

Re: spanning tree

Topology diagram will help here.... In general, you'd want to make the path to the root bridge shorter thru the bottom switch by manipulating Bridge Priority, Port Cost, and Port Priority. It's quite fiddly to do this if your connections b/w the 3524s and the 6509s are in what I'd call "criss-cross" configurations. If you want strictly primary/secondary set up for your 3524s, "Box"/"Square" configurations are probably better. Edit: "Box"/"Square" configuration is basically where you trunk both core switches, then uplink each access switch to 1 core switch, and connect the two access switches between them with another trunk (I tried to depict this before, but didn't quite come out right when I posted it).

Assuming you're using "box"/"square" configurations to connect your 3524s to the 6509s, here's what you should do:

- Configure 6509-1 with spanning tree priority of 8192 for all VLANs

- Configure 6509-2 with spanning tree priority of 16384 for all VLANs

- Configure bottom 3524 with spanning tree priority of 20480 for all VLANs

- Configure the top 3524's spanning tree priority value to 32768 for all VLANs.

Personally, I prefer "criss-cross", where the 2 core switches are trunked between them, then each access switch has a trunk to each of the 2 core (no trunk to other access switch). Then configure both core switches as primary/secondary root bridges for all VLANs, while all other switches are left to their default priority values of, usually, 32768.

Community Member

Re: spanning tree

hi! I think that would be a major changes :). The current settings do have trunk in between these 2 core switches and all every stacks of switches will have 1 link to core1 and one link to core2. What i;m puzzle here, is that why does both of these links from the stack of switches to the cores is in forwarding state? My understaning is that without etherchannel, spanning tree will only allow one forwarding link from switch to switch. The "standby" one will only be active when the current forwarding link is down.

Please advise further how can i make this happens. Thanks.

Community Member

Re: spanning tree

hi! need help on this....how do i increase the priority of the bottom link (3524sw) over the top link(3524sw) to the core switches(6509) for access on server resources (not just the root bridge). The reason of doing this is i suspect of intermittent connection problem on the top link. Probably because of old cabling. So, i would want to redirect the traffic from the edge switch through the bottom link to the core switch. Please advise. Thanks.

Hall of Fame Super Silver

Re: spanning tree

Hello David,

the command you need is given in global configuration mode:

spanning-tree priority

follow the suggestions of the post sent by the other person

Hope to help

Giuseppe

Community Member

Re: spanning tree

So, if I understand you correctly, your set up is "box" configurations with trunk between the two cores, trunk between the two 3524s, and then 1 uplink from each 3524 to one of the cores. In this case, it's quite normal for both uplinks to core to be FORWARDING. Simply because the 3524s "realize" that the shortest path to the root bridge is via its direct uplink to the core rather than thru the trunk link between themselves.

Command to change spanning tree priority is:

switch(config)# spanning-tree vlan priority

can be a single VLAN ID, or a range of VLAN IDs, e.g.:

switch(config)# spanning-tree vlan 1-1005 priority 8192

Edit: btw, use the command above along with the priority values that I suggested before. Once you've done that, all traffic should flow through your bottom switch.

Cisco Employee

Re: spanning tree

If you look closely your bottom 3524 switch is the Root bridge for Vlan 16. The Root bridge ID's MAC address and the Designated Bridge ID is the same MAC address of "00d0.0091.081f". So that tells me the the two Core Switches 1 and 2 have a higher bridge priority than 100. So you simply need to make the Core switches bridge priorities lower than 100 and be Root bridges for VLAN 16. I would suggest priority 0 for Core 1 (Root) and Priority 1 for Core 2 (Standby Root), so there is no possibility another switch plugged into this STP domain could potentially win root election away from your two Core switches.

Bottom Switch

Gi0/2 is the one connected to the core switch 1

Interface Gi0/2 (port 48) in Spanning tree 16 is FORWARDING

Port path cost 3004, Port priority 128

Designated root has priority 100, address 00d0.0091.081f <----- Same

Designated bridge has priority 100, address 00d0.0091.081f <----- Same

Hall of Fame Super Silver

Re: spanning tree

hello,

you found the reason but I don't think bottom switch is the route bridge for vlan 16

Gi0/2 is the one connected to the core switch 1

>>>Interface Gi0/2 (port 48) in Spanning tree 16 is FORWARDING

Port path cost 3004, Port priority 128

Designated root has priority 100, address 00d0.0091.081f

Designated bridge has priority 100, address 00d0.0091.081f

>>>>Designated port is 8, path cost 0

Timers: message age 2, forward delay 0, hold 0

>>>BPDU: sent 1208, received 2308085

gi0/2 is a root port because it is receiving a lot of BPDUs and is connected to a switch that is the root bridge so the two MACs of root bridge and designated bridge id are the same.

the designated port on segment is the number 8 on the root bridge.

Hope to help

Giuseppe

Community Member

Re: spanning tree

hi! I think you're right, the core switch that the bottom sw is connected to is the root bridge. I did a spanning tree command on core1 and shows that the root and designated brige's MACs are the same.

I don't quite get the explanation on the bottom switch whereby you explain why the root and designated bridge are the same for the bottom sw. Doesn't it suppose to show different MAC for root and designated MAC since the bottom sw is not an root bridge?

Re: spanning tree

Don't play with the priority. It's almost irrelevant here. Here, you have a ring of bridges (from the perspective of STP). STP is going to block the link that is the further away from the root bridge, so it's probably going to block around the middle of your stack.

It's very simple, if you want to block a port in particular, just increase the cost of this port. So go to your last switch, where you want the vlans to be blocked, and configure a huge cost on it (if you don't want to think, the put the highest value possible). That's all you need.

Regards,

Francois

Hall of Fame Super Silver

Re: spanning tree

Hello,

the show is about a specific port of the bottom switch and tells that root bridge-id = designated bridge-id

Root bridge is a vlan-wide concept.

the last is a local concept: a designated port is elected on every network segment.

The designated bridge-id simply is the bridge-id present on the best BPDU heard on segment that is the one sent out by the designated port.

So you can understand that in this case the two can be the same: because gi0/2 of bottom switch is its root port for vlan16 and it is directly connected to the core switch that is the root.

For another vlan where the root bridge is the other core switch you could see what you expect: that is the top switch to have a forwarding port.

I think what you see is normal given the inter-switch connections and the STP configuration.

Hope to help

Giuseppe

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