Spanning tree loop

[shayan_2k@hotmail.com]

Hi 

I am running some spanning tree loop issue on my switches 

We have 2 sites connected with metro ethernet link 

On each site we have cisco 3850 stack switches 

On each site we have 2 links from different service providers connecting site A and site B switches 

We are making trunk link between between Site A and Site B 

So we have 2 trunks 

1 trunks is on  ISP A and 1 on ISP B 

We have enabled rstp on both switches 

We prefer isp1 over isp2 so we increased cost on isp2 link on site B switch 

So ideally isp2 link shoud go into Alt path Blocking state 

But non of the link is going on blocking state 

And i see output of  

#show spanning tree 

Both trunks on both switches are in Desg/forwarding 

Means non of the port on both switches are in forwarding which is causing loop in network and ports on site B switch are flapping 

I would need some assistance please

[paul driver]

Hello

You could utlilise a flex link which will put the specified isp b link into a backup link so to use the isp a as the primary link This should negate your switching looP

int x/x

description primary isp a

switchport backup interface xxx ( isp b)

[rasmus.elmholt]

What kind of metro ethernet link are you running?

Most likely STP is not send correctly through the ISPs.
On Site B do you see the correct root bridge of Site A or is your STP "blocked" my the ISPs?

[shayan_2k@hotmail.com]

I m not sure
But in spanning tree output i see link says P2P and other one says Shr
Also ill try flex interface tomorrow

[paul driver]

Hello

---

shayan_2k@hotmail.com wrote:
i see link says P2P and other one says Shr

---

Check the speed/duplex settings usually you see this when the port is running half duplex

[shayan_2k@hotmail.com]

i tried changing speed and duplex full but didnt worked

also i tried flex interface but i think this command is not supported on 16.3.6 

switch is not accepting switchport backup interface gx/x

[rasmus.elmholt]

I can't find it in 16.6.4a either.

This configuration guide mentions flexlinks but it is not available:
https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9300/software/release/16-6/configuration_guide/sec/b_166_sec_9300_cg/configuring_port_based_traffic_control.html

[Cisco Freak]

Do you see BPDU sending and receiving at the switchport?

LAB-SW-01#sh spanning-tree interface g1/0/24 detail  | i BPDU
   BPDU: sent 123888, received 5045088