Posted on November 5, 2019 6:30 pm
 |  Asked by Félix Bouynot
 |  123 views
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Hello,

I’m trying to setup the following topology (Backbone is Arista 7280SR, Cust is Cisco N3K):

Bacbone A =========== Backbone B
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Cust site A ========= Cust site B

Cust is using some vlan, let’s say 100 to 200.
We share some vlans for internal usages, let’s say 2 and 3.
I’m q-in-qing all others vlan in let’s say vlan 10.

So, here are conf for Backbone A and B to respectively Cust A and B:

switchport trunk allowed vlan 2,3,10
switchport mode trunk
switchport vlan mapping 100-200 dot1q-tunnel 10
spanning-tree link-type point-to-point
spanning-tree guard loop

Backbone A to backbone B (and B to A):

switchport trunk allowed vlan 2,3,10
switchport mode trunk
spanning-tree link-type point-to-point
spanning-tree guard loop

And the conf for Cust site A and B to every other:

switchport mode trunk
switchport trunk allowed vlan 2,3,100-200
spanning-tree link-type point-to-point
spanning-tree guard loop

This topology create a L2 loop which is not mitigated by spannin-tree.
Vlan 2-3 doesn’t create loop.
Vlan 100-200 / 10 are.

If EOS were NX-OS, I could just do on Arista to Cisco interfaces:
l2protocol-tunnel stp
Well, i would need to be able to do that on only vlan 100-200 encapsulation in 10 anyway.

Is there a way for me to keep this redundancy but avoid the l2 loop?

Thank you

1
Answered on November 5, 2019 8:55 pm

Hi Felix,

Thank you for writing to the forum. May I know what is the EOS version on the 7280SR?

We do support L2 forwarding on the platform. More details are here:

https://eos.arista.com/eos-4-22-0f/l2-protocol-forwarding/
https://eos.arista.com/eos-4-21-0f/l2-protocol-forwarding/

Thank you,

Hello, Thank you very much for pointing me this! [Note: because of bad formatting, we don't see but Backone B and Cust site B are linked too] The version is 4.21.3F, so it looks like I need to upgrade to 4.22.0F to be able to support Ethernet ports and tag / untag keywords with L2 forwarding. Then, I need to create a specific hardware tcam profile with what's in your first link and apply it. Then, I need to configure a l2-protocol forwarding profile, and apply it to the interface. The documentation example is: switch(config-l2-protocol)# forwarding profile abc switch(config-l2-protocol-abc)# lacp forward switch(config-l2-protocol-abc)# stp untagged forward switch(config-l2-protocol-abc)# exit switch(config-l2-protocol)# With rapid-pvst, PVST+ BPDU are sent tagged except for vlan 1. In my example, how am I supposed to differentiate vlan 2 & 3 BPDU from vlan 100-200 BPDU? Wouldn't "stp untagged forward" create a loop for vlan 2 & 3 ? Thank you,
(Félix Bouynot at November 6, 2019 11:26 am)
Hi Felix, The switches that forward the L2 frames without trapping it do not differentiate in the tags. However, the switches that consume the packet does. The STP on these switches that consume the packet will take care of creating the loop-free topology. Thanks,
(Dharmesh Mithbavkar at November 19, 2019 9:39 pm)

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