Broadcast switching on VC

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KernelPanic
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Posts: 4
Joined: 25 Jul 2019 16:16

Broadcast switching on VC

Post by KernelPanic » 26 Jul 2019 15:36

Hello Alcatel-friends,

i'm quite new to switching in general and Alcatel in special. At the moment i'm struggling with maybe a bit stupid question.
Would be nice if you could shed some light 8)

So i'm trying to understand how the switches handle broacasts and/or unknown unicast packets - especially in a VC and in combination with Spanning Tree.

Let's imagine we have a default VC with two chassis, connected over the VFL, only the default vlan is configured on all ports.
On port 1/1/1 arrives a broadcast and with my basic understanding i would say - the switch has to flood the broadcast out of all connected interfaces, except 1/1/1. So the switch has to transmit the broadcast over the VFL and also flood it on the second chassis. So far so good.

Now there comes a second VC with two chassis.
Port 1/1/2 VC 1 is connected with 1/1/2 VC 2
Port 2/1/2 VC 1 is connected with 2/1/2 VC 2

So we have two physical inks between the VCS, no aggregation, on the same vlan.

Now my question - loop or no loop? What happens with broadcasts arriving on VC 1 Port 1/1/1.

Thanks for you help and patience.

KernelPanic

silvio
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Re: Broadcast switching on VC

Post by silvio » 27 Jul 2019 13:47

you fully understand correctly BC and UUCAST traffic - it has to be flooded at all ports (within the same vlan) beside the incoming port. A VC is like one big switch (like a chassis switch) with 2 or more slots. This is called virtual chassis. The mac-table, arp-table etc. will be shared between all the chassis. Thats why the link between is named VFL - virtual fabric link. So you can trust alcatel that a VC (same to a stack) handles the traffic correctly. So no loop. In your example with all ports in vlan 1. Incoming BC at 1/1/1 will be flooted at all other ports off all chassis (member of vc) beside 1/1/1 and the vfl-ports.
regards
Silvio

KernelPanic
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Posts: 4
Joined: 25 Jul 2019 16:16

Re: Broadcast switching on VC

Post by KernelPanic » 27 Jul 2019 15:09

Thanks silivo, but sry - i'm still a bit confused with your statement that there is no loop.
I attached a little image to maybe make it more clear.

As you said, VC 1 has to flood the BC on 1/1/2 and 2/1/2.
What will VC 2 do with the BCs received on port 1/1/3 and 2/1/3?
Definitely no loop and STP will not block a port here? :?

Regards.

KernelPanic
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Posts: 4
Joined: 25 Jul 2019 16:16

Re: Broadcast switching on VC

Post by KernelPanic » 27 Jul 2019 15:10

VC.jpg
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silvio
Alcatel Unleashed Certified Guru
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Posts: 1358
Joined: 01 Jul 2008 10:51
Location: Germany

Re: Broadcast switching on VC

Post by silvio » 28 Jul 2019 12:44

Hi, you are correct - I haven't seen the VC2.
VC1 is like one switch and VC2 like an other switch. So there are 2 cable between 2 switches - therefore this is a loop (if all in same vlan). And in this case one of the 4 ports will be blocked by STP. If f.e. VC1 is calculated as root than 2/1/3 will be blocked (if both links have the same speed). So VC2 will forward the BC at all ports - beside 1/1/3 (BC is incomming from VC1) and 2/1/3 (is blocked).
regards
Silvio

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