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Connecting Managed Switches with Unmanaged Switches

chefkoch_1
Occasional Visitor

Connecting Managed Switches with Unmanaged Switches

Hello,
actually I am dealing with a task for my student work where I have to test the abilities of RSTP for a specific network scenario. The network scenario itself is quite simple. We have two MANAGED switches (RSTP is activated), one unmanaged switch (Netgear 5 port) and a Desktop PC used for pinging the root node. The ASCII figure below should sketch the scenario. SW1 and SW2 are managed switches. They form a ring together with SW3. The desktop PC serves as traffic source. SW1 is the root node

|SW1| ----- |SW2|
| L1 | L2
| |
-----------
|
|SW3|
|
Desktop PC

When I configure SW1 and SW2 with RSTP (minimum timer parameters hello time 1 s / forward delay 4 s ), all ports (roles and states) are set correctly. The Desktop PC reaches SW1 via L1. When I cut of L1, SW1 and SW2 recognize the change, however the traffic will not come through to SW1 for 10 s. First, I thought the MAC tables in SW3 have to update themselves. However there seems to be a dependency on the RSTP timing parameters. In case I increase hello and forwarding timer the convergence time increases too (always around twice the forwardin gtime) as if the Switches operate in STP mode. However, as the port roles and port states are defined as for RSTP I think there must be another reason for this dependency... Does anyone of you have an idea? How does a RSTP network react in case it is REDUNDANTLY connected to a LAN that is unaware of RSTP... unfortunately I read the IEE standard but I must missed it
2 REPLIES
JOB CACKA
Advisor

Re: Connecting Managed Switches with Unmanaged Switches

I wonder if all of your switches support RSTP. If they don't then I believe it will default to the Legacy STP if available, which is much slower. As to your general protocol knowledge question check this out.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml
chefkoch_1
Occasional Visitor

Re: Connecting Managed Switches with Unmanaged Switches

Thanks for your answer,

- SW3 does not support RSTP; it is a common unmanaged switch posessing of an own MAC table but no additional managing functionality

- It was also my first impression that the managed switches SW1 and SW2 switch over to STP functionality on the ports where they are connected to SW3. However, I was not sure as I did not find a statement in the standard. But I think you are right and the switches move back to STP processing

I appreciated your help!
Thanks