Configuring RSTP on Switch

[chrisvio48]

i have a topology look like this 

 and i already turn on the rstp configuration to each switch. then i set up priority each switch like this

Lantai 6 (up right) : 28672

Lantai 2 (up left) : 32768

and Multi SW 10 (down) 40960

but, why Lantai 2 - Multi SW still can connected (it still can ping each other) ? isn't it suppose they can't ping ??? im so confused...

thank you, and im sorry for my bad english

[Joseph W. Doherty]

If your 3 switches are all interconnected in the same L2 domain, you've formed a L2 loop.  What STP does is logically block one of the connections, which allows ping to work between the 3 switches.

All setting switch priorities does is determine which link will be blocked.

[MHM Cisco World]

Stp will stop l2 loop not stop traffic between SW's

MHM

[Joseph W. Doherty]

"Stp will stop l2 loop not stop traffic between SW"

@MHM Cisco World 's statement might be misunderstood.

STP will block (most) traffic on one of the 3 switches interconnecting links, like pings, but such switches can still intercommunicate via another path.

For example, if STP blocks traffic, like ping, on the Lantai 2 <> Multi Sw 10 link, those two switches would exchange traffic, like ping, via Lantai 2 <> Lantai 6 <> Multi Sw 10.

[MHM Cisco World]

there is no direct link between SW2 and SW10 it BLK by STP 
but there is path via SW6 and this path SW6 is use for ping 
so STP not drop traffic it drop loop between SW's

MHM

[Joseph W. Doherty]

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@MHM Cisco World wrote:

there is no direct link between SW2 and SW10 it BLK by STP 
but there is path via SW6 and this path SW6 is use for ping 
so STP not drop traffic it drop loop between SW's

---

Again, the terminology being used by @MHM Cisco World might be misunderstood, such as:

"there is no direct link between SW2 and SW10 it BLK by STP "

The link is still there, and "up", and even passing some, as @Giuseppe Larosa describes, non "user traffic".

And with "so STP not drop traffic it drop loop between SW's", he's correct, user traffic is not dropped, and what dropping the loop really means, is user traffic doesn't "see" the physical loop.  As far as user traffic is concerned, there's no loop.  However, STP knows there's a loop, and for redundancy purposes, if one of the other active links physically goes down, STP will unblock the currently blocked link, recreating a non-looped topology between the 3 switches.

Also, if the previously active failed link comes back on-line, STP still once again, block the lower priority link and begin to reuse the restored/earlier path.

BTW, without STP (or something like it), if you create a loop, frames will continue loop, generally making that L2 segment unusable.  (The latter is why, even when a topology doesn't, by design, have any redundant paths, the general recommendation is to have STP active to preclude an accidental L2 loop creation taking down that part of the network.) 

[MHM Cisco World]

There is cable and there is link' link I mention is virtual' and there is no direct because on SW10 the port color is orange i.e. it stp blk. So the stp block direct connection

And SW2 use SW6 to reach SW10.

MHM

[Joseph W. Doherty]

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@MHM Cisco World wrote:

There is cable and there is link' link I mention is virtual' and there is no direct because on SW10 the port color is orange i.e. it stp blk. So the stp block direct connection

And SW2 use SW6 to reach SW10.

---

Perhaps just me, but again, I understand what you meant, starting with your initial reply, but possibly, the terminology might confuse others, such as the OP.

Again, possibly, something like "no direct" might be misunderstood that STP blocking a link is the same as downing the link.  It's not, well except for user traffic.

For example, in PT, just built:

If we example Switch2, we see:

Switch#sh spanning-tree 
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     000C.CFD5.AD72
             Cost        19
             Port        1(FastEthernet0/1)
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     00D0.FF20.ADDD
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1            Root FWD 19        128.1    P2p
Fa0/2            Altn BLK 19        128.2    P2p

STP has blocked f0/2, the interface with the orange dot.

But:

Switch#sh interfaces status 
Port      Name               Status       Vlan       Duplex  Speed Type
Fa0/1                        connected    1          auto    auto  10/100BaseTX
Fa0/2                        connected    1          auto    auto  10/100BaseTX

Interface f0/2 is connected (or "up") and . . .

Switch#sh cdp ne
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone
Device ID    Local Intrfce   Holdtme    Capability   Platform    Port ID
Switch       Fas 0/1          134            S       2960        Fas 0/2
Switch       Fas 0/2          139            S       2960        Fas 0/2

CDP is passing traffic across this, f0/2, STP blocked port.

Again, I strongly suspect, you ( @MHM Cisco World  ) know this, that the STP blocking doesn't block all traffic, just most traffic (i.e. the traffic that would have a problem with a L2 loop).  Also again, the way your replies were worded, this distinction might be unclear; possibly, and especially, to the OP, because the nature of the OP question also possibly shows very little knowledge about STP (which is fine, generally we all at one time knew nothing about networking).

[Giuseppe Larosa]

Hello @chrisvio48 ,

>> but, why Lantai 2 - Multi SW still can connected (it still can ping each other) ?

what you see is normal and expected.

The topology is a triangle made of three switches and three links. STP will make one link blocked to prevent bridging loop to form. However, for user traffic the remaining links are enough to allow ping from Lantai 2 to Multi SW to go through the current root bridge that is Lantai 6 for its lowest STP priority  . lowest = best for STP root bridge election.

You can easily check this by yourself using

show arp | inc x.x.x.x where x.x.x.x is the IP address of the remote device.

then you issue

show mac address-table address aaaa.bbbb.cccc    where aaaa.bbbb.cccc is the MAC address in hex that you see in the previous output.

You should be able to track that the destination MAC address is learned via the port to the root bridge a.k.a as the root port.

Hope to help

Giuseppe

[paul driver]

@chrisvio48 wrote:

 isn't it suppose they can't ping

I would say No because spanning-tree is to negate(stop) L2 loops and NOT reachabiltiy.
In your diagram it looks like the Multi SW<>Lantai2 interconnect is the segment being blocked by spanning-tree, which makes sense as that segment is not the best path to the root switch Lantai6, both Multi SW & Lantai2 will go via their direct connection to Lantai6