| Author |
Message |
Josh T
Guest
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 Posted:
Mon Dec 05, 2005 5:20 pm Post subject:
How exactly does STP traverse the network? |
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Supposed I have three Cisco switches connected to each other
like so:
Switch 1 <===> Switch 2 <===> Switch 3
And I'm running STP on all ports as per default settings. Do STP packets
traverse the network or do they only need to go to the next switch? For
example, in my diagram, for STP to work from Switch 1 to Switch 3, do
STP packets need to go directly from Switch 1 to Switch 3, or do they go
from Switch 1 to the STP processor on Switch 2, then from Switch 2 to
Switch 3?
What would happen if switch 2 was configured to only forward
ethernet packets of types ARP and IPv4 and drop everything else (but could
still send/receive all packets to/from itself)? Also what would happen if
STP was disabled on Switch 2 and someone also directly connected Switch 1
and Switch 3?
I've looked at STP with a packet sniffer - it looks like they are sent
from a mac address unique to the port the packet sniffer is connected to
and sent to a special mac address for STP. Is this correct?
I'm simplified my example to try to understand how things work - in
reality Switch 1 and Switch 3 would be VLANs and Switch 2 would be a
Linux-based firewalling bridge. I had some hardware problems with either
one of my switches or a device plugged into it which as far as I can tell
caused two of my VLANs to be bridged on that switch as well as on the
Linux bridge. I did not have STP enabled on the Linux bridge as I didn't
think I need it, but when that happened I had address flaps occur on about
half my switches and some of my users lost network connectivity for a
couple minutes.
Thanks a lot for any help,
Josh |
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stephen
Guest
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| Posted:
Tue Dec 06, 2005 4:33 am Post subject:
Re: How exactly does STP traverse the network? |
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"Josh T" <josht@ihatespam.invalid> wrote in message
news:pan.2005.12.05.14.56.25.824301@ihatespam.invalid...
| Quote: | Supposed I have three Cisco switches connected to each other
like so:
Switch 1 <===> Switch 2 <===> Switch 3
And I'm running STP on all ports as per default settings. Do STP packets
traverse the network or do they only need to go to the next switch? For
example, in my diagram, for STP to work from Switch 1 to Switch 3, do
STP packets need to go directly from Switch 1 to Switch 3, or do they go
from Switch 1 to the STP processor on Switch 2, then from Switch 2 to
Switch 3?
|
spanning tree goes from switch to switch (with possibly multiple spanning
trees if you use vlans)
| Quote: |
What would happen if switch 2 was configured to only forward
ethernet packets of types ARP and IPv4 and drop everything else (but could
still send/receive all packets to/from itself)? Also what would happen if
STP was disabled on Switch 2 and someone also directly connected Switch 1
and Switch 3?
|
depends on implementation - some will just "eat" spanning tree packets, some
pass them thru.
most dangerous thing in a spanning tree network is a link which eats
spanning tree packets within a loop - you usually get a very efficient
broadcast packet generator.
next common problem is a 1 way link - this has a similar effect if it is in
the right place within the current spanning tree.
| Quote: |
I've looked at STP with a packet sniffer - it looks like they are sent
from a mac address unique to the port the packet sniffer is connected to
and sent to a special mac address for STP. Is this correct?
|
it can be (and usually is on modern hardware), but it doesnt have to be.
some older bridges and switches had a single MAC.
Also, you may find devices with a different MAC source per VLAN, or per VLAN
/ per port....
| Quote: |
I'm simplified my example to try to understand how things work - in
reality Switch 1 and Switch 3 would be VLANs and Switch 2 would be a
Linux-based firewalling bridge. I had some hardware problems with either
one of my switches or a device plugged into it which as far as I can tell
caused two of my VLANs to be bridged on that switch as well as on the
Linux bridge. I did not have STP enabled on the Linux bridge as I didn't
think I need it, but when that happened I had address flaps occur on about
half my switches and some of my users lost network connectivity for a
couple minutes.
|
sounds like a loop, or some variation.
if you have a sniffer look for duplicate packets with broadcast addresses.
if you switch is
| Quote: |
Thanks a lot for any help,
Josh
-- |
Regards
stephen_hope@xyzworld.com - replace xyz with ntl |
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Josh T
Guest
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| Posted:
Tue Dec 06, 2005 5:20 pm Post subject:
Re: How exactly does STP traverse the network? |
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On Mon, 05 Dec 2005 22:33:43 +0000, stephen wrote:
| Quote: | "Josh T" <josht@ihatespam.invalid> wrote in message
news:pan.2005.12.05.14.56.25.824301@ihatespam.invalid...
Supposed I have three Cisco switches connected to each other
like so:
Switch 1 <===> Switch 2 <===> Switch 3
And I'm running STP on all ports as per default settings. Do STP packets
traverse the network or do they only need to go to the next switch? For
example, in my diagram, for STP to work from Switch 1 to Switch 3, do
STP packets need to go directly from Switch 1 to Switch 3, or do they go
from Switch 1 to the STP processor on Switch 2, then from Switch 2 to
Switch 3?
spanning tree goes from switch to switch (with possibly multiple spanning
trees if you use vlans)
|
Thanks - this one statement and some docs on the Cisco site helped me
figure it out.
Josh |
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Hansang Bae
Guest
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| Posted:
Thu Dec 08, 2005 9:21 am Post subject:
Re: How exactly does STP traverse the network? |
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Josh T wrote:
[stp questions]
| Quote: | Thanks - this one statement and some docs on the Cisco site helped me
figure it out.
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IMO, Clark and Kennedy's Cisco LAN Switching explains STP better than
*ANY* other book. And I'm comparing books like Interconnections, The
Switch Book, and all the other industry bibles.
--
hsb
"Somehow I imagined this experience would be more rewarding" Calvin
**************************ROT13 MY ADDRESS*************************
Due to the volume of email that I receive, I may not be able to
reply to emails sent to my account. Please post a followup instead.
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