Why Hubs cause loops when connected to just one switchport and portfast is

[ricardbadia]

I'm trying to find why when I connect a hub to a swicthport with portfast on, causes loops?

I have a 6509 port connected to a hub and 2 PCs connected to the hub. Why does it cause a loop? If I disable portfast is fine.

[krishna.kg]

Hi, portfast is used for access ports. In normal STP senario the port has to undergo 5 steps before it can be put into forwarding state. The normal STP steps takes around 50 secs to put a port into forwarding state. Since access ports are considered to be in forwarding as soon as it comes up so this portfast is used taking that into consideration. So portfast on an uplink will cause a loop as it will help redundant ports to be in forwarding at the same time.

[Georg Pauwen]

Hello,

normally, as long as the hub is only connected to one access port on the switch, a loop should not occur. What kind of hub is that (which brand) is that ?

Regards,

GP

[ricardbadia]

is a regular netgear with 4 regular ports, no uplink ports.

thanks

Ricard

[Francois Tallet]

Where is occurring this loop? If there is just one connection from the switch to the hub and then two pcs connected to the hub, where is that data looping? Are your PCs doing any kind of bridging? I doubt because removing portfast on the switch should not fix the issue in that case.

From now on I assume that you must have a redundant connection from the switch to the hub, and in which case, portfast should not be configured (when you configure portfast on your c6k, you have a big warning message displayed on the console to warn about that;-).

When a port is coming up, STP blocks it by default. It will take 2xforward_delay (typically 30 seconds) for this port to go forwarding. The idea is that STP needs to make sure that there is no loop through this port. Portfast is a feature that allows to skip this initial blocking stage of 30 seconds on ports leading to hosts... understand on ports where there cannot be a loop.

So you basically have the choice of:

-1- no portfast, and regular spanning tree convergence of 30 seconds.

-2- portfast, and a temporary loop.

Generally, option 1 is recommended;-)

Regards,

Francois

[Georg Pauwen]

Hello Ricard,

this is going to be somewhat fuzzy, and I might be totally off, but I have seen Netgears (especially with older firmware) assuming the role of root in the spanning tree topology. Can you try and make the VLAN on the port where the Netgear is connected to the root on the 6509 ? So, if your Netgear is connected to e.g. FastEthernet0/3, and this port is assigned to VLAN 3, on the 6509 the following global command would configure the 6509 as the root for that VLAN:

spanning-tree vlan 3 root primary

Regards,

GP