UplinkFast

[asepiacci]

From the CCNP SWITCH Book:

"The UplinkFast feature on Catalyst switches enables leaf-node switches or switches at the ends of the spanning-tree branches to have a functioning root port while keeping one or more redundant or potential root ports in Blocking mode. When the primary root port uplink fails, another blocked uplink immediately can be brought up for use.

[...]

UplinkFast also makes some modifications to the local switch to ensure that it does not become the root bridge and that the switch is not used as a transit switch to get to the root bridge. In other words, the goal is to keep UplinkFast limited to leaf-node switches that are farthest from the root."

I don't get the point why the Uplinkfast should be enabled only on leaf-node switches. Can you give me an example of a transit switch where putting the port that receives suboptimal BPDUs into FW state, immediately after the failure of the link connected to the Root port, could bring to a loop?

Thanks in advance for help.

Alessandro.

[asepiacci]

Sorry guys, I've already caused two headaches and I feel a bit guilty...

Seriously speaking, I just think there's a lack of good scenarios in books: they always show very simple situations and, in that way, they end up in not being exhaustive at all.

Anyway, no rush, take your time, and thanks again!

[andtoth]

The example is correct if we assume that the STP cost on the link between SW2 and SW3 (root) is set to an artificially high value that is higher than the cumulative cost of the best path to the root.

However, the theory behind that example is not precise because in that example SW2 does not have a "real" alternate root port, as the alternate RP is going back to itself. In other words, imagine that as soon as link between SW1 and SW2 goes down, SW2 does not hear about the root anymore, not even through the alternate port as there's only one path to the root from that perspective. By the way, in reality the alternate root port would be on SW3's link which goes to SW2. And SW2 would have designated ports towards SW3 and SW4 as he's advertising the best path towards the root (still assuming the artificially high cost on the SW2-SW3 link on top).

By the way, if the link on SW2 to the root would go down, SW2 would either consider itself as the new root (if we assume the link on top would be shut down) and other switches would start calculating new port roles immediately as they receive new BPDU from SW2. In reality SW3 would enable it's port as root port and start sending better BPDU on it's designated port causing other switches to recalculate their port roles.

Andras

[droeun141]

I thought Uplinkfast increases the bridge priority & port costs to incredibly high values.  How would S2 become new the root?  Ok, check out this diagram.  Cisco says uplinkfast should only be on enabled on edge switches, not core or distribtion.  But why?  Let's presume A is the root and C is a distro switch with uplinkfast enabled.  If link AC goes down, why would transitioning to BC immediately cause any problems? I've tried it numerous ways but can't get it to form any sort of loop.

[Jon Marshall]

In your scenario i don't believe it could form a loop. I told you it took me sometime to actually come up with a scenario where it would

I think Cisco are not saying that using Uplinkfast on any transit switch will always cause a loop. What they are saying is that using it on transit switches could cause a loop and therefore it is recommended not to use it.

Jon

[Mohamed Sobair]

Hello,

The Uplink fast and backbone fast features are PVST+ features , that is used to spead up the convergence time in STP.

Let me try to navigate on what Cisco says here and Why?

Cisco Says , the Uplink fast feature is not recommended to be applied on the transit Switches and here is why, In your example, If Switch C link to the root bridge fails, then Switch C shall wait for the max age timer to expire before start to accept Superior BPDUs from Switch B on its blocking port. Once the max age timer expires , the blocking port goes through normal STP states before it becomes Root Forwarding. This operation takes about 50 sec in normal STP timers, However , when you apply Uplinkfast, the Switch doesnt have to wait for the max age timer to expire and would therfor choose alternative root port towards switch B.

In transit Switches on some specific design approach it could be a risk to have a transit switch not to wait for the Max age timer expires and transit its port to root forwarding state, However, your Example doesnt show any kind of Switching loop risk when its applied in your Scenario.

I shall come up with Specfic design that could introduce loop when uplinkfast is enabled and post it here.

HTH

Mohamed