15. UplinkFast BackboneFast

hello and welcome to this tutorial we're going to look at the uplink fast and the backbone fast spanning tree protocol features now we're all aware that there's a significant amount of time required in order to transition a port from the blocking state to the forwarding state that can be up to 50 seconds that's a long time both of these features are designed to decrease the amount of time required in order to do just that so we're going to go ahead and take a look at both of these and see exactly how they make converging in spanning tree a bit faster let's start with uplink fast this is a Cisco proprietary feature and basically what it does is it enables a secondary or a backup root port to transition to the forwarding State more quickly than it normally would so when uplink fast is enabled on a switch it enables that switch to identify a blocked port that could be used as a new root port in the event of a failure so let's say this is our current root port which would mean our second link is in the blocking State but just remember even though it's in the blocking State this port can still receive be PDUs from the network so switch a does have knowledge of another route to get to the root bridge but here it's just - it's just chosen the current report has the better way to do that now when our active port our root port fails the switch by using uplink fast can quickly move this blocked port to the forwarding state and reestablish a new root port so it does this by looking at all the blocked ports now here we only have one but if it had multiple blocked ports it would go ahead and compare the root path cost in each one and it would choose the lowest one as the new root port now like I said we just have one here so it's kind of easy the switch will go ahead and take this port out of the blocking State and instead of going through the listening in the learning States it moves it immediately directly into the forwarding state so we actually bypassed the listening in the learning States this means we establish a new route port almost immediately in the path to the root bridge is reestablished very quickly we didn't have to wait 15 seconds in the listening state and then another 15 seconds in the learning State when you use uplink fast just keep in mind it is only recommended to use uplink fast on switches in the access layer that means it's not recommended for switches in the core or the distribution layers if you're not yet familiar with the Cisco hierarchical design model please go ahead and refer to some of the IC and IC M v1 videos we have to go over those basics okay so only use it on access which is and the second point to keep in mind is it's really only appropriate to use this on an access switch that has at least one port in the blocking state in other words it has redundant up links to the network now let's look at backbone fast this is another Cisco proprietary feature and it's concerned with addressing failures on links that are not directly connected to a particular switch and by using it we can save up to about 20 seconds on the STP convergence process so it's pretty significant let's take a look at how it works we have switches a B and C a is our root bridge so this will be the root port on switch B this will be the root port on switch C and for the shared length lan segment this will be our designated port which would mean this port is in a blocking state on switch B now let's suppose there's a link failure between a and C now that would result in switzer it's not receiving B PDUs from switch B because that blocking port can only receive B PDUs it cannot source them that would mean switch c is going to there itself as the root bridge and start sending out its new hello BPD use well switch be would receive that hello BPD you from Switzer it to its current knowledge of the root bridge now switch B is going to quickly realize that this is inferior information meaning root bridge the sorry switch a is the root bridge and it should remain the root bridge it has better information better credentials it has won the election process so this bpdu from suet see is inferior now normally switch B would have to wait twenty seconds the Max age time before it can start to transition the blocking port into the listening learning and forwarding states however with backbone fast enabled as soon as it receives that inferior bpdu from its designated port on switch C switch B can go ahead and bypass the max aged timer and then start immediately sourcing B PDUs out to switch C so that way switch C would go ahead and relearn that switch a is the root bridge and then it can go ahead and go through the process of determining its new root port and switch B would become the new designated port for this particular land segment and our network has reconverge and we just shaved off 20 seconds off of the process because which B was able to transition past the max age timer immediately okay so here we had a failure on a link between a and C it had nothing to do directly with switch B but with backbone fast enabled switch B was able to speed up the convergent process even though it wasn't directly affected and keep in mind one important fact backbone fast has to be enabled on all switches in the network in order for this to work okay so to quickly summarize what we covered we now know that uplink fast is a cisco proprietary feature and it enables us to quickly transition to a new route port when the current route port fails and remember this is really only appropriate to be used on the access layer switches and then we talked about backbone fast and by using this we can detect indirect failures between other switches and we bypass the max age timer in order to speed up the transition of the spanning tree process and to more quickly send out B PDUs okay so that's it that is the uplink fast and the backbone fast spanning tree features for cisco thanks for watching

Channel: System Engineer

Views: 8,120

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Id: 1wnhWt7zRtY

Length: 7min 15sec (435 seconds)

Published: Tue Apr 25 2017