Came across this question in my BCMSN studies Link is here:
The question is:
spanning-tree guard root command is configured on interface G0/0 on S2 and S5. Global command spanning-tree uplinkfast has also been configured on both S2 and S5. The link between S4 and S5 fails, Will host A be able to reach host B.
I'm just not sure how the answer is D.
I know that root guard prevents a switch with a superior priority from becoming the root bridge. Root guard is configured on both the Gi0/0 ports of S2 and S5.
Uplink fast helps protect against direct link failures by keeping a backup link in 'standby' and moving it straight to the forwarding state.
So in the question, S5 would have either its link to S4 in blocking or the link to S2. It must be the Gi0/0 link on S5 blocking because if the link to S4 was blocked and it failed nothing would happen as far as STP goes.
So the link between S4 and S5 fails, the Gi0/0 link between S5 and S2 is put into forwarding. If thats the case then this would create a loop between S5, S2, S3 and S6. Root guard shouldn't come into play here since the link between S4 and S5 is now down, S1 can't be configured with a better priority then the root (whatever that is) or there would be no connectivity at all to host B.
So im assuming now that STP would block the link between S5 and S6 making the correct answer D.
Does this rambling of mine seem like correct logic?