Hi,
I hope you might have already read the below link which gives detailed explanation of what these two stp features does:
http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/stp_enha.html#wp1052528
http://www.netcraftsmen.net/blogs/entry/what-is-bridge-assurance.html
2)
There are a few scenarios where LoopGuard would not be effective at detecting loops and/or unidirectional links.
- can only be enabled on root & alternate ports. it CANNOT run on 'designated ports'.
- ineffective at detecting a port that has been unidirectional since link-up.
Bridge Assurance (BA) is effective at mitigating those remaining scenarios that LoopGuard could not.
BA works because it turns STP into operating more like a routing protocol where BPDUs now go both ways on a given link verifying device health/awareness / lack of braindeadness.
i.e. it turns STP from traditional "fail open" behavior to "fail closed".
compare figure 1 to figure 3 in
<http://www.cisco.com/en/US/docs/switches/datacenter/sw/4_2/nx-os/layer2/configuration/guide/Cisco_Nexus_7000_Series_NX-OS_Layer_2_Switching_Configuration_Guide_Release_4.2_chapter7.html#con_1285149>
and it should be clear.
HTH
Regards
Inayath
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