This is a follow on post from OTV (Part I).
Edge Devices do take part in STP by sending and receiving BPDUs on their internal interface as would any other layer2 switch.
But an OTV Edge Device will not originate or forward BPDUs on the overlay network. OTV thus limits the STP domain to the boundaries of each site. This means a STP problem in the control plane of a given site would not produce any effect on the remote data centers. This is one of the biggest benefits of OTV in comparison to other DCI technologies. This is made possible because MAC reachability information is advertised and learned via the control plane protocol instead of learned using typical MAC flooding behavior.
With the STP separation between sites, the ability for different sites to use different STP technologies is made possible with OTV. I.e., one site can run MSTP while another runs RSTP. In the real world this is a nifty enhancement.
OTV allows multiple Edge Devices to co-exist in the same site for load-sharing purposes. (With NX-OS 5.1 that is limited to 2 OTV Edge Devices per site.)
With multiple OTV Edge Devices per site and no STP across the overlay to shut down redundant links, the possibility of an end-to-end site loops are created. The absence of STP between sites holds valuable benefits, but a loop prevention mechanism is still required, so an alternative method was used. The boys who wrote OTV, decided on electing a master device responsible for traffic forwarding (similar to some non-STP protocols).
With OTV this master elected device is called an AED (Authoritative Edge Device).
An AED is an Edge Device that is responsible for forwarding the extended VLAN frames in and out of a site, from and to the overlay network. It is a very important to understand this before carrying on. Only the AED will forward traffic out of the site onto the overlay. With optimal traffic replication in a transport network, a site’s broadcast and multicast traffic will reach every Edge Device in the remote site. Only the AED in the remote site will forward traffic from the overlay into the remote site. The AED thus ensures that traffic crossing the site-overlay boundary does not get duplicated or create loops when a site is multi-homed.
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