Руководство По Проектированию для Cisco Cisco Nexus 5010 Switch
Design Guide
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Figure 13. vPC Does Not Introduce Duplicate Frames
It is also important to realize that a topology based on PortChannels does not introduce loops, even if the peer link is
lost and all the ports are forwarding. Figure 14 shows why.
Figure 14 shows the worst-case scenario of a vPC dual-active failure in which both peer-link and peer-keepalive-link
connectivity are lost. In this particular case, one switch is running spanning tree (switch 4) with links that are not in
PortChannel mode, and the other switches are configured in PortChannel mode.
With all links forwarding, a broadcast frame or an unknown unicast generated on switch 4, for example, is forwarded
on both links directed to switches 1 and 2. When these two frames arrive on switch 3, they are not sent back to the
PortChannel because that breaks the basic rule of Layer 2 forwarding: a frame cannot return to the port from which it
originated.