Cisco Cisco Nexus 5010 Switch Guida Alla Progettazione
Design Guide
Figure 5.
Traffic Flows with vPC
Dual-Control Plane with Single Layer 2 Node Behavior
While still operating with two separate control planes, vPC ensures that the neighboring devices connected in vPC-
mode perceive the vPC peers as a single spanning-tree and Link Aggregation Control Protocol (LACP) entity. For
this to happen, the system has to perform IEEE 802.3ad control plane operations in a slightly modified way (which is
not noticeable to the neighbor switch).
mode perceive the vPC peers as a single spanning-tree and Link Aggregation Control Protocol (LACP) entity. For
this to happen, the system has to perform IEEE 802.3ad control plane operations in a slightly modified way (which is
not noticeable to the neighbor switch).
The Link Aggregation Group Identifier
IEEE 802.3ad specifies the standard implementation of PortChannels. Port channeling specifications provide LACP
as a standard protocol, which makes it possible to negotiate port bundling.
as a standard protocol, which makes it possible to negotiate port bundling.
LACP makes misconfiguration less likely, in that if ports are mismatched, they will not form a PortChannel.
Consider example A in Figure 7, in which switch 1 connects to switch 2. Port 1 on switch 1 connects to port 4 on
switch 2, and port 2 on switch 1 connects to port 6 on switch 2.
switch 2, and port 2 on switch 1 connects to port 6 on switch 2.
Now imagine that the administrator configured PortChannel on switch 1 between ports 1 and 2, while on switch 2 the
PortChannel is configured between ports 5 and 3. Without LACP, it would be possible to just put the ports in channel-
group mode, and you would not discover that this is a misconfiguration until you notice that traffic has dropped.
PortChannel is configured between ports 5 and 3. Without LACP, it would be possible to just put the ports in channel-
group mode, and you would not discover that this is a misconfiguration until you notice that traffic has dropped.
LACP discerns that the only ports that can be bundled are port 1 going to port 4. According to the IEEE
specifications, to allow Link Aggregation Control Protocol to determine whether a set of links connect to the same
system, and to determine whether those links are compatible from the point of view of aggregation, it is necessary to
be able to establish:
specifications, to allow Link Aggregation Control Protocol to determine whether a set of links connect to the same
system, and to determine whether those links are compatible from the point of view of aggregation, it is necessary to
be able to establish:
●
A globally unique identifier for each system that participates in link aggregation (that is, the switch itself needs
to be “unique.”) This number is referred to as system ID and is composed of a priority and a MAC address that
uniquely identifies the switch. Figure 6 illustrates what the System ID looks like.
to be “unique.”) This number is referred to as system ID and is composed of a priority and a MAC address that
uniquely identifies the switch. Figure 6 illustrates what the System ID looks like.
© 2010 Cisco Systems, Inc. All rights reserved. This document is Cisco Public Information.
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