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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 27 Configuring EtherChannel and Link State Tracking
Understanding Link-State Tracking
Figure 27-3 on page 27-25 shows a network configured with link-state tracking. To enable link-state
tracking, create a link-state group, and specify the interfaces that are assigned to the link-state group. An
interface can be an aggregation of ports (an EtherChannel), a single physical port in access or trunk
mode, or a routed port. In a link-state group, these interfaces are bundled together. The downstream
interfaces are bound to the upstream interfaces. Interfaces connected to servers are referred to as
downstream interfaces, and interfaces connected to distribution switches and network devices are
referred to as upstream interfaces.
The configuration in Figure 27-3 ensures that the network traffic flow is balanced as follows:
• For links to switches and other network devices
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Server 1 and server 2 use switch A for primary links and switch B for secondary links.
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Server 3 and server 4 use switch B for primary links and switch A for secondary links.
• Link-state group 1 on switch A
–
Switch A provides primary links to server 1 and server 2 through link-state group 1. Port 1 is
connected to server 1, and port 2 is connected to server 2. Port 1 and port 2 are the downstream
interfaces in link-state group 1.
–
Port 5 and port 6 are connected to distribution switch 1 through link-state group 1. Port 5 and
port 6 are the upstream interfaces in link-state group 1.
• Link-state group 2 on switch A
–
Switch A provides secondary links to server 3 and server 4 through link-state group 2. Port 3 is
connected to server 3, and port 4 is connected to server 4. Port 3 and port 4 are the downstream
interfaces in link-state group 2.
–
Port 7 and port 8 are connected to distribution switch 2 through link-state group 2. Port 7 and
port 8 are the upstream interfaces in link-state group 2.
• Link-state group 2 on switch B
–
Switch B provides primary links to server 3 and server 4 through link-state group 2. Port 3 is
connected to server 3, and port 4 is connected to server 4. Port 3 and port 4 are the downstream
interfaces in link-state group 2.
–
Port 5 and port 6 are connected to distribution switch 2 through link-state group 2. Port 5 and
port 6 are the upstream interfaces in link-state group 2.
• Link-state group 1 on switch B
–
Switch B provides secondary links to server 1 and server 2 through link-state group 1. Port 1 is
connected to server 1, and port 2 is connected to server 2. Port 1 and port 2 are the downstream
interfaces in link-state group 1.
–
Port 7 and port 8 are connected to distribution switch 1 through link-state group 1. Port 7 and
port 8 are the upstream interfaces in link-state group 1.
In a link-state group, the upstream ports can become unavailable or lose connectivity because the
distribution switch or router fails, the cables are disconnected, or the link is lost. These are the
interactions between the downstream and upstream interfaces when link-state tracking is enabled:
• If any of the upstream interfaces are in the link-up state, the downstream interfaces can change to or
remain in the link-up state.
• If all of the upstream interfaces become unavailable, link-state tracking automatically puts the
downstream interfaces in the error-disabled state. Connectivity to and from the servers is
automatically changed from the primary server interface to the secondary server interface.
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