Cisco Systems IGX 8400 Manual De Usuario
4-16 Cisco IGX 8400 Series Reference
Universal Switching Module Enhanced
Adding and Removing IMA Group Links
When you configure an IMA trunk through Cisco WAN Manager or the cnftrk command, you enter
the number of retained links. This represents the number of ports that must remain active for the IMA
trunk itself to remain active. If a physical line goes out of service, but the number of active lines is
at least as great as the retained links value, the IMA trunk remains active even though the node goes
into major alarm. Also, the available load bandwidth is adjusted according to physical line status, so
the switch does not reroute connections after a line failure unless the used bandwidth becomes
greater than the available bandwidth.
the number of retained links. This represents the number of ports that must remain active for the IMA
trunk itself to remain active. If a physical line goes out of service, but the number of active lines is
at least as great as the retained links value, the IMA trunk remains active even though the node goes
into major alarm. Also, the available load bandwidth is adjusted according to physical line status, so
the switch does not reroute connections after a line failure unless the used bandwidth becomes
greater than the available bandwidth.
In Figure 4-6, the number of retained links is set to 3. If line 1 fails for example, trunk 8.1-4 will
continue functioning, however, the node will show major alarm due to the physical line failure.
continue functioning, however, the node will show major alarm due to the physical line failure.
Figure 4-6
Line Redundancy
The transmit and receive rate of an IMA trunk is the sum of all physical lines minus the IMA protocol
overhead. The overhead for up to four lines is one DS0. Using the previous IMA trunk example, the
maximum rates are as follows:
overhead. The overhead for up to four lines is one DS0. Using the previous IMA trunk example, the
maximum rates are as follows:
for trunk 8.1-4 (with T1 lines):
TX rate = Rx rate = 24 * 4 DS0s – 1 DS0 = 95 DS0s
You could configure the line receive rate to be the maximum bandwidth allowed on this trunk:
total bandwidth = RX rate = 95 DS0s
You could configure the line transmit rate to be the maximum bandwidth allowed on this trunk:
total bandwidth = RX rate = 95 DS0s
However, if a physical line fails (and the number of retained links are still active), the switch adjusts
the total bandwidth. Using the example IMA trunk consisting of four physical T1 lines:
the total bandwidth. Using the example IMA trunk consisting of four physical T1 lines:
total bandwidth is 95 DS0s – 24 DS0s = 71 DS0s
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IGX-Hub
Trunk 8.1-4
IGX-Feeder
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