Cisco Systems IGX 8400 manual Adding and Removing IMA Group Links, Line Redundancy

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.

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.

Figure 4-6	Line Redundancy
		IGX-Hub		4		1
6	1	U	U
		X	X
				5	2
		M-	M-
		E	E	6		3

Trunk 8.1-4

IGX-Feeder

U	U
X	X
M-	M-
E	E

6 1

29452

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:

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:

total bandwidth is 95 DS0s – 24 DS0s = 71 DS0s

4-16Cisco IGX 8400 Series Reference