Nortel Networks NN47200-503 manual R1 = CRC32 SIP, DIP, Ecmpindex = R2 % ecmpcount +

204IP Routing Configuration and Management

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Equal Cost Multipath (ECMP)

Equal Cost Multipath (ECMP) is an IP feature for load-balancing routed IP traffic across up to four equal-cost paths for each supported protocol. ECMP supports OSPF, RIP, and static routes. Some benefits of using ECMP:

•Supported protocols will rerun when an ECMP path fails, and the other configured paths will automatically take the load.

•Load sharing implies better use of network facilities.

ECMP is selected based on the source and destination IP address in the packet. The hash_control register has a HASH_SELECT field which is set to 5 (lower CRC-32).

R1 = CRC32 (SIP, DIP)

R2 = R1 & 0x1F(The Least Significant 5 bits are selected)

ecmp_index = R2 % (ecmp_count + 1)

Note: The value ecmp_count above is zero-based in the hardware so if four paths are present then the value is three. This is why the value is ecmp_count + 1.

The ECMP traffic distribution algorithm is demonstrated in the following example:

Consider two network devices, Device 1 at the IP address 192.1.1.3 and Device 2 at 192.1.1.4. Device 1 send to Device 2 so that 192.1.1.3 is the source IP address (SIP) and 192.1.1.4 is the destination IP address (Device 2).

To calculate the CRC32 for the example source and destination IP address noted above, the following calculations would be made:

•CRC32 polynomial : x ^ 32 + x ^ 28 + x ^ 23 + x ^ 22 + x ^ 16 + x ^ 12 + x ^ 11 + x ^ 10 + x ^ 8 + x ^ 7 + x ^ 5 + x ^ 4 + x ^ 2 + x ^ 1 + 1

•R1 = CRC32 ( 0xc0010103, 0xc0010104 ) = 0xf474b549

•R2 = ( 0xf474b549 & 0x1f ) = 9

If, for the purposes of this example, it is assumed that the ECMP count is 4 (hardware entries 0 though 3), the following calculation is then made:

•ecmp_index = 9 % ( 4+1 ) = 1

This means that in this example, the second path at hardware index 1 in the ECMP table will be used.

Nortel Ethernet Routing Switch 5500 Series

Configuration-IP Routing Protocols

NN47200-503 03.01 Standard

5.127 August 2007

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