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204 IP Routing Conﬁguration and Management

interface fast-Ethernet 1/48

spanning-tree stp 1 learning disable

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 shar ing 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

5.1 27 August 2007