Wednesday, 4 May 2011

EIGRP metric calculation

EIGRP uses a composite metric, meaning it’s made up of several smaller metrics:
Bandwidth (minimum along path)
Delay (cumulative along path)
Reliability
Load
MTU
These correspond to numbers that come from the show interface command, and can be directly read from the show ip eigrp topology network/mask command.
Router#show int e0
Ethernet0 is up, line protocol is up
Hardware is Lance, address is 0060.5cf3.bb1e (bia 0060.5cf3.bb1e)
Internet address is 10.50.0.1/24
MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set

------- rest output omitted ---------




Router#show ip eigrp topology 10.50.0.0/24
IP-EIGRP (AS 44): Topology entry for 10.50.0.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 281600
Routing Descriptor Blocks:
0.0.0.0 (Ethernet0), from Connected, Send flag is 0x0
Composite metric is (281600/0), Route is Internal
Vector metric:
Minimum bandwidth is 10000 Kbit
Total delay is 1000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 0
The bandwidth is calculated as 10E7/BW(in kbit, 10E7 means 10 to the power of 7) , and the lowest along the path is chosen. The delay is the sum of all the delays in tens of microseconds. Thus, 1000 microseconds is 100 “tens of microseconds”. Reliability, MTU, and Load can be easily seen.
The metric formula is
metric = [K1 * bandwidth + (K2 * bandwidth) / (256 - load) + K3 * delay] * [K5 / (reliability + K4)]
Where K1 through K5 are constants that can be changed with the metric weights command, but default is K1=K3=1 and K2=K4=K5=0. This reduces the equation to
metric = ((10E7/minbw)+(sum of delays))*256
In above example, the metric is(10E7/10000+100)*256 = 281600.
Verify from Router#show ip eigrp topology 10.50.0.0/24
Composite metric is (281600/0), Route is Internal

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