Why did Cisco add EIGRP wide metrics, and how do the numbers change?
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The classic metric runs out of resolution on fast links. Bandwidth is 10^7 divided by kbps, so a 10 Gbps link already scores 1 and faster links cannot be told apart, and delay is counted in tens of microseconds with the same 10 usec default from gigabit upward. Wide metrics, in named mode, use a 64-bit composite with throughput and latency scaled by 65536 and delay kept in picoseconds. The topology table shows the 64-bit value; the routing table shows it divided by the RIB scale, 128 by default.
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The two main terms
- Throughput = 65536 x 10^7 / slowest bandwidth in kbps
- Latency = 65536 x total delay in picoseconds / 10^6 With default K values (K1=1, K3=1, and K2, K4, K5, K6 all 0) the composite is throughput plus latency.
Worked numbers for gigabit
- Throughput: 65536 x 10^7 / 1000000 = 655360
- Latency for one gigabit interface (10 usec = 10^7 ps): 65536 x 10^7 / 10^6 = 655360
- Connected gigabit route: 655360 + 655360 = 1310720, shown in the RIB as 1310720 / 128 = 10240
- Gigabit LAN one gigabit hop away: 655360 + 2 x 655360 = 1966080, RIB value 15360
R1# show eigrp address-family ipv4 topology
EIGRP-IPv4 VR(CAMPUS) Topology Table for AS(100)/ID(10.255.0.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.2.0.0/24, 1 successors, FD is 1966080
via 10.0.12.2 (1966080/1310720), GigabitEthernet0/0/0R1# show ip route eigrp | include 10.2.0.0
D 10.2.0.0/24 [90/15360] via 10.0.12.2, 00:05:02, GigabitEthernet0/0/0K6
Named mode adds K6 for extended attributes such as jitter and energy. It defaults to 0 and I have not seen it used in production.
They will ask next
Why is there a RIB scale at all?
The routing table metric field is 32 bits. A 64-bit composite can be larger than that, so EIGRP divides by the RIB scale (128 by default, set with
metric rib-scale) before installing the route.Can you compare a wide-metric topology table with a classic one?
No, they are different scales. A gigabit connected route is 2816 classic and 1310720 wide. Compare paths only within the same mode.
Traps
- Reading the topology table FD and the routing table metric as different paths. In named mode they differ by the RIB scale.