4RF Aprisa LTE, User Manual

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132 | Aprisa LTE User Manual
Metric
This parameter specifies the metric (or priority) of a route. If two routes can reach the same target, the
one with the lower metric is used. This can be used for failover between routes
– where the preferred route
gets lower metric, but it the interface goes down, then the second route with higher metric will be used.
The following description distinguish between the ‘GUI metric’ configured in SuperVisor versus the ‘kernel
metric’ that can be seen and configure in the CLI.
This ‘GUI metric ’ value represents both the ‘kernel administrative distance ’ (AD) and the ‘kernel metric ’ of
the route (i.e. presented in the format of AD/Metric). In case two or more routes are almost the same, the
routing table will forward the packet based on the following match priority order:
1. Longest prefix match (i.e. if packet destination address is 10.10.10.10 and routing table includes
two routes with target IP (a) 10.10.0.0/16 and (b) 10.10.10.0/24 then route (b) with the longest prefix
match will be chosen to forward this packet).
2. Kernel administrative distance (AD) – AD defines the reliability of a dynamic routing protocol, static
and directly connected routes. These routes are prioritized in order of most to least reliable with the help
of an AD value, where the lowest AD has the highest priority route. AD value range is 0 to 255, where 0 is
connected interface, 20 = eBGP, 110 = OSPF, 120 = RIP, and 255 is unreachable route. Example: if packet
destination address is 10.10.10.10 and routing table includes two routes with {target IP, AD} = {(a)
10.10.10.0/24, 254 and (b) 10.10.10.0/24, 250} then route (b) with the same prefix match, but with the
lowest AD will be selected as the best path/route to forward this packet.
3. Kernel metric - router metrics is another value that help the router choose the best route among
multiple feasible routes to a destination. The metric value based on information such as path length,
bandwidth, load, hop count, path cost, delay that may be used by the different routing protocols. The
lowest Metric value has the highest priority route. Metric value range is 0 to 16,777,215. Example: if packet
destination address is 10.10.10.10 and routing table includes two routes with {target IP, AD, Metric} = {(a)
10.10.10.0/24, 250, 16,000 and (b) 10.10.10.0/24, 250, 1000} then route (b) with the same prefix match,
same AD, but with the lowest metric will be selected as the best path/route to forward this packet.
Converting ‘kernel AD/ M etric’ to ‘GUI M etric’
Use the following example to convert from ‘kernel AD/Metric’ to ‘GUI Metric’. Note: for efficien t and quick
conversion you can use the Windows calculator App in ‘Programmer’ mode. To convert kernel {AD/Metric}
= {254/1000}, use the following steps:
1. Use left arithmetic shift of (AD) 254 by 24 (i.e. 254 << 24). The results should be 4,261,412,864.
2. Add the Metric 1000 to the results (i.e. 4,261,412,864 + 1000 = 4,261,413,864)
3. Thus ‘kernel AD/Metric’ of 254/1000 equals to ‘GUI Metric’ of 4,261,413,864
Converting ‘GUI M etric’ to ‘kernel AD/ M etric’
Use the following example to convert from ‘GUI Metric’ to ‘kernel AD/Metric’ . Note: for efficient and quick
conversion, you can use the Windows calculator App in ‘Programmer’ mode. To convert ‘GUI Metric =
4,261,413,864 ’ to kernel {AD/Metric}, use the foll owing steps:
1. Use right arithmetic shift of ‘GUI metric’ 4,261,413,864 by 24 (i.e. 4,261,413,864 >> 24). The results
should be 254. This is the ‘kernel AD’ value.
2. Use left arithmetic shift of the previous result 254 by 24 (i.e. 254 << 24). The results should be
4,261,412,864.
3. Subtract the ‘GUI Metric’ value ( 4,261,413,864) from result #2 (4,261,412,864) (i.e. 4,261,413,864
- 4,261,412,864 = 1000). This is the ‘kernel Metric’ value.
4. Thus ‘GUI metric’ of 4,261,413,864 equals to ‘kernel AD/Metric’ of 254/1000.