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Tuesday, March 8, 2011

Does the command works correctly with distance 255 only?

We have a simple network (see figure). On R3 we have a route 10.10.10.0/24 from two sources: 1.1.1.1 and 2.2.2.2. R3 need to choose the route from 1.1.1.1. And we must do it with administrative distance for some reason. We can't change any OSPF metrics (the network is more complex, than at the pictue; we didn't draw all network elements for simplicity).

I have the following configuration at R3:

router ospf 1

dstance 140 2.2.2.2 0.0.0.0 NET

!

ip access-list standard NET

permit 10.10.10.0 0.0.0.255

But the distance of route from R2 is 110 as before.And if we input the command 'distance 255 2.2.2.2 0.0.0.0 NET', the route from R2 disappear as expected. Could you tell us if this command work correctly with distance 255 only?

Tips:

This is an expected behavior for changes in AD within a process will not have an effect on the routes.

Please check here for the link

So changing the AD will not influence in Path selection in OSPF.

The explanation for what happens when you set the distance to 255 is a bit different. Most of the setting of AD are to compare prefixes learned from different sources. And the bug referenced in the link is quite clear that this is for comparing between multiple processes and should have no effect within a single routing process. The setting of AD to 255 is a special case of AD. Set to 255 is an indicator that the prefix is not trustworthy and should not be used in routing at all. This is why setting it to 255 changes the behavior.

It is perhaps a subtle bit of logic but setting AD to some value less than 255 helps guide the choice when there are two alternatives (and works between two routing processes) but setting AD to 255 assures that there will only be a single alternative to choose from.

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