Primary route confusion

[Kenth Eriksson]

On Fri, 2018-09-21 at 12:44 +0200, Ondrej Zajicek wrote:
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> On Fri, Sep 21, 2018 at 06:51:17AM +0000, Kenth Eriksson wrote:
> > Hi!
> > 
> > The primary route in BIRD, marked by '*', confuses me. Consider the
> > following two default routes as displayed by BIRD (2.0.2) and the
> > Linux
> > kernel (4.14.51+).
> > 
> > bird> show route
> > Table master4:
> > 0.0.0.0/0            unicast [ospf1 19:54:43.687] * E1 (110/350)
> > [172.20.4.41]
> >       via 172.20.4.41 on p1-1-1-1-2
> >                      unicast [kernel1 19:47:39.563] (40)
> >       via 172.20.0.1 on eth1
> > 
> > 
> > # route -n
> > Kernel IP routing table
> > Destination     Gateway         Genmask         Flags Metric
> > Ref    Use
> > Iface
> > 0.0.0.0         172.20.0.1      0.0.0.0         UG    0      0     
> >    0
> > eth1
> > 0.0.0.0         172.20.4.41     0.0.0.0         UG    32     0     
> >    0
> > p1-1-1-1-2
> > 
> > BIRD shows that the OSPF route with administrative distance of 110
> > is
> > primary, and not the kernel route with administrative distance of
> > 40.
> 
> Hi
> 
> BIRD on Linux cannot overwrite alien/kernel routes in kernel routing
> table. It uses kernel metric 32 to avoid collisions with them.
> 
> > When BIRD pushes the OSPF route to the kernel, it uses the metric
> > 32 by
> > default, whereas the default kernel route has metric 0. Both routes
> > are
> > installed to the FIB, but the kernel will elect the route via eth1.
> > Why
> > is not the default route over eth1 shown as primary in BIRD?
> 
> That is because kernel protocol has low preference. Set it to higher
> value. Set preference for kernel protocol to higher value than OSPF.
> I see you have 40 and 110, which are not default values anyway.

So the preference value in BIRD is not the same as administrative
distance? I believe both Juniper and Cisco treats lower preference
value / administrative distance value as a more preferred route.


https://www.juniper.net/documentation/en_US/junos/topics/reference/general/routing-protocols-default-route-preference-values.html

Is preference value inverted in BIRD?

> 
> > Can we control if BIRD pushes the OSPF route into the kernel here?
> > The
> > kernel already has a more preferred default route.
> 
> If kernel route wins in BIRD table, then OSPF route will not be
> pushed
> to kernel.
> 
> --
> Elen sila lumenn' omentielvo
> 
> Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
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> "To err is human -- to blame it on a computer is even more so."