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Re: Thinking about "branes" for netbsd...

On Fri, May 04, 2012 at 09:45:32AM +0200, Gert Doering wrote:
> Hi,
> On Thu, May 03, 2012 at 10:37:35PM -0500, David Young wrote:
> > > Except for the notion that a route can belong to one domain,
> > > this is otherwise in agreement with what's proposed. More
> > > than one domain may have a specific route, for example, two
> > > domains may have the same default route.
> > 
> > How/why do two domains share a route, even the default?
> > 
> > It might help to see a made-up routing table for a couple of domains.
> In Cisco IOS, you have <n> independent routing tables, but you can
> have routes pointing out of a routing instance by specifying a target
> interface that belongs to another routing instance.  Like:
>   ip route vrf BLUE gige3/6
> so if "gige3/6" belongs to "vrf RED", the default route for packets 
> in "vrf BLUE" will make them change the vrf.
> What you cannot easily do in IOS is "keep the packets on the same box,
> but have them change vrf" - but that's something, for example, Juniper
> ScreenOS can do with their "virtual routers" - you can point a route 
> at another vrouter, to make it jump routing tables
> set vrouter "trust-vr"
>   set route vrouter "untrust-vr"
> (a "vrouter" is, basically, a collection of routing table entries plus
> interfaces - same as a VRF in Cisco-Speak)
> So, for example, you could have one network, which is
> reachable from all the virtual routing systems
> set vrouter "vr-blue"
>   set route vrouter "vr-management"
> set vrouter "vr-green"
>   set route vrouter "vr-management"
> set vrouter "vr-yellow"
>   set route vrouter "vr-management"
> set vrouter "vr-management"
>   set route interface ethernet0/3 gateway


The Cisco way is awful.  The Juniper way seems much more rational.

I wonder how all of this works under the hood in Cisco & Juniper.  A
naive implementation would make one routing-table lookup in vr-blue
( -> vrouter "vr-management") and a second lookup in
vr-management ( -> nexthop), but somehow I think that it
doesn't actually work that way.


David Young    Urbana, IL    (217) 721-9981

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