From: Pavel Bykov (slidersv@gmail.com)
Date: Fri Nov 14 2008 - 17:38:25 ARST
What would be the problem for recursive routing beyond /14 ??
There are no special rules that I am aware of, except with the /0 prefix,
which is considered "default" and then route has a special meaning.
But in case of recursive lookups, router will use same logic as when routing
a packet. So if you have "ip classless" then it will use even B 0.0.0.0, if
C, O, and E are down.
The problem could be with iterations.
1. It's a static pointing to IP address
2. IP address points to BGP route with BGP NEXT_HOP well known attribute
3. BGP NEXT_HOP points to address that was learned from IGP (Connected,
Static or dynamically). THIS HAS TO EXIST, otherwise BGP will not be even
installed: there is a process that checks this every 60 seconds.
4. Address points out of an interface.
So that's a lot of recursing.
Router writes only the final data in CEF. With process switching it would
have to do this for every packet.
On Fri, Nov 14, 2008 at 6:46 PM, Joe <joe.clyde@utah.edu> wrote:
> I have a question on recursive lookups, hopefully I can phrase it in a way
> that makes sense. Thanks
>
> If I have a static route like:
> Ip route 11.11.11.0 255.255.255.0 10.10.10.1
>
> And I have a route table that looks something like this:
> C 10.10.10.0/24 via int X
> O 10.10.0.0/16 via x.x.x.x
> E 10.0.0.0/8 via x.x.x.x
> B 0.0.0.0
>
> Hopefully you get the idea of the route table...doesn't matter how the
> routes are learned... just the idea of multiple routes (each less
> specific).
>
> What happens when I lose my directly connected interface? How "un-specific"
> of a route will the router use for a recursive lookup? *I know in this
> example if I lost the /24 then the next most specific /16 in this case is
> next in line but at what point will the router say it won't use a valid
> route (as far as the route table) for a recursive lookup?
>
> I don't believe it will ever use a default route but it seems like I've
> seen
> it try to recurse off a /14 route. In my opinion that is an undesirable
> behavior (let's say you have a summary address to null 0, you wouldn't want
> you statics still showing up as accessible because the next hop is
> reachable
> via a /16 net lets say). I know, I can avoid that issue by adding a
> specific
> interface to the static route, but would still like to know at what point
> do
> I not do a recursive lookup. Thanks for the help
>
>
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-- Pavel Bykov ------------------------------------------------- Stop the braindumps! http://www.stopbraindumps.com/Blogs and organic groups at http://www.ccie.net
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