Subject: Re: [Isis-wg] ISIS routing on DA circuit

From: "Vijay Nath" <[email-masked]>
Date: 4 Feb 2002 06:52:02 -0000

Thanks Jeff for your input.
I have another query regarding the Sec. 8.3.3 "RPF on DA Circuits".RPF Cache looks more or like Forwarding Cache at the Control plane.

My doubt is, why the forwarding information is stored in ISIS. 
Assume there are Forwarding Cache, Routing table in Data plane and ISIS in control plane.How a packet is forwarded , if RPF information is stored in control plane?. Or the spec discusses generally why RPF is needed in DA circuits...

"So where sits RPF Cache ?" is MY ULTIMATE QUESTION

Thanks,
V Nath

On Fri, 01 Feb 2002 Jeff Learman wrote :
> 
> Another way of looking at this is, when you do manually 
> configure a route,
> you don't want to have to manually configure that route 
> on every router,
> do you?  Of course not.  You only configure it at the 
> point of origin,
> and IS-IS disseminates it throughout the rest of the 
> area/domain.
> 
> Unfortunately, you can't use it to flood this 
> information across a
> DA circuit.  I think the idea was that in any sizeable 
> network, there
> would be enough chatter on any link where IS-IS was run 
> that it
> wouldn't make any sense for it to be a DA circuit.  
> Also, when the
> link connects significant portions of a network, it 
> would normally
> have enough traffic to justify a statically assigned 
> circuit.  So,
> while I said "unfortunate", I haven't seen any cases 
> where another
> method didn't work better (e.g., static routing, BGP).
> 
> Regards,
> Jeff
> 
> At 03:07 AM 2/1/2002, mike shand wrote:
> >At 04:47 01/02/2002 +0000, Vijay  Nath wrote:
> >
> >>mike,
> >>
> >>     when only manual route is the solution, what is 
> need of router sending those manual route to other 
> routers.Even other routers too, have manual information 
> in their Reachable address prefix table, so it knows 
> where to send the pkt.
> >
> >No they don't. They may have reachable address 
> prefixes for OTHER prefixes, but not (necessarily) for 
> THIS prefix. Manual routing is NOT used (except under 
> VERY strange circumstances, and then usu
only as a 
> result of misconfiguration) within the core of the 
> network. It is used to hang reachable addresses as 
> leaves off the the core tree. Often an RA will only 
> appear hung off one router, although sometimes they 
> will be "multi-homed". But the point is that you need 
> the propagation of the routing information about them 
> to make it all work.
> >
> >It may help you to think of reachable address prefixes 
> in exactly the same way as IP subnets. In a sense they 
> are "manual" routes since they are defined on the local 
> interface. But you still need to propagate this 
> information across the network so that other routers 
> can route to them.
> >
> >Reachable addresses on DA circuits are similar
> >
> >
> >>In sec 8.3.5
> >>
> >>"There causes this information to be included in 
> subsequently generated LSPs as described in 
> 7.3.3.2.Routeing PDUs (LSPs and SN PDUs) shall not be 
> sent on DA circuits"
> >>
> >>Two contradictory statements in a clause.Pl. explain 
> on this.
> >
> >Not contradictory at all! The information is included 
> in LSPs which are flooded, but NOT flooded over the DA 
> circuits themselves.
> >
> >        Mike
> >
> >
> >
> >>TIA,
> >>V Nath
> >>
> >>
> >>
> >>
> >>On Fri, 01 Feb 2002 mike shand wrote :
> >>> At 10:18 29/01/2002 +0000, Vijay  Nath wrote:
> >>>
> >>>
> >>> >Hi All,
> >>> >
> >>> >   As per spec 10589, DA circuits shall not send
> >>> Hellos, LSPs and
> >>> > SNPs.Then how dynamic routing is achieved ??.
> >>> >or it is not possible (dynamic routing) in DA
> >>> circuits??.
> >>>
> >>> It's not possible. A DA circuit has a reachable 
> address
> >>> prefix (aka manual
> >>> route) attached to it, and the circuit is only 
> brought
> >>> up when there is
> >>> traffic to that destination. The manual route is of
> >>> course advertised in
> >>> LSPs and propagated to other routers so that they 
> know
> >>> where to send the
> >>> traffic. It was primarily designed for use with X.25
> >>> SVCs(which tells you
> >>> how old it is), where you only want to

> >>>
> >>>          Mike
> >>>
> >>>
> >>>
> >>> >TIA,
> >>> >V Nath
> >>> >
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