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List:       ms-ospf
Subject:    Re: [OSPF] ospfv3: next hop calculation for adjacent router's
From:       Acee Lindem <acee () redback ! com>
Date:       2007-04-27 12:59:17
Message-ID: 84B70973-59C3-424B-8811-B84F8EA037A8 () redback ! com
[Download RAW message or body]

Hi Manoj,

I agree that is not unclear. I've modified section 3.8.2 in the RFC  
2740 update.

***************
*** 2698,2713 ****

      In IPv6, the calculation of the next hop's IPv6 address (which  
will
      be a link-local address) proceeds along the same lines as the IPv4
!    next hop calculation (see Section 16.1.1 of [OSPFV2]).  The only
!    difference is in calculating the next hop IPv6 address for a router
!    (call it Router X) which shares a link with the calculating router.
!    In this case the calculating router assigns the next hop IPv6  
address
!    to be the link-local interface address contained in Router X's  
Link-
!    LSA (see Appendix A.4.9) for the link.  This procedure is necessary
!    for some link types, for example NBMA, where the two routers  
need not
!    be neighbors and might not be exchanging OSPF Hello packets.  For
!    other link types, the next hop address may be determined via the  
IPv6
!    source address in the neighbor's Hello packet.

   3.8.3.  Calculating the inter-area routes

--- 2698,2724 ----

      In IPv6, the calculation of the next hop's IPv6 address (which  
will
      be a link-local address) proceeds along the same lines as the IPv4
!    next hop calculation (see Section 16.1.1 of [OSPFV2]).  However,
!    there are some differences.  When calculating the next hop IPv6
!    address for a router (call it Router X) which shares a link with  
the
!    calculating router, the calculating router assigns the next hop  
IPv6
!    address to be the link-local interface address contained in Router
!    X's Link- LSA (see Appendix A.4.9) for the link.  This procedure is
!    necessary for some link types, for example NBMA, where the two
!    routers need not be neighbors and might not be exchanging OSPF  
Hello
!    packets.  For other link types, the next hop address may be
!    determined via the IPv6 source address in the neighbor's Hello
!    packet.
!
!    Additionally, when calculating routes for the area's intra-area-
!    prefix-LSAs, the parent vertex can be either a router-LSA or  
network-
!    LSA.  This is in contrast to the second stage of the OSPFv2 intra-
!    area SPF (Section 16.1 in [OSPFV2]) where the parent vertex is  
always
!    a router-LSA.  In the case where the intra-area-prefix-LSA's
!    referenced LSA is a directly connected network-LSA, the prefixes  
are
!    also considered to be directly connected.  In this case, the  
next-hop
!    is solely the outgoing link and no IPv6 next hop address is  
selected.

   3.8.3.  Calculating the inter-area routes

Thanks,
Acee
On Apr 26, 2007, at 9:47 PM, Goyal, Manoj wrote:

> Hi,
> I have a doubt in the determination of the next hop
> for the adjacent router's prefixes on a Broadcast link.
>
>
>                 R2 (DR, pfx: n2)
>                 |L2
>                 |      L3
>     R1-------------------R3 (pfx: n3)
>
> The router R1 is the calculating router and R2 is the DR.
> R2 has prefix n2 on the link and R3 has n3 and the link local
> address are L2 and L3 respectively.
>
> Here the DR R2 generates the Intra-Area-Prefix(IAP) Lsa
> for the network with prefixes n2 and n3 in that.
>
> While calculating the routing table, R1 goes through the
> IAP lsa of R2 and calculates the next hop for both
> n2 and n3 as L2 only.
>
> This will cause wrong forwarding
> from R1 to R2 for the packets destined to n3.
> In the draft, there does not seem to be a mention of
> this situation.
>
> In this case, to calculate the right next hop, the R1
> has to look at the Link-Lsa of R3 and see that
> the prefix n3 appears in it, then install the route
> n3 with next-hop as Link-Local address from this Link Lsa.
>
> Is there something I am missing here?
> Though the purpose 2 of the link-lsa indirectly explains it.
>
> Thanks,
> Manoj
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