[Xorp-users] Questions on OSPF

Atanu Ghosh atanu at ICSI.Berkeley.EDU
Thu Sep 13 14:58:01 PDT 2007


>>>>> "kristian" == kristian  <kristian at spritelink.net> writes:

    kristian> On Thu, 13 Sep 2007 12:18:42 -0700, Atanu Ghosh
    kristian> <atanu at icsi.berkeley.edu> wrote:
    >>>>>>> "kristian" == kristian <kristian at spritelink.net> writes:
    >>
    kristian> On Thu, 13 Sep 2007 11:51:32 -0700, Atanu Ghosh
    kristian> <atanu at icsi.berkeley.edu> wrote:
    >> >>>>>>> "Kristian" == Kristian Larsson <kristian at spritelink.net>
    >> >>>>>>> writes:
    >> >>
    Kristian> Hansi wrote:
    >> >> >> Hello All,
    >> >> >>
    >> >> >> I'm currently learning how to configure OSPFv2 on two XORP
    >> >> >> machines just to establish adjacency with one another. In a
    >> p2p >> >> link type, is it still necessary to explicitly set the
    >> >> 'neighbor' >> parameter of each machine before adjacency is >>
    >> established?  >> Furthermore, would it be possible to set the >>
    >> router-id to its >> loopback address? instead of say.. the ip >>
    >> address of the >> interface on which ospf will be used?
    >> >>
    Kristian> The neighbor command is only useful if you are using a
    Kristian> medium on which the routers cannot broadcast and thus
    Kristian> cannot discover each other.  If you're using ethernet
    Kristian> (which I presume from your NIC names) you do not have to
    Kristian> use the neighbor statements. I would advice configuring
    Kristian> the interfaces as link-type p2p as this avoids DR election
    Kristian> and unnecessary CPU load.
    >>  >> I am fairly sure that it is necessary to use the neighbour >>
    >> statements.
    >> 
    kristian> Are you serious?  I haven't used the XORP code in quite
    kristian> some time now.. but at least I thought XORP implemented
    kristian> the OSPF standard. AFAIK, that includes being able to
    kristian> discover neighbors and turn up adjacencies to them. Is
    kristian> this not the case?  Observe that he is running an Ethernet
    kristian> point-to-point link, ie, it is not a non-broadcast medium.
    kristian> Or are you saying that you can't do link-type p2p without
    kristian> configuring neighbours ?

    >>  If the link-type is set to "broadcast" then the neighbours will
    >> be correctly discovered. If the link-type is set to "p2p"
    >> (Point-to-point) or "p2m" (Point-to-multipoint) then it is
    >> necessary to configure the neighbours. It has been argued that it
    >> should not be necessary to configure the neighbours if the
    >> routers are connected via a true Point-to-point link, but
    >> unfortunately even in this case it is necessary to configure the
    >> neighbour.

    kristian> Okey, that "kinda" makes sense. I apparently forgot or
    kristian> missed the conversation on this.  What I want to configure
    kristian> with link-type p2p is not whether or not the router should
    kristian> try to broadcast but if it should setup one of those
    kristian> virtual router thingys, hehe. I'm not very familiar with
    kristian> the terminology but (as you know) on a broadcast medium
    kristian> you first have a DR selection and all that and then you're
    kristian> gonna run your SPF. Since SPF can't handle the concept of
    kristian> a broadcast medium it creates a "virtual router" to
    kristian> represent the broadcast medium and connects all routers in
    kristian> that broadcast domain as adjacencies to the virtual
    kristian> router.  When I configure 'isis network point-to-point' on
    kristian> a Cisco router I expect it to not setup one of these
    kristian> "virtual routers" in it's SPF topology. And this is
    kristian> different with XORP?

Setting the link type to "broadcast" or "p2p" will both result in the
hello packets being broadcast, the distinction is that if the link-type
is set to "p2p" no DR election will be attempted. The XORP OSPF behaves
as specified in the relevant RFCs and interoperates with other OSPF
implementations, the only difference is in configuration of a "p2p"
where we require the neighbour to be specified, which as I mentioned
before should not strictly be necessary.

    Atanu.



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