[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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