[Xorp-users] Questions on OSPF

[kristian at spritelink.net]

On Thu, 13 Sep 2007 12:18:42 -0700, Atanu Ghosh <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.

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

Perhaps the increase in simplicity to the SPF topology that 'isis network
point-to-point' brings is so small that it's negligable. I think SPF runs
take in the order of 10ms or so for a network with a couple of hundred
routers on a normal routing engine these days.

  -K