[Xorp-users] Questions on OSPF

[Kristian Larsson]

Hansi wrote:
> Hello Kristian, Atanu,
> 
> Thank you answering for my queries. Let me see if I understood it clearly.
> 
> For link-types: p2p or p2m, it is necessary to explicitly set the 
> neighbor parameter in order for the router running OSPF to establish 
> adjacency with another router. Broadcast link-types on the other hand 
> does not require the neighbor parameter to be explicitly set, am I 
> correct? :)
> 
> I concur with Atanu that p2p link-types requires the neighbor statement 
> to be explicitly stated. My initial configuration does not include 
> setting the neighbor parameter, upon invoking "show ospf4 neighbor", 
> nothing comes up even though dumps from the network shows OSPF hello 
> packets have been multicast already.. The neighbor router only displays 
> [upon invoking show ospf4 neighbor] after setting the neighbor parameter 
> on both routers.

Yepp, I was simply wrong. I expected XORP to work like Cisco or Juniper.


> Regarding setting router-ID parameters to loopback 127.0.0.1 
> <http://127.0.0.1>, would it be possible for two routers running OSPF to 
> use the same router-ID? that is both of them are configured to 127.0.0.1 
> <http://127.0.0.1>? Since conventionally the router-ID is usually set to 
> the loopback, would it be possible to configure all routers in an OSPF 
> network to have the same router-ID of 127.0.0.1 <http://127.0.0.1>?

No, you cannot use 127.0.0.1, at least not on both routers.
Router-id have to be unique within your OSPF domain, one common way of 
ensuring this is to use the loopback address that you assign to a 
router. Although you are correct that 127.0.0.1 is a loopback adress, 
routes normally get one assigned from your address pool. iBGP session 
for example are normally established between loopback addresses to not 
be dependant upon a specific interface being up.
So assign 172.16.0.1-254 (if your are using private addressing) or 
something to your loopbacks as well and you can use those.

   -K

> On 9/14/07, *kristian at spritelink.net <mailto:kristian at spritelink.net>* < 
> kristian at spritelink.net <mailto:kristian at spritelink.net>> wrote:
> 
>     On Thu, 13 Sep 2007 14:58:01 -0700, Atanu Ghosh <
>     atanu at icsi.berkeley.edu <mailto:atanu at icsi.berkeley.edu>>
>     wrote:
>      >>>>>> "kristian" == kristian  <kristian at spritelink.net
>     <mailto:kristian at spritelink.net>> writes:
>      >
>      >     kristian> On Thu, 13 Sep 2007 12:18:42 -0700, Atanu Ghosh
>      >     kristian> <atanu at icsi.berkeley.edu
>     <mailto:atanu at icsi.berkeley.edu>> wrote:
>      >     >>>>>>> "kristian" == kristian < kristian at spritelink.net
>     <mailto:kristian at spritelink.net>> writes:
>      >     >>
>      >     kristian> On Thu, 13 Sep 2007 11:51:32 -0700, Atanu Ghosh
>      >     kristian> < atanu at icsi.berkeley.edu
>     <mailto:atanu at icsi.berkeley.edu>> wrote:
>      >     >> >>>>>>> "Kristian" == Kristian Larsson
>     <kristian at spritelink.net <mailto: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.
> 
>     Alright, just as I expected.
> 
>      > 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.
> 
>     Okey, not what I expected. Why is it so? Just lack of time to do the
>     actual
>     implementation (although I don't see how it would actually be more code
>     than it is today) or has there been a policy decision against it?
> 
>       -K
> 
>