[Xorp-users] Fwd: Questions on OSPF
Atanu Ghosh
atanu at ICSI.Berkeley.EDU
Tue Sep 18 12:06:31 PDT 2007
Hi,
OSPF will advertise the the subnets for the interfaces on which it is
configured. In the case of OSPFv2 one subnet has to be explicitly
configured in the config, for OSPFv3 just specifying the interface will
advertise all associated subnets (the subnets can also be explicitly
configured).
Routes from other protocols must be explicity exported using policy.
Atanu.
>>>>> "Hansi" == Hansi <hantongs at gmail.com> writes:
Hansi> looping in the mailing list. Thank you. One more question
Hansi> though.. I noticed that before RIP can be configured, the
Hansi> policy parameter must be set first in order for RIP to either
Hansi> advertise static and/or connected routes. Although RIP
Hansi> already sends out udp packets once you configure it, it does
Hansi> not send out its routing table entries not until after a
Hansi> policy is either imported/exported to it. Does this also
Hansi> apply to OSPF? Thanks. Hansi.
Hansi> On 9/14/07, Kristian Larsson < kristian at spritelink.net>
Hansi> wrote:
Hansi> Hansi wrote:
>> Hello Kristian, Atanu,
>>
>> Thank you answering for my queries. Let me see if I understood it
Hansi> clearly.
>> For link-types: p2p or p2m, it is necessary to explicitly set
>> the neighbor parameter in order for the router running OSPF to
Hansi> establish
>> adjacency with another router. Broadcast link-types on the other
Hansi> hand
>> does not require the neighbor parameter to be explicitly set, am
Hansi> I
>> correct? :)
>>
>> I concur with Atanu that p2p link-types requires the neighbor
Hansi> statement
>> to be explicitly stated. My initial configuration does not
Hansi> include
>> setting the neighbor parameter, upon invoking "show ospf4
Hansi> neighbor",
>> nothing comes up even though dumps from the network shows OSPF
Hansi> hello
>> packets have been multicast already.. The neighbor router only
Hansi> displays
>> [upon invoking show ospf4 neighbor] after setting the neighbor
Hansi> parameter
>> on both routers.
Hansi> Yepp, I was simply wrong. I expected XORP to work like
Hansi> 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
Hansi> OSPF to
>> use the same router-ID? that is both of them are configured to
Hansi> 127.0.0.1
>> < http://127.0.0.1>? Since conventionally the router-ID is
Hansi> usually set to
>> the loopback, would it be possible to configure all routers in an
Hansi> OSPF
>> network to have the same router-ID of 127.0.0.1
Hansi> <http://127.0.0.1>? No, you cannot use 127.0.0.1, at
Hansi> least not on both routers. Router-id have to be unique
Hansi> within your OSPF domain, one common way of ensuring this is
Hansi> to use the loopback address that you assign to a
Hansi> router. Although you are correct that 127.0.0.1 is a loopback
Hansi> adress, routes normally get one assigned from your address
Hansi> pool. iBGP session for example are normally established
Hansi> between loopback addresses to not be dependant upon a
Hansi> specific interface being up. So assign 172.16.0.1-254 (if
Hansi> your are using private addressing) or something to your
Hansi> loopbacks as well and you can use those. -K
>> On 9/14/07, * kristian at spritelink.net <mailto:
Hansi> 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
Hansi> Ghosh
>> > kristian> <atanu at icsi.berkeley.edu <mailto:
>> atanu at icsi.berkeley.edu>> wrote: > >>>>>>> "kristian" == kristian
>> <
Hansi> kristian at spritelink.net
>> <mailto: kristian at spritelink.net>> writes:
>> > >>
>> > kristian> On Thu, 13 Sep 2007 11:51:32 -0700, Atanu
Hansi> 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
Hansi> OSPFv2 on
>> two XORP > >> >> >> machines just to establish adjacency with one
>> another. In a > >> p2p >> >> link type, is it still necessary to
Hansi> explicitly
>> set the > >> >> 'neighbor' >> parameter of each machine before
>> adjacency is >> > >> established? >> Furthermore, would it be
>> possible
Hansi> to set
>> the >> > >> router-id to its >> loopback address? instead of
Hansi> say.. the
>> ip >> > >> address of the >> interface on which ospf will be
Hansi> used?
>> > >> >> > Kristian> The neighbor command is only useful if you
Hansi> are using a
>> > Kristian> medium on which the routers cannot broadcast
Hansi> and thus
>> > Kristian> cannot discover each other. If you're using
Hansi> ethernet
>> > Kristian> (which I presume from your NIC names) you do
Hansi> not
>> have to > Kristian> use the neighbor statements. I would advice
Hansi> configuring
>> > Kristian> the interfaces as link-type p2p as this
Hansi> avoids DR
>> election > Kristian> and unnecessary CPU load. > >> >> I am
>> fairly sure that it is necessary to use
Hansi> the
>> neighbour >> > >> statements.
>> > >>
>> > kristian> Are you serious? I haven't used the XORP
Hansi> code in
>> quite > kristian> some time now.. but at least I thought XORP
Hansi> implemented
>> > kristian> the OSPF standard. AFAIK, that includes
Hansi> being able to
>> > kristian> discover neighbors and turn up adjacencies
Hansi> to them. Is
>> > kristian> this not the case? Observe that he is
Hansi> running an
>> Ethernet > kristian> point-to-point link, ie, it is not a
Hansi> non-broadcast
>> medium. > kristian> Or are you saying that you can't do
Hansi> 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
Hansi> "p2p"
>> > >> (Point-to-point) or "p2m" (Point-to-multipoint)
Hansi> then it is
>> > >> necessary to configure the neighbours. It has been
Hansi> argued
>> that it > >> should not be necessary to configure the neighbours
Hansi> if the
>> > >> routers are connected via a true Point-to-point
Hansi> link, but
>> > >> unfortunately even in this case it is necessary to configure
>> the > >> neighbour.
>> >
>> > kristian> Okey, that "kinda" makes sense. I apparently
Hansi> forgot or
>> > kristian> missed the conversation on this. What I
Hansi> want to
>> configure > kristian> with link-type p2p is not whether or not
>> the
Hansi> router
>> should > kristian> try to broadcast but if it should setup one
Hansi> of those
>> > kristian> virtual router thingys, hehe. I'm not very
Hansi> familiar
>> with > kristian> the terminology but (as you know) on a
Hansi> broadcast medium
>> > kristian> you first have a DR selection and all that
Hansi> and then
>> you're > kristian> gonna run your SPF. Since SPF can't handle
Hansi> the
>> concept of > kristian> a broadcast medium it creates a "virtual
Hansi> router" to
>> > kristian> represent the broadcast medium and connects
Hansi> all
>> routers in > kristian> that broadcast domain as adjacencies to
>> the
Hansi> virtual
>> > kristian> router. When I configure 'isis network
>> point-to-point' on > kristian> a Cisco router I expect it to not
>> setup one
Hansi> of these
>> > kristian> "virtual routers" in it's SPF topology. And
Hansi> this is
>> > kristian> different with XORP?
>> >
>> > Setting the link type to "broadcast" or "p2p" will both
Hansi> result in
>> the > hello packets being broadcast, the distinction is that if
Hansi> 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
Hansi> other OSPF
>> > implementations, the only difference is in configuration
Hansi> of a "p2p"
>> > where we require the neighbour to be specified, which as I
Hansi> mentioned
>> > before should not strictly be necessary.
>>
>> Okey, not what I expected. Why is it so? Just lack of time to
Hansi> do the
>> actual implementation (although I don't see how it would actually
>> be
Hansi> more code
>> than it is today) or has there been a policy decision against
Hansi> it?
>> -K
>>
>>
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