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From: Sina Mirtorabi (sinaCISCO.COM)
Date: Tue Jun 05 2001 - 15:50:42 CDT
Michael Yeung wrote:
> Hi all,
> Sina, thanx for your suggestion. With P2P
> interface, can you have more than one neighbor?
no of corse not :-)
> means there will be 3 routers in the network. I did
> say the network will ever have most 2 routers, but I
> want to make sure there is a backup plan to support
> additional routers without reconfiguring both existing
> routers. From RFC2328, I could not find restriction
> that P2P interface can only have at most 1 neighbor.
ok if you have 3 routers you could configure the interface as
point-to-multipoint but you will carry /32 see below
> > you will carry extra /32 routes by configuring p2p
> Where does the extra 32 routes with P2P
> configuration come from?
this is mentioned in RFC
188.8.131.52. Describing Point-to-MultiPoint interfaces
For operational Point-to-MultiPoint interfaces, one or
more link descriptions are added to the router-LSA as
o A single Type 3 link (stub network) is added with
Link ID set to the router's own IP interface
address, Link Data set to the mask 0xffffffff
(indicating a host route), and cost set to 0.
o For each fully adjacent neighbor associated with the
interface, add an additional Type 1 link (point-to-
point) with Link ID set to the Router ID of the
neighboring router, Link Data set to the IP
interface address and cost equal to the interface's
configured output cost.
you will get the /32 route because of the first point.
> It seems obvious to me to use "broadcast" interface
> type for Ethernet medium. But if I know the network
> will ever going to have only 2 and some at most 3
> routers. I'm not sure if broadcast is the way to go
> versus P2P. Is there other benefits (other than
> DR/BDR, type 2 LSA elimination) for using P2P type
> versus broadcast in a confined broadcast network?
I wouldn't go with p2mp for 3 routers beacuse as said before you will
caryy /32 route, more over the RFC specify that for point-2-multipoint
netwroks, hello are sent to each neighbor as unicast which means a
configuration might be necessary for neighbor discovery.
Cisco use both method depending the way you configure it,
if you configure " ip ospf network point-to-multipoint " in this case
neighbor discovery is dynamic and hello are sent to AllSPFRouter address
if you configure " ip ospf network point-to-multipoint non-broadcast"
in this case you would need configuration for neighbor discovery
also the default of HelloInterval for p2mp link in Cisco routers is 30s
where as for broadcast and p2p its 10s so your convergenece will be
for 2 routers on a broadcast network it make sense to use p2p however
knowing all the above points you can configure p2mp if you wish ;-)
> thanx in advance,
> --- Sina Mirtorabi <sinaCISCO.COM> wrote:
> > Michael
> > Michael Yeung wrote:
> > > Hi all,
> > > The following is quoted from RFC2328:
> > >
> > > "Each broadcast and NBMA network that has at least
> > two
> > > attached routers has a Designated Router"
> > >
> > This is because if only one router is attached to a
> > multi-acess netwrok
> > ( broadcast or NBMA ) the netwrok is considered as
> > stub and you won't
> > have DR / BDR
> > on the other hand 2 router never know in advance
> > how many routers will
> > be joining the netwrok so as soon as you have 2
> > router we run the DR /
> > BDR election
> > >
> > > I was wonder if it is worth the trouble of using
> > DR
> > > and BDR in a broadcast environment where there
> > will
> > > ever only be 2 possible routers.
> > >
> > > Given the RFC does not impose restriction on
> > using
> > > "broadcast" type for broadcast medium. I can
> > easily
> > > configure the interface as "point-to-multipoint"
> > type.
> > > Does anyone see a problem of doing this
> > (remember,
> > > there are only 2 possible routers therefore
> > routing
> > > traffic should not be an implication)?
> > >
> > > Personally I see benefit of using
> > > point-to-multipoint configuration if the network
> > is
> > > only ever going to deal with 2 routers. Just
> > because
> > > the process of DR and BDR election is eliminated
> > and
> > > the process of database synchronization with DR is
> > > eliminated.
> > >
> > > Please give you your opinion and insight.
> > >
> > why you want to use point-to-multipoint for only 2
> > routers and not
> > point-2-point, you will carry extra /32 routes
> > by configuring p2p your database size will be
> > reduced as there would be
> > no type 2 LSA and no DR/BDR election
> > Sina
> > >
> > > thanx in advance,
> > > Michael
> > >
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