From: Brian Dennis (bdennis@internetworkexpert.com)
Date: Sat Dec 29 2007 - 17:02:38 ART
OSPF point-to-multipoint non-broadcast was designed to allow for the
assignment of the cost on a per neighbor basis as opposed to using the
interface's cost. This
is useful on a multipoint Frame Relay interface where there are two
neighbors advertising the same route but the CIRs for the DLCIs to reach
each neighbor is different or these two neighbors that are advertising the
same route have different port speeds to the Frame Relay network.
Remember that the cost is based on your "incoming" interface's bandwidth
and not the bandwidth of the neighbor's interface that connects to you.
As an example say we have two remote routers over Frame Relay and the
remote routers are both connected to and advertising the same Ethernet
segment. Our router is connected to these two routers via Frame Relay.
One of the remote routers has a T1 Frame Relay connection and the other
has a 64k Frame Relay connection. Since our cost to the Ethernet
segment advertised by these two routers will be calculated based on the
cost of the Ethernet segment plus the cost of our incoming interface,
both routes appear to be equal cost. Obviously this is not what we
would want. We would want to prefer the route from the router with the
T1 connection over the 64k connection.
Here is an example with two remote routers advertising the same network
(loopback interfaces):
Rack2R4#show ip ospf interface s0/0 | include Cost
Process ID 1, Router ID 150.1.4.4, Network Type POINT_TO_MULTIPOINT,
Cost: 64
Rack1R4#sho run int s0/0
interface Serial0/0
ip address 154.1.0.4 255.255.255.0
encapsulation frame-relay
ip ospf network point-to-multipoint
frame-relay map ip 154.1.0.3 403 broadcast
frame-relay map ip 154.1.0.5 405 broadcast
no frame-relay inverse-arp
end
Rack2R4#sho ip route 150.1.0.0 255.255.255.0
Routing entry for 150.1.0.0/24
Known via "ospf 1", distance 110, metric 65, type intra area
Last update from 154.1.0.3 on Serial0/0, 00:00:30 ago
Routing Descriptor Blocks:
* 154.1.0.3, from 150.1.3.3, 00:00:30 ago, via Serial0/0
Route metric is 65, traffic share count is 1
154.1.0.5, from 150.1.5.5, 00:00:30 ago, via Serial0/0
Route metric is 65, traffic share count is 1
Rack2R4#
As you can see both 154.1.0.3 (router-ID 150.1.3.3) and 154.1.0.5
(router-ID 150.1.5.5) are advertising the 150.1.0.0/24 network with an
OSPF
cost of 1 (total cost minus our interface's cost, 65-64=1). If both of
these routers have the same port speed to the Frame Relay network then
this
is what we would want to see, two equal cost paths. But if they have
different port speeds, then we would want to prefer the route from the
router with the higher port speed, theoretically. The problem is that
OSPF does not take into account the cost of the remote router's interface
to us.
We only take into account the cost of the loopback and our interface's
cost to reach the remote neighbor.
To prefer the route from the router with the higher port speed, we are
going to use OSPF point-to-multipoint non-broadcast to specify the cost
on a per neighbor basis. In this example we are going to add a cost of 25
to the routes from 154.1.0.5 and 50 to the routes from 154.1.0.3.
Rack1R4#sho run | be router ospf
router ospf 1
network 154.1.0.0 0.0.255.255 area 0
neighbor 154.1.0.5 cost 25
neighbor 154.1.0.3 cost 50
Rack1R4#sho run int s0/0
interface Serial0/0
ip address 154.1.0.4 255.255.255.0
encapsulation frame-relay
ip ospf network point-to-multipoint non-broadcast
frame-relay map ip 154.1.0.3 403 broadcast
frame-relay map ip 154.1.0.5 405 broadcast
no frame-relay inverse-arp
end
Rack1R4#sho ip route 150.1.0.0 255.255.255.0
Routing entry for 150.1.0.0/24
Known via "ospf 1", distance 110, metric 26, type intra area
Last update from 154.1.0.5 on Serial0/0, 00:06:13 ago
Routing Descriptor Blocks:
* 154.1.0.5, from 150.1.5.5, 00:06:13 ago, via Serial0/0
Route metric is 26, traffic share count is 1
^^^^^^^^^^^^^^^^^^^
Now we can see that we prefer the route from 154.1.0.5 (router-ID
150.1.5.5).
Brian Dennis, CCIE4 #2210 (R&S/ISP-Dial/Security/SP)
bdennis@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Direct: 775-745-6404 (Outside the US and Canada)
>----- Original Message -----
Subject: OSPF network type : ip ospf network point-to-multipoint
non-broadcast
Date: Sat, December 29, 2007 10:23
From: "srinivas pv" <vsrinivas.paturi@gmail.com>
> Hi,
>
> Under exactly which scenario/circumstances do we need OSPF network type 'ip
> ospf network point-to-multipoint non-broadcast'? (i.e where other OSPF
> network types CAN NOT be useful)
>
> AFAIK, we can accomplish OSPF adjacencies with 'ip ospf network
> point-to-multipoint' or 'ip ospf network non-broadcast' (apart from other
> network types like broadcast etc), but could not understand at which
> specific circumstance we need 'ip ospf network point-to-multipoint
> non-broadcast'
>
> I checked OSPF Config guide by William R. Parkhurst, but I did not see
above
> network type.
>
> Thanks,
> Srinivas
>
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