From: Christian Sica (csica@liweb.net)
Date: Fri Sep 09 2005 - 14:58:18 GMT-3
Think of "ip ospf network point-to-multipoint" as a way to get around a
partial mesh topology with limited layer 2 connectivity. Say for example
that you have a frame-relay network:
(R1) ---- (R2)
|
|
(R3)
From a layer 2 perspective, R3 can reach R1, however, R3 has no layer 2
connectivity to R2 and vice versa. So when you add OSPF onto this network,
R2 neighbors up with R1 and R3 neighbors up with R1. Then routes learned
from R2 are sent to R1 and R1 passes these routes to R3. However, on R3, if
you look at the next hop address for routes sourced from R2, you will see
R2's ip address there. Since you have no layer 2 connectivity to R2
directly from R3, any traffic destined to R2 will be dropped. If you do a
"debug frame-relay", you will see encapsulation failed error messages.
This is where the "point-to-multipoint" feature of OSPF comes in handy. It
tells the router not to assume that you have full layer 2 connectivity. As
such, when routes sourced from R2 are passed to R1, R1 will pass them to R3,
however it will now show that R1 is the next hop for routes generated by R2.
This way when traffic from R3 gets sent to R2, R3 looks up in it routing
table and sees that the next hop is R1. R3 has layer 2 connectivity to R1
and forwards the packet. R1 receives the packet and sees that it's destined
for R2 and forwards it along. This operates the same way from R2's
perspective.
Hope this helps,
Christian
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Jaycee Cockburn - BCX SS
Sent: Tuesday, September 06, 2005 9:02 AM
To: Rajib Khan; ccielab@groupstudy.com
Subject: RE: ip ospf netw point-to-multipoint
Hi R,
This is the behaviour of the "point-to-multipoint" network type...It
happens because when configured it treats the Frame Relay network as
multiple point-to-point links (hench the /32 masks)
This is to circumvent the creation of DR/BDR's and to overcome the
limitations of Broadcast and NBMA network types...
HTH
Cheers
JC
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Rajib Khan
Sent: 06 September 2005 02:51 PM
To: ccielab@groupstudy.com
Subject: ip ospf netw point-to-multipoint
HI Group
I am trying to understand default behavior of "ip ospf netw
point-to-multipoint"
Folloing is my diagram
R1
ospf area 0
R2 R3
All router is running "ip ospf netw point-to-multipont"
Under R1's ip routing table I can see R2 and R3's ip address as /32 bit
mask eventhough they area in the same network subnet. Same applies to R2
and R3's routing table.
My question is
When using "ip ospf netw point-to-multipoint" does interface ip address
is shown as /32 bit mask on the neighbor router.
Can this be configured as /24 bit
Thanks is advance
Raj
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