From: Kenneth Wygand (KWygand@customonline.com)
Date: Mon May 17 2004 - 15:15:02 GMT-3
Ahmed,
The only reason to set OSPF priorities is when you want (or need) to
influence the "DR capabilities" of a particular router. Keep in mind
that a DR/BDR election only occurs on Broadcast and Non-Broadcast
network types (with no DR/BDR elections occurring on P2P and P2MP
links).
In your case, you mentioned that R1 and R2 are connected via Frame
Relay. Are you implying that they are connected back-to-back, meaning
that there are no other routers on that particular network segment? If
so, then think about how the DR and BDRs work. Does it matter to you
which becomes the DR or BDR? If so, then set the "ip ospf priority" to
be '0' (ineligible) on the interfaces of the routers you do _NOT_ want
to become the DR or BDR (effectively making it always a 'DR-other' as
per your "show ip ospf neighbor" commands).
The only time this "DR/BDR stuff" becomes an issue is when you have a FR
cloud running in a hub-and-spoke fashion at layer 2 (the PVC-level).
Unless a layer-2 establishment (PVC mapping of the DLCI) is present, two
routers will _not_ be able to communicate directly. Typically, there is
only a PVC mapping of a DLCI to an IP address from the spokes to the
hub, not from the spokes to the other spokes. Consequently, all traffic
that needs to go from one spoke to another spoke needs to be sent to the
hub first.
If during the OSPF DR/BDR election, one of the spoke routers becomes
either a DR or a BDR, there is a problem because that router will not be
able to communicate directly (layer 2) with all other routers on that
segment (namely, that router will not be able to communicate with the
other spokes unless mappings are present). By its very nature, both a
DR and a BDR need to be able to communicate with _ALL_ other routers on
its segment, otherwise OSPF will not be stable.
Now, to answer your question, if the requirements limit you to running
OSPF on the physical interface (as opposed to subinterfaces) and
prohibit you from issuing the "ip ospf network" command, you have no
choice but to use the OSPF default network type of a physical
frame-relay interface, which is NON-BROADCAST. So even though you don't
need to worry about the DR/BDR election, you will need to issue a
"neighbor" command within your OSPF process so one router can find the
other router since the non-broadcast OSPF network type has no other
discovery mechanisms it can use to locate eligible neighbors. Also
note, you only need to (and should) put the "neighbor" command on _ONE_
side of the connection.
Hope this helps!
Kenneth E. Wygand
Systems Engineer, Project Services
CISSP #37102, CCNP, CCDP, ACSP, Cisco IPT Design Specialist, MCP, CNA,
Network+, A+
Custom Computer Specialists, Inc.
"The only unattainable goal is the one not attempted."
-Anonymous
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Ahmed Mustafa
Sent: Monday, May 17, 2004 1:41 PM
To: ccielab@groupstudy.com
Subject: Neighbor command
If R1 and R2 are connected via Frame-Relay.
The task is to use only physical interfaces, and run ospf. Note: Don't
use
ip ospf network command.
What be the best approach. Assign the neighbor statement on either R1
or R2
and set the ip ospf priority to 0 either R1 or R2.
Regards,
Ahmed
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