From: Waters, Kivas (UK72) (Kivas.Waters@xxxxxxxxxxxxx)
Date: Sun Nov 18 2001 - 12:39:14 GMT-3
Hi Neil, I hope that the text below answers your question. From Cisco
documentation ....
Serial Interfaces
Serial interfaces, which are by default multipoint, are non-broadcast media,
while point-to-point subinterfaces are broadcast. If you are using static
routes, you can point to either the next hop or the serial subinterface. For
multipoint, you need to point to the next hop. This concept is very
important when doing Open Shortest Path First (OSPF) over Frame Relay. The
router needs to know that this is a broadcast interface for OSPF to work.
OSPF and Multipoint
OSPF and multipoint can be very troublesome. OSPF needs a Designated Router
(DR). If you start losing PVCs, some routers may lose connectivity and try
to become a DR even though other routers still see the old DR. This causes
the OSPF process to malfunction.
Overhead associated with OSPF is not as obvious and predictable as that with
traditional distance vector routing protocols. The unpredictability comes
from whether or not the OSPF network links are stable. If all adjacencies to
a Frame Relay router are stable, only neighbor hello packets (keepalives)
will flow, which is comparatively much less overhead than that incurred with
a distance vector protocol (such as RIP and IGRP). If, however, routes
(adjacencies) are unstable, link-state flooding will occur, and bandwidth
can quickly be consumed. OSPF also is very processor-intensive when running
the Dijkstra algorithm, which is used for computing routes.
In earlier releases of Cisco IOS software, special care had to be taken when
configuring OSPF over multiaccess nonbroadcast medias such as Frame Relay,
X.25, and ATM. The OSPF protocol considers these media like any other
broadcast media such as Ethernet. Nonbroadcast multiaccess (NBMA) clouds are
typically built in a hub and spoke topology. PVCs or switched virtual
circuits (SVCs) are laid out in a partial mesh and the physical topology
does not provide the multiaccess that OSPF believes is there. For the case
of point-to-point serial interfaces, OSPF always forms an adjacency between
the neighbors. OSPF adjacencies exchange database information. In order to
minimize the amount of information exchanged on a particular segment, OSPF
elects one router to be a DR, and one router to be a backup designated
router (BDR) on each multiaccess segment. The BDR is elected as a backup
mechanism in case the DR goes down.
The idea behind this setup is that routers have a central point of contact
for information exchange. The selection of the DR became an issue because
the DR and BDR needed to have full physical connectivity with all routers
that exist on the cloud. Also, because of the lack of broadcast
capabilities, the DR and BDR needed to have a static list of all other
routers attached to the cloud. This setup is achieved using the neighbor
command:
neighbor ip-address [priority number] [poll-interval seconds]
In later releases of Cisco IOS, different methods can be used to avoid the
complications of configuring static neighbors and having specific routers
becoming DRs or BDRs on the nonbroadcast cloud. Which method to use is
influenced by whether the network is new or an existing design that needs
modification.
A subinterface is a logical way of defining an interface. The same physical
interface can be split into multiple logical interfaces, with each
subinterface being defined as point-to-point. This scenario was originally
created in order to better handle issues caused by split horizon over NBMA
and vector based routing protocols.
A point-to-point subinterface has the properties of any physical
point-to-point interface. As far as OSPF is concerned, an adjacency is
always formed over a point-to-point subinterface with no DR or BDR election.
OSPF considers the cloud a set of point-to-point links rather than one
multiaccess network. The only drawback for the point-to-point is that each
segment belongs to a different subnet. This scenario might not be acceptable
because some administrators have already assigned one IP subnet for the
whole cloud. Another workaround is to use IP unnumbered interfaces on the
cloud. This scenario also might be a problem for some administrators who
manage the WAN based on IP addresses of the serial lines.
regards Ki
------------------------------------------------------
-----Original Message-----
From: Neil Garcia Legada [mailto:nlegs@visto.com]
Sent: 18 November 2001 14:59
To: ccielab@groupstudy.com
Subject: OSPF Point-to-Multipoint 'non-broadcast' keyword
Hi Group,
Anybody tried using the 'non-broadcast' keyword when configuring 'ip ospf
network point-to-multipoint' ??? It say's in the doc CD that 'neighbor'
statement is necessary for this type of OSPF network to work. Just curious
as to when should one use this option and to what advantage ???
Appreciate any feedback.
Thanks and regards,
Neil
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