User`s guide

9-2 Preliminary November 23, 1998 MultiVoice Gateway for the MAX— User’s Guide
Configuring OSPF Routing
Introduction to OSPF
Ascend implementation of OSPF
The primary goal of OSPF at this release is to enable the MultiVoice Gateway to communicate
with other routers within a single autonomous system (AS).
The MultiVoice Gateway acts as an OSPF internal router with limited border router capability.
At this release, Ascend does not recommend an area border router (ABR) configuration for the
MultiVoice Gateway, so the Ethernet interface and all of the MultiVoice Gateway WAN links
should be configured in the same area.
The MultiVoice Gateway does not function as a full AS border router (ASBR) at this release.
However, it performs ASBR calculations for external routes such as WAN links that do not
support OSPF. It imports external routes into its OSPF database and flags them as ASE
(autonomous system external). It redistributes those routes by means of OSPF ASE
advertisements, and propagates its OSPF routes to remote WAN routers that are running RIP.
The MultiVoice Gateway supports null and simple password authentication.
OSPF features
This section provides a brief overview of OSPF routing to help you properly configure the
MultiVoice Gateway. For full details about how OSPF works, see RFC 1583, “OSPF Version
2,” 03/23/1994, J. Moy.
An Autonomous System (AS) is a group of OSPF routers exchanging information, typically
under the control of one company. An AS can include a large number of networks, all of which
are assigned the same AS number. All information exchanged within the AS is interior.
Exterior protocols are used to exchange routing information between autonomous systems.
They are referred to by the acronym EGP (exterior gateway protocol). The AS number can be
used by border routers to filter out certain EGP routing information. OSPF can make use of
EGP data generated by other border routers and added into the OSPF system as ASEs, as well
as static routes configured in the MultiVoice Gateway or RADIUS.
Excessive routing traffic
and slow convergence
RIP creates a routing table and then propagates it throughout the
internet of routers, hop by hop. The time it takes for all routers to
receive information about a topology change is called
convergence. A slow convergence can result in routing loops and
errors.
A RIP router broadcasts its entire routing table every 30 seconds.
On a 15-hop network, convergence can be as high as 7.5 minutes.
In addition, a large table can require multiple broadcasts for each
update, which consumes a lot of bandwidth.
OSPF uses a topological database of the network and propagates
only changes to the database (as described in “Exchange of
routing information” on page 9-4).
Problem Description and solution