User`s guide

Configuring OSPF Routing
Introduction to OSPF
MultiVoice Gateway for the MAX— User’s Guide Preliminary November 23, 1998 9-3
Security
All OSPF protocol exchanges are authenticated. This means that only trusted routers can
participate in the AS’s routing. A variety of authentication schemes are available. In fact,
different authentication types can be configured for each area. In addition, authentication
provides added security for the routers that are on the network. Routers that do not have the
password will not be able to gain access to the routing information, because authentication
failure prevents a router from forming adjacencies.
Support for variable length subnet masks
OSPF enables the flexible configuration of IP subnets. Each route distributed by OSPF has a
destination and mask. Two different subnets of the same IP network number may have
different sizes (different masks). This is commonly referred to as variable length subnet masks
(VLSM), or Classless Inter-Domain Routing (CIDR). A packet is routed to the best (longest or
most specific) match. Host routes are considered to be subnets whose masks are all ones
(0xFFFFFFFF).
Note: Although OSPF is very useful for networks that use VLSM, Ascend recommends that
you attempt to assign subnets that are as contiguous as possible in order to prevent excessive
link-state calculations by all OSPF routers on the network.
Interior gateway protocol (IGP)
OSPF keeps all AS-internal routing information within the AS. All information exchanged
within the AS is interior.
For communication with other autonomous systems, an AS border router (ASBR) is required
to use an external gateway protocol (EGP), as shown in Figure 9-1. An EGP acts as a shuttle
service between autonomous systems.
Figure 9-1. Autonomous system border routers
ASBRs perform calculations related to external routes. The MultiVoice Gateway imports
external routes from RIP (for example, when it establishes a WAN link with a caller that does
not support OSPF) and always performs the ASBR calculations.
If you must prevent the MultiVoice Gateway from performing ASBR calculations, you can
disable them in Ethernet > Mod Config > OSPF Global Options.
EGP