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Page 301 / 944 Scroll up to view Page 296 - 300
Chapter 14 Routing Protocols
ZyWALL USG 50 User’s Guide
301
Each type of area is illustrated in the following figure.
Figure 181
OSPF: Types of Areas
This OSPF AS consists of four areas, areas 0-3. Area 0 is always the backbone. In
this example, areas 1, 2, and 3 are all connected to it. Area 1 is a normal area. It
has routing information about the OSPF AS and networks X and Y. Area 2 is a stub
area. It has routing information about the OSPF AS, but it depends on a default
route to send information to networks X and Y. Area 3 is a NSSA. It has routing
information about the OSPF AS and network Y but not about network X.
OSPF Routers
Every router in the same area has the same routing information. They do this by
exchanging Hello messages to confirm which neighbor (layer-3) devices exist, and
then they exchange database descriptions (DDs) to create a synchronized link-
state database. The link-state database contains records of router IDs, their
associated links and path costs. The link-state database is then constantly
updated through Link State Advertisements (LSA). Each router uses the link state
database and the Dijkstra algorithm to compute the least cost paths to network
destinations.
Like areas, each router has a unique 32-bit ID in the OSPF AS, and there are
several types of routers. Each type is really just a different role, and it is possible
for one router to play multiple roles at one time.
•
An internal router (IR) only exchanges routing information with other routers in
the same area.
•
An Area Border Router (ABR) connects two or more areas. It is a member of all
the areas to which it is connected, and it filters, summarizes, and exchanges
routing information between them.
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Chapter 14 Routing Protocols
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•
An Autonomous System Boundary Router (ASBR) exchanges routing information
with routers in networks outside the OSPF AS. This is called redistribution in
OSPF.
•
A backbone router (BR) has at least one interface with area 0. By default, every
router in area 0 is a backbone router, and so is every ABR.
Each type of router is illustrated in the following example.
Figure 182
OSPF: Types of Routers
In order to reduce the amount of traffic between routers, a group of routers that
are directly connected to each other selects a designated router (DR) and a
backup designated router (BDR). All of the routers only exchange information with
the DR and the BDR, instead of exchanging information with all of the other
routers in the group. The DR and BDR are selected by priority; if two routers have
the same priority, the highest router ID is used.
The DR and BDR are selected in each group of routers that are directly connected
to each other. If a router is directly connected to several groups, it might be a DR
in one group, a BDR in another group, and neither in a third group all at the same
time.
Virtual Links
In some OSPF AS, it is not possible for an area to be directly connected to the
backbone. In this case, you can create a virtual link through an intermediate area
Table 83
OSPF: Redistribution from Other Sources to Each Type of Area
SOURCE
\
TYPE OF AREA
NORMAL
NSSA
STUB
Static routes
Yes
Yes
No
RIP
Yes
Yes
Yes
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Chapter 14 Routing Protocols
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303
to logically connect the area to the backbone. This is illustrated in the following
example.
Figure 183
OSPF: Virtual Link
In this example, area 100 does not have a direct connection to the backbone. As a
result, you should set up a virtual link on both ABR in area 10. The virtual link
becomes the connection between area 100 and the backbone.
You cannot create a virtual link to a router in a different area.
OSPF Configuration
Follow these steps when you configure OSPF on the ZyWALL.
1
Enable OSPF.
2
Set up the OSPF areas.
3
Configure the appropriate interfaces. See
Section 11.3.1 on page 221
.
4
Set up virtual links, as needed.
14.3.1
Configuring the OSPF Screen
Use the first OSPF screen to specify the OSPF router the ZyWALL uses in the OSPF
AS and maintain the policies for redistribution. In addition, it provides a summary
of OSPF areas, allows you to remove them, and opens the
OSPF Add/Edit
screen
to add or edit them.
Page 304 / 944
Chapter 14 Routing Protocols
ZyWALL USG 50 User’s Guide
304
Click
Configuration > Network
>
Routing > OSPF
to open the following
screen.
Figure 184
Configuration > Network > Routing > OSPF
The following table describes the labels in this screen. See
Section 14.3.2 on page
306
for more information as well.
Table 84
Configuration > Network > Routing Protocol > OSPF
LABEL
DESCRIPTION
OSPF Router ID
Select the 32-bit ID the ZyWALL uses in the OSPF AS.
Default
- the highest available IP address assigned to the interfaces
is the ZyWALL’s ID.
User Defined
- enter the ID (in IP address format) in the field that
appears when you select
User Defined
.
Redistribute
Active RIP
Select this to advertise routes that were learned from RIP.
The
ZyWALL advertises routes learned from RIP to
Normal
and
NSSA
areas but not to
Stub
areas.
Type
Select how OSPF calculates the cost associated with routing
information from RIP. Choices are:
Type 1
and
Type 2
.
Type 1
- cost = OSPF AS cost + external cost (
Metric
)
Type 2
- cost = external cost (
Metric
); the OSPF AS cost is ignored.
Metric
Type the external cost for routes provided by RIP. The metric
represents the “cost” of transmission for routing purposes. The way
this is used depends on the
Type
field. This value is usually the
average cost in the OSPF AS, and it can be between 1 and 16777214.
Active Static
Route
Select this to advertise routes that were learned from static routes.
The ZyWALL advertises routes learned from static routes to all types
of areas.
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Type
Select how OSPF calculates the cost associated with routing
information from static routes. Choices are:
Type 1
and
Type 2
.
Type 1
- cost = OSPF AS cost + external cost (
Metric
)
Type 2
- cost = external cost (
Metric
); the OSPF AS cost is ignored.
Metric
Type the external cost for routes provided by static routes. The metric
represents the “cost” of transmission for routing purposes. The way
this is used depends on the
Type
field. This value is usually the
average cost in the OSPF AS, and it can be between 1 and 16777214.
Area
This section displays information about OSPF areas in the ZyWALL.
Add
Click this to create a new OSPF area.
Edit
Double-click an entry or select it and click
Edit
to open a screen
where you can modify the entry’s settings.
Remove
To remove an entry, select it and click
Remove
. The ZyWALL confirms
you want to remove it before doing so.
#
This field is a sequential value, and it is not associated with a specific
area.
Area
This field displays the 32-bit ID for each area in IP address format.
Type
This field displays the type of area. This type is different from the
Type
field above.
Authentication
This field displays the default authentication method in the area.
Apply
Click this button to save your changes to the ZyWALL.
Reset
Click this button to return the screen to its last-saved settings.
Table 84
Configuration > Network > Routing Protocol > OSPF (continued)
LABEL
DESCRIPTION

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