Advanced Juniper Networks Routing
Release 5.1, Revision 0
Module 2: OSPF Stub Areas
Copyright © 2001, Juniper Networks, Inc.
Module Objectives
 Review the basic components of the OSPF Protocol
 Discuss the OSPF neighbor hello protocol and
adjacency formation process
 Examine the function of the Link-State Database
(LSDB) and how it contributes routes to the routing
table
 Look at the different kinds of OSPF packet types
 Configure OSPF on a Juniper Networks router
Copyright © 2001, Juniper Networks, Inc.
LSA Flooding: Default
Area 0
LSA 1
Area 0
LSA 2
Area 1
LSA 2
Area 0
Area 0
LSA 3
LSA 4
Area 2
Area 3
LSA 3
LSA 4
Area 3
Area 0
LSA 3
LSA 5
Area 3
LSA 5
Area 1
External
Routes
Area 0
LSA 5
Area 3
LSA 5
Area 1
LSA 3
Area 1
LSA 1
Backbone
(0.0.0.0)
Area 2
LSA 3
Area 3 Area 3
LSA 3 LSA 4
Area 2
LSA 1
Area 3
LSA 1
Area 3
LSA 2
Area 0
Area 0
LSA 3
LSA 4
Area 1
Area 0
LSA 3
LSA 5
Area 2
LSA 3
Area 3
LSA 5
Area 2
LSA 2
Area 0
LSA 3
Area 1
LSA 3
Area 3
LSA 3
Area 0
LSA 4
Area 3
LSA 4
Area 0
LSA 5
Area 3
LSA 5
Area 2
External
Routes
Copyright © 2001, Juniper Networks, Inc.
Area 3
OSPF Stub Areas
 Reduces the size of the link-state database
– ABR does not inject Type 4 LSA into area
– ABR does not flood Type 5 LSA into area
 Reachability for routes external to OSPF is achieved
via a 0/0 default route injected by the ABR
– Manual configuration step for administrator control
 ASBR in a stub area can’t flood LSAs for those routes
 Virtual links can’t transit a stub area
Copyright © 2001, Juniper Networks, Inc.
LSA Flooding: Stub Area
Area 0
LSA 1
Area 0
LSA 2
Area 1
LSA 2
Area 0
LSA 3
Area 2
LSA 3
Area 3
LSA 3
Area 2
LSA 3
Area 0
LSA 5
Area 3 Area 3
LSA 3 LSA 4
Area 2
LSA 1
Area 3
LSA 1
Area 3
LSA 2
Area 0
Area 0
LSA 3
LSA 4
Area 1
Area 0
LSA 3
LSA 5
Area 2
LSA 3
Area 3
LSA 5
Area 2
LSA 2
Area 0
LSA 3
Area 1
LSA 3
Area 3
LSA 3
Area 1
Stub
External
Routes
Area 3
LSA 5
Area 1
LSA 3
Area 1
LSA 1
Backbone
(0.0.0.0)
Area 0
LSA 4
Area 3
LSA 4
Area 0
LSA 5
Area 3
LSA 5
Area 2
External
Routes
Copyright © 2001, Juniper Networks, Inc.
Area 3
OSPF Stub No-Summaries Areas
 Further reduces the size of the link-state database
– ABR does not inject Type 3 LSA into area
– ABR does not inject Type 4 LSA into area
– ABR does not flood Type 5 LSA into area
 Reachability for external routes is still via a 0/0 default
route injected by the ABR
– Again, a manual configuration step for administrator control
 ASBR in a no-summaries area can’t flood LSAs for
those routes
 Virtual links can’t transit a no-summaries area
Copyright © 2001, Juniper Networks, Inc.
LSA Flooding: Stub No-Summaries
Area 0
LSA 1
Area 0
LSA 2
Area 1
LSA 3
Area 1
LSA 1
Area 1
LSA 2
Area 0
LSA 3
Area 2
LSA 3
Area 3
LSA 3
Backbone
(0.0.0.0)
Area 2
LSA 3
Area 0
LSA 5
Area 3 Area 3
LSA 3 LSA 4
Area 2
LSA 1
Area 3
LSA 1
Area 2
LSA 2
Area 0
LSA 3
Area 1
LSA 3
Area 3
LSA 3
Area 1
Stub
External
Routes
Area 0
LSA 4
Area 3
LSA 4
Area 0
LSA 5
Area 2
External
Routes
Copyright © 2001, Juniper Networks, Inc.
Area 3
LSA 2
Area 3
Stub
No-Summaries
Stub Area Configuration
 Each router in the area is required to configure itself
as a stub router
 The ABR can optionally inject a default route via the
default-metric command
[edit protocols ospf]
user@area-1-router# set area 1 stub
user@area-1-router# show
area 0.0.0.1 {
stub;
interface so-0/0/0.0;
}
[edit protocols ospf]
user@area-1-abr# set area 1 stub default-metric 10
user@area-1-abr# show
area 0.0.0.1 {
stub default-metric 10;
interface so-0/1/1.0;
}
Copyright © 2001, Juniper Networks, Inc.
No-Summaries Area Configuration
 Only the ABR is required to configure itself to support
a no-summaries area
 The ABR can optionally inject a default route via the
default-metric command
[edit protocols ospf]
user@area-1-abr# set area 1 stub no-summaries
user@area-1-abr# show
area 0.0.0.1 {
stub default-metric 10 no-summaries;
interface so-0/1/1.0;
}
Copyright © 2001, Juniper Networks, Inc.
Area-Range
 All local area routes are forwarded to the backbone by
default
– Stub only changes what enters an area, not what leaves
 To summarize routing information headed to the
backbone, use the area-range command
 Only a single Type 3 LSA will be injected into the
backbone
 Configured on the ABR only
[edit protocols ospf]
user@host# set area 1 area-range 192.168.16/20
[edit protocols ospf]
user@host# show
area 0.0.0.1 {
area-range 192.168.16.0/20;
interface at-1/0/1.100;
}
Copyright © 2001, Juniper Networks, Inc.
Area-Range Restrict
 Adding the restrict keyword to the area-range
command stops all routes in the range from entering
the backbone
 No Type 3 LSA will be injected into the backbone
 Configured on the ABR only
[edit protocols ospf]
user@host# set area 1 area-range 192.168.16/20 restrict
[edit protocols ospf]
user@host# show
area 0.0.0.1 {
area-range 192.168.16.0/20 restrict;
interface at-1/0/1.100;
}
Copyright © 2001, Juniper Networks, Inc.
Review Questions
Copyright © 2001, Juniper Networks, Inc.