Understanding Stubbiness
in OSPF
Configuration of ABR router and other routers which are the
part of OSPF process.
Scenario1: General ospf configuration in ABR and area 1
router
1. ABR configuration:
T1 is the bankbone area
router which is ABR also.
Configuration
root@T1# run show
configuration protocols ospf
area 0.0.0.0 {
================è Backbone Area
configuration
interface ge-0/0/0.0 {
priority 255;
}
interface lo0.0 {
passive;
}
}
area 0.0.0.1 { ====================è
Connected area 1
interface ge-0/0/1.0 {
interface-type p2p;
}
}
T3 is area 1 router
which has p2p connectvity with ABR
Configuration
area 0.0.0.1 {
interface ge-0/0/0.0 {
interface-type p2p;
}
interface lo0.0 {
passive;
}
}
Results from ABR
after OSPF Configuration:
OSPF database, Area 0.0.0.0
Type ID Adv Rtr Seq Age
Opt Cksum Len
Router *10.1.1.1 10.1.1.1 0x8000000a 76
0x22 0x20fa 48 – Type 1 LSA
Router 10.1.1.2 10.1.1.2 0x80000007 89
0x22 0x3adf 48
Network *192.168.1.1
10.1.1.1 0x80000003 323
0x22 0x584f 32 – Type 2 LSA
Summary *10.1.1.3
10.1.1.1 0x80000001 606
0x22 0x6cb3 28--- Type 3 LSA
Summary 10.1.1.4 10.1.1.2 0x80000003 907
0x22 0x58c3 28
Summary *192.168.2.0
10.1.1.1 0x80000007 678
0x22 0x506d 28
Summary
192.168.3.0 10.1.1.2 0x80000004 1452
0x22 0x4579 28
Summary
192.168.16.0 10.1.1.2 0x80000003 634
0x22 0xc1ef 28
ASBRSum 10.1.1.4 10.1.1.2 0x80000003 362
0x22 0x4ad0 28 - - Type 4 LSA
OSPF database,
Area 0.0.0.1
Type ID Adv Rtr Seq Age
Opt Cksum Len
Router *10.1.1.1 10.1.1.1 0x80000002 606
0x22 0xcb54 48 – Type1
Router 10.1.1.3 10.1.1.3 0x80000003 607
0x22 0x13ea 60
Summary *10.1.1.1
10.1.1.1 0x80000001 678
0x22 0x76ac 28—Type 3
Summary *10.1.1.2
10.1.1.1 0x80000001 678
0x22 0x76aa 28
Summary *10.1.1.4
10.1.1.1 0x80000001 678
0x22 0x6cb1 28
Summary *192.168.1.0
10.1.1.1 0x80000001 678
0x22 0x5572 28
Summary *192.168.3.0
10.1.1.1 0x80000001 678
0x22 0x5b66 28
Summary *192.168.16.0
10.1.1.1 0x80000001 678
0x22 0xd5dd 28
ASBRSum *10.1.1.4
10.1.1.1 0x80000001 678
0x22 0x5ebe 28—Type 4
OSPF AS SCOPE link
state database
Type ID Adv Rtr Seq Age
Opt Cksum Len
Extern
192.168.29.0 10.1.1.4 0x80000003 969
0x22 0x8990 36--- Type 5
Extern
192.168.30.0 10.1.1.4 0x80000002 2929
0x22 0x8099 36 --- Type 5
Ospf Database from
Area 1 router:
root@T3> show ospf database
OSPF database,
Area 0.0.0.1
Type ID Adv Rtr Seq Age
Opt Cksum Len
Router 10.1.1.1 10.1.1.1 0x80000002 829
0x22 0xcb54 48 – Type 1
Router *10.1.1.3 10.1.1.3 0x80000003 828
0x22 0x13ea 60
Summary 10.1.1.1 10.1.1.1 0x80000001 901
0x22 0x76ac 28--- Type 3
Summary 10.1.1.2 10.1.1.1 0x80000002 52
0x22 0x74ab 28
Summary 10.1.1.4 10.1.1.1 0x80000001 901
0x22 0x6cb1 28
Summary
192.168.1.0 10.1.1.1 0x80000001 901
0x22 0x5572 28
Summary
192.168.3.0 10.1.1.1 0x80000001 901
0x22 0x5b66 28
Summary
192.168.16.0 10.1.1.1 0x80000001
901 0x22 0xd5dd 28
ASBRSum 10.1.1.4 10.1.1.1 0x80000001 901
0x22 0x5ebe 28 ---Type 4
OSPF AS SCOPE link
state database
Type ID Adv Rtr Seq Age
Opt Cksum Len
Extern 192.168.29.0 10.1.1.4 0x80000003 1192
0x22 0x8990 36 –Type 5
Extern
192.168.30.0 10.1.1.4 0x80000003 213
0x22 0x7e9a 36
Conclusion: It
states that when you defined basics ospf with connected area0 , area1, area2
and so on we have database synchronization on all the routers and Type1, Type2,
Type 3, Type 4 & Type 5 database populates over all the routers.
So when an organization is big like we take an example of
AON Hewitt which has almost 165 countries in and if we need to populate the
routes from all 165 countries to every router it is not at all feasible and
very costly. So a Network Engineer Need to optimize the routing tables so that
database can be maintained. First progress to optimize the OSPF routing tables
is creating STUB areas.
STUB Areas:
Stub areas are areas through which or into which OSPF does
not flood AS external link-state advertisements (Type 5 LSAs). You might create
stub areas when much of the topology database consists of AS external
advertisements and you want to minimize the size of the topology databases on
the internal routers in the stub area.
The following restrictions apply to stub areas:
•You cannot create a virtual link through a stub area.
•A stub area cannot contain an AS boundary router.
•You cannot configure the backbone as a stub area.
•You cannot configure an area as both a stub area and an
not-so-stubby area (NSSA).
So, Type 5 routes have been removed from the opsf topology
when we define area 1 as stub. Which certainly reduces the size of database so
all the smaller branches which are connected to main branch through ospf domain
and were considered in external routes were all are now traversed through
default route? So we have reach ability to branches but the database is all
populated via default route which reduces the size of database and is easy to
manage for Network Engineer.
Totally Stubby
Areas: In Totally Stub areas routes, all Type-3 LSA are also further
removed from Ospf Database and only default route populated. All Type-3 routes
are also learned from default routes.
On ABR, we need to change the configuration of routes and
only “no-summaries” field is added to stub area.
Not- So- Stubby
Area:
The most common deployment for a not-so-stubby area occurs
when a newly acquired or merged company needs connectivity into an existing network.
Connecting it directly to the core of the OSPF network is not commonplace, so
using an outlying router to connect this new network to the existing network is
the best practice.
So, When a NSSA area is
introduced in the network area 0 did not populates the external routes to the
NSSA but External router beyond NSSA area are learned in NSSA.
When NSSA area is used,
ABR become the NSSA ASBR which populates the Type 5 converted routes from Type
7 into the ABR. This is really same as when during the merger of small
companies with AON, All new companies router are kept in NSSA area and external
routes are populated to AON Data Centre Core and routing table have the desired
routes. NSSA Routes from AREA 0:
NSSA AREA routes in ABR router.
NSSA Summary routes are also converted to default router
when we use no summary command in OSPF NSSA configuration. NSSA area is now
become Totally Stubby Area NSSA
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