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Enterprises typically use default routes in two different cases:
■ To direct remote-site routers at the edge of the Enterprise network to send all packets
toward the core of the Enterprise, with the core routers knowing all the more specific
routes to Enterprise destination addresses.
■ To direct traffic on all Enterprise routers toward an Internet-facing router so that all
traffic destined for the Internet eventually arrives at the Enterprise’s Internet-connected
routers.
Engineers could achieve both of these goals by using route summarization with the area
range and summary-address commands. For example, consider a case in which the goal is
to drive all packets destined for Internet hosts to one of two equal Internet routers for an
Enterprise, as shown in Figure 7-6. The design shows two ASBRs connected to the Internet.
Both ASBRs could learn routes with BGP. Rather than redistribute all BGP routes into
the Enterprise, the ASBRs summarize to the ultimate summary, 0.0.0.0/0. The two OSPF
ASBRs flood the Type 5 LSA for a summary route–one from ASBR1 and one from
ASBR2–throughout the Enterprise. As a result, all OSPF routers choose a default route,
with the packets destined for locations in the Internet eventually reaching one of the two
ASBRs, which then forwards the packets into the Internet.
To meet the other design goal for using defaults–to get the routers in an area to use default
routing to deliver packets to the ABR–the ABR could use the area range command
to flood a default route into a single area. Again in Figure 7-6, if the design called for the
routers in area 1 to use a default route to reach other destinations in the Enterprise, the
ABRs connected to area 1, like ABR1, could use the area 0 range 0.0.0.0 0.0.0.0 com
■ With the always parameter, the default route is advertised even if there is no default
route in the router’s routing table.
■ The metric keyword defines the metric listed for the default route (default 1).
■ The metric-type keyword defines whether the LSA is listed as external type 1 or external
type 2 (default). (Chapter 9’s section “Redistribution into OSPF” discusses
OSPF external route types.)
■ The decision of when to advertise, and when to withdraw, the default route is based
on matching the referenced route-map with a permit action.
When configured, OSPF will flood the default route throughout the OSPF routing domain,
drawing traffic to each ASBR, as shown earlier in Figure 7-6.
Note: The type of external OSPF route (Type 1 or Type 2) is explained more fully in
Chapter 9.
Stubby Areas
As mentioned earlier, the two most common reasons to consider using default routes are
to drive all Internet-destined traffic toward the Internet-connected routers in the Enterprise
and to drive traffic inside an area toward one of the ABRs in that area. This second
design choice allows the routers in an area to use default routes for forwarding packets to
ABRs, rather than more specific routes. Using default routes inside an area reduces memory
consumption and CPU processing time on the routers inside the area, because the
routers in that area can have fewer LSAs in their LSDBs.
The OSPF stub router feature provides engineers a very simple way to enable the function
of flooding default routes inside an area, with those default routes driving IP packets back
toward the ABRs attached to that area. ABRs in stub areas advertise a default route into
the stub area. At the same time, the ABR chooses to not advertise external routes (5 LSAs)
into the area, or even instead to no longer advertise interarea routes (in Type 3 LSAs) into
the area. As a result, all routers in the stub area can still route to the destinations (based on
the default route), but the routers require less memory and processing.
The following list summarizes these features of stub areas for easier study and review:
■ ABRs create a default route, using a Type 3 LSA, listing subnet 0.0.0.0 and mask
0.0.0.0, and flood that into the stub area.
■ ABRs do not flood Type 5 LSAs into the stub area.
■ ABRs may not flood other Type 3 LSAs into the area.
■ The default route has a metric of 1 unless otherwise configured using the OSPF subcommand
area area-num default-cost cost.
■ Routers inside stub areas cannot redistribute external routes into the stubby area, because
that would require a Type 5 LSA in the area.
11.11.0.0/16, 11.12.0.0/16, and 11.13.0.0/16 would not exist in the LSDBs for area 34. Finally,
because the area is not a totally stubby area, the ABRs do create and flood Type 3
LSAs for interarea routes as usual, so they flood LSAs for the 10.16.11.0/24, 10.16.12.0/24,
and 10.16.13.0/24 subnets listed in the figure.
Next, consider a similar scenario but with a totally stubby area for area 5. As for all stubby
area types, the ABRs each advertise a default route into area 5. As for all stubby area
types, the ABRs filter all Type 5 LSAs, which means that the three Type 5 LSAs for
11.11.0.0/16, 11.12.0.0/16, and 11.13.0.0/16 would not exist in the LSDBs for area 5. The
key difference exists in that the ABRs also would not create and flood Type 3 LSAs for interarea
routes as usual, so they would not advertise Type 3 LSAs for the 10.16.11.0/24,
10.16.12.0/24, and 10.16.13.0/24 subnets listed in the figure into area 5.
The other difference in stubby area types relates to whether the name uses NSSA (NSSA
or totally NSSA) or not (stubby, totally stubby). Stubby area types that use the NSSA
name can redistribute external routes into the area; stubby area types without NSSA in
the name cannot.
Configuring and Verifying Stubby Areas
Configuring stub and totally stubby areas requires only three commands, but with at least
one command on each router, as listed in Table 7-3:
Note: For totally stubby areas, only the ABR must have the no-summary keyword on the
area area-id stub no-summary command. However, including this keyword on internal
routers does not cause a problem.
Figure 7-9 shows a more detailed view of area 34 from Figure 7-8, so by making area 34 a
stub area, ABRs R1 and R2 will not flood Type 3 LSAs into area 34–other than the Type 3
LSA for the default routes. Example 7-4 shows the configuration on Routers R1, R2, and
R3 from Figure 7-9.
Key
Topic
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R2
R3
R4
R1
Fa0/0
10.10.34.3/24
Fa0/0
10.10.34.4/24
S0/0/0.1
14.4
S0/0/0.2
24.4
S0/0/0.1
13.3
S0/0/0.2
23.2
Area 34 (Stubby)
S0/0/0.4
14.1
S0/0/0.3
13.1
S0/0/0.4
24.2
S0/0/0.3
23.2
Figure 7-9 Detailed View of Area 34
Example 7-4 Stub Area Configuration
! On Router R1:
router ospf 1
area 34 stub
auto-cost reference-bandwidth 1000
!
interface s0/0/0.3 point-to-point
ip ospf area 34
!
interface s0/0/0.4 point-to-point
ip ospf area 34
! On Router R2:
router ospf 2
area 34 stub
auto-cost reference-bandwidth 1000
!
interface s0/0/0.3 point-to-point
ip ospf area 34
!
interface s0/0/0.4 point-to-point
iip ospf area 34
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Chapter 7: OSPF Route Summarization, Filtering, and Default Routing 243
! On Router R3:
router ospf 3
area 34 stub
auto-cost reference-bandwidth 1000
!
interface s0/0/0.1 point-to-point
ip ospf area 34
ip ospf cost 500
!
interface s0/0/0.2 point-to-point
ip ospf area 34
!
interface fa0/0
ip ospf area 34
With the configuration as shown, both R1 and R2 will inject a default route, represented
as a Type 3 LSA, with default metric 1. They will also not flood the Type 5 LSAs into
area 34. Example 7-5 confirms these facts, showing the Type 3 LSA for the summary, and
the absence of Type 5 LSAs in the output of the show ip ospf database command on
router R3.
Example 7-5 Evidence of Type 5’s Existing, Disappearing, and Defaults Appearing
! Before making Area 34 stubby:
R3#show ip ospf database | begin AS External
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
11.11.0.0 7.7.7.7 929 0x80000001 0x00016D 0
12.12.0.0 7.7.7.7 845 0x80000001 0x00E784 0
13.13.0.0 7.7.7.7 835 0x80000001 0x00CE9B 0
! After making area 34 stubby – no output from the next command.
R3#show ip ospf database | begin AS External
R3#
! The database for area 34 now has two Type 3 LSAs for default routes.
R3#show ip ospf database
OSPF Router with ID (3.3.3.3) (Process ID 3)
Router Link States (Area 34)
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! Lines omitted for brevity – skipped to “Summary Net” (Type 3) section
Summary Net Link States (Area 34)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 1.1.1.1 692 0x80000001 0x0093A6
0.0.0.0 2.2.2.2 686 0x80000001 0x0075C0
10.10.5.0 1.1.1.1 692 0x8000000E 0x00445C
10.10.5.0 2.2.2.2 686 0x8000000F 0x002477
10.10.12.0 1.1.1.1 692 0x8000000E 0x0054AF
10.10.12.0 2.2.2.2 686 0x8000000E 0x0036C9
! Many Type 3 LSA’s omitted for brevity’s sake
Example 7-5 shows the existence of the Type 5 external LSAs before area 34 became a
stubby area, and the disappearance of those same LSAs once it was made a stubby area.
The show ip ospf database command then shows two LSAs that list default routes, one
learned from RID 1.1.1.1 (R1), and one learned from RID 2.2.2.2 (R2).
Example 7-6 continues the verification of how stub areas work with three more commands.
Example 7-6 Three External Routes Before and None Afterward Changing to Stubby
! Next, R3 confirms it thinks area 34 is a stub area
R3#show ip ospf
Routing Process “ospf 3” with ID 3.3.3.3
Start time: 00:00:38.756, Time elapsed: 07:51:19.720
! lines omitted for brevity
Area 34
Number of interfaces in this area is 3
It is a stub area
Area has no authentication
SPF algorithm last executed 00:11:21.640 ago
SPF algorithm executed 18 times
Area ranges are
Number of LSA 29. Checksum Sum 0x0D3E01
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
! The next command shows all Type 3 (summary) LSAs of prefix 0.0.0.0
R3#show ip ospf database summary 0.0.0.0
OSPF Router with ID (3.3.3.3) (Process ID 3)
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Chapter 7: OSPF Route Summarization, Filtering, and Default Routing 245
Summary Net Link States (Area 34)
Routing Bit Set on this LSA
LS age: 879
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 0.0.0.0 (summary Network Number)
Advertising Router: 1.1.1.1
LS Seq Number: 80000001
Checksum: 0x93A6
Length: 28
Network Mask: /0
TOS: 0 Metric: 1
LS age: 873
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 0.0.0.0 (summary Network Number)
Advertising Router: 2.2.2.2
LS Seq Number: 80000001
Checksum: 0x75C0
Length: 28
Network Mask: /0
TOS: 0 Metric: 1
! The next command lists statistics of the number of LSAs of each type –
! note a total of 0 Type 5 LSAs, but many Type 3
R3#show ip ospf database database-summary
OSPF Router with ID (3.3.3.3) (Process ID 3)
Area 34 database summary
LSA Type Count Delete Maxage
Router 4 0 0
Network 1 0 0
Summary Net 24 0 0
Summary ASBR 0 0 0
Type-7 Ext 0 0 0
Prefixes redistributed in Type-7 0
Opaque Link 0 0 0
Opaque Area 0 0 0
Subtotal 29 0 0
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Process 3 database summary
LSA Type Count Delete Maxage
Router 4 0 0
Network 1 0 0
Summary Net 24 0 0
Summary ASBR 0 0 0
Type-7 Ext 0 0 0
Opaque Link 0 0 0
Opaque Area 0 0 0
Type-5 Ext 0 0 0
Prefixes redistributed in Type-5 0
Opaque AS 0 0 0
Non-self 28
Total 29 0 0
Following are the three commands in Example 7-6 in order:
■ show ip ospf–Confirms with one (highlighted) line that the router believes that the
area is a stub area.
■ show ip ospf database summary 0.0.0.0–By definition, this command lists all
summary (Type 3) LSAs with prefix 0.0.0.0. It lists two such LSAs, created by R1 and
R2 (RIDs 1.1.1.1 and 2.2.2.2, respectively), both with metric 1 (the default setting).
■ show ip ospf database database-summary–This command lists statistics about
the numbers of and types of LSAs in the database. The counters show 0 Type 5 LSAs,
and a few dozen Type 3s–confirming that the area, while stubby, is not totally stubby.
Configuring and Verifying Totally Stubby Areas
Configuring totally stubby areas requires almost no additional effort as compared with
stubby areas. As listed earlier in Table 7-3, the only difference for totally stubby configuration
versus stubby configuration is that the ABR’s include the no-summary keyword on
the area stub command. (no-summary refers to the fact that ABRs in totally stubby areas
do not create/flood Type 3 summary LSAs.)
Example 7-7 shows another example configuration, this time with area 34 as a totally
stubby area. Additionally, the default routes’ metrics have been set so that both R3 and
R4 will use R1 as their preferred ABR, by setting R2’s advertised summary to a relatively
high metric (500). Example 7-7 just shows the changes to the configuration shown in
Example 7-4.
Example 7-7 Totally Stubby Area Configuration
! On Router R1:
router ospf 1
area 34 stub no-summary
auto-cost reference-bandwidth 1000
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Chapter 7: OSPF Route Summarization, Filtering, and Default Routing 247
! On Router R2:
router ospf 2
area 34 stub no-summary
area 34 default-cost 500
auto-cost reference-bandwidth 1000
The configuration of a totally stubby area reduces the size of the LSDB in area 34, because
the ABRs no longer flood Type 3 LSAs into area 34, as shown in Example 7-8. R3
displays its LSDB, listing only two Summary (Type 3) LSAs–the two default routes advertised
by the two ABRs, respectively. No other Type 3 LSAs exist, nor do any external
(Type 5) or ASBR summary (Type 4) LSAs.
Also, note that the example lists the OSPF routes known to R3. Interestingly, in the topology
shown for area 34, R3 learns only three OSPF routes: the two intra-area routes for the
subnets between R4 and the two ABRs, plus the best default route. The default route has a
metric of 501, based on R3’s S0/0/0.1 interface cost plus the cost 1 listed for R1’s Type 3
LSA for the default route.
Example 7-8 Confirmation of the Effects of a Totally Stubby Area
! On Router R3: show ip ospf database, show ip route ospf, get show ip ospf
database database-summary –
R3#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is 10.10.13.1 to network 0.0.0.0
10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
C 10.10.13.0/29 is directly connected, Serial0/0/0.1
O 10.10.14.0/29 [110/657] via 10.10.34.4, 00:57:37, FastEthernet0/0
C 10.10.23.0/29 is directly connected, Serial0/0/0.2
O 10.10.24.0/29 [110/657] via 10.10.34.4, 00:57:37, FastEthernet0/0
C 10.10.34.0/24 is directly connected, FastEthernet0/0
O*IA 0.0.0.0/0 [110/501] via 10.10.13.1, 00:24:35, Serial0/0/0.1
R3#show ip ospf database database-summary
OSPF Router with ID (3.3.3.3) (Process ID 3)
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! lines omitted for brevity
Process 3 database summary
LSA Type Count Delete Maxage
Router 4 0 0
Network 1 0 0
Summary Net 2 0 0
Summary ASBR 0 0 0
Type-7 Ext 0 0 0
Opaque Link 0 0 0
Opaque Area 0 0 0
Type-5 Ext 0 0 0
Prefixes redistributed in Type-5 0
Opaque AS 0 0 0
Non-self 6
Total 7 0 0
R3#show ip ospf database | begin Summary
Summary Net Link States (Area 34)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 1.1.1.1 1407 0x80000003 0x008FA8
0.0.0.0 2.2.2.2 1506 0x80000004 0x00FF3E
Following are the three commands in Example 7-8 in order:
■ show ip route–It lists a single interarea route–a default route, with destination
0.0.0.0/0. The output also lists this same next-hop information as the gateway of
last resort.
■ show ip ospf database database-summary–The statistics still show no External
Type 5 LSAs, just as when the area was stubby, but now shows only 2 Type 3 LSAs,
whereas before a few dozen existed.
■ show ip ospf database | begin Summary–This command shows the output beginning
with the Type 3 Summary LSAs. It lists two default route LSAs: one from R1
and one from R2.
Examples 7-6 and 7-8 demonstrate the key differences between stub (they do see Type 3
LSAs) and totally stubby areas (which do not see Type 3 LSAs). Next, this section looks at
the different types of not-so-stubby areas.
The Not-So-Stubby Area (NSSA)
Stub and totally stubby areas do not allow external routes to be injected into a stubby
area–a feature that originally caused some problems. The problem is based on the fact that
stub areas by definition should never learn a Type 5 LSA, and OSPF injects external routes
Example 7-9 shows a sample with the configuration of a totally NSSA area 34 from the
network represented in the last four figures. Note that as with the area stub command, the
area nssa command’s no-summary option is required only on the ABRs.
Example 7-9 Totally NSSA Area Configuration and Verification
! On Router R1:
router ospf 1
area 34 nssa no-summary
! On Router R2:
router ospf 2
area 34 nssa no-summary
area 34 default-cost 500
! On Router R3:
router ospf 3
area 34 nssa
! On Router R4:
router ospf 4
area 34 nssa
The same verification steps and commands can be used for NSSA areas as were shown in
the earlier examples for stub areas. In particular, the show ip ospf command states that
the area is an NSSA area. You can also see Type 7 LSAs in the OSPF LSDB after redistribution
has been configured, as shown in Chapter 9.
Stubby Area Summary
Table 7-4 summarizes the key points regarding stubby areas.
Note: Both types of totally stubby areas (totally stubby, totally NSSA) are Cisco proprietary
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