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 (TOTALLY stub/NSSA).
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.
■ All routers in the area must be configured to be stubby; if not, neighbor relationships cannot form between potential neighbors based on this mismatched configuration.
Four types of stubby areas exist: stub, totally stubby, not-so-stubby areas (NSSA), and totally NSSA.
Two types of stubby areas have the word “totally” as part of the name, and two do not.
The differences between those with the word “totally” and those without have to do with whether Type 3 LSAs are flooded into the area.
The rules are
■ For all types of stubby areas, the ABR always filters Type 5 (external) LSAs.
■ For totally stubby and totally NSSA areas, the ABR also filters Type 3 LSAs.
■ For stubby and NSSA areas–those without the word “totally” in the name–the ABRs do not filter Type 3 LSAs, advertising Type 3 LSAs as normal.
Stubby area types that use the NSSA name can redistribute external routes into the area; stubby area types without NSSA in the name cannot.
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.
A. Stub area (Type 5 LSA: NO, Type 3 LSA: YES)
General syntax: area area-id stub (configured on each router in the area)
We have Area 1 as a stub area:
R2#show run | s ospf
router ospf 1
router-id 2.2.2.2
log-adjacency-changes
area 1 stub
network 192.168.0.0 0.0.0.7 area 0
network 192.168.168.0 0.0.0.3 area 1
R13#show run | s ospf
router ospf 1
router-id 13.13.13.13
log-adjacency-changes
area 1 stub
network 192.168.168.0 0.0.0.3 area 1
network 192.168.169.0 0.0.0.255 area 1
On R13 a default route is injected by ABR R2, represented as a Type 3 LSA:
R13#show ip route
...
Gateway of last resort is 192.168.168.1 to network 0.0.0.0
...
O*IA 0.0.0.0/0 [110/11] via 192.168.168.1, 00:00:08, FastEthernet0/0
R13#show ip ospf database
OSPF Router with ID (13.13.13.13) (Process ID 1)
...
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 2.2.2.2 357 0x80000001 0x0075C0
Router R2 also has Type 5 LSAs generated by ASBR R4 for the EIGRP networks, but does not advertise them to the stub area.
R2 advertises only Type 3 LSAs to R13:
R2#show ip ospf database
OSPF Router with ID (2.2.2.2) (Process ID 1)
...
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 2.2.2.2 495 0x80000001 0x0075C0
10.0.1.1 2.2.2.2 430 0x80000001 0x00BA26
10.0.1.2 2.2.2.2 403 0x80000001 0x00336C
10.0.1.3 2.2.2.2 431 0x80000001 0x002975
10.0.10.10 2.2.2.2 403 0x80000001 0x008904
10.0.11.11 2.2.2.2 431 0x80000001 0x007417
192.168.0.0 2.2.2.2 492 0x80000001 0x00F3D6
192.168.0.12 2.2.2.2 431 0x80000001 0x001664
192.168.0.16 2.2.2.2 431 0x80000001 0x00ED88
192.168.0.20 2.2.2.2 431 0x80000001 0x00C5AC
192.168.3.0 2.2.2.2 431 0x80000001 0x00D1A7
192.168.13.13 2.2.2.2 431 0x80000001 0x001692
192.168.33.33 2.2.2.2 431 0x80000001 0x0057DE
192.168.100.100 2.2.2.2 431 0x80000001 0x00EB0E
...
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
172.16.0.0 4.4.4.4 305 0x80000001 0x00D7FE 0
172.16.0.4 4.4.4.4 307 0x80000001 0x00AF23 0
172.16.0.8 4.4.4.4 307 0x80000001 0x008747 0
172.16.0.12 4.4.4.4 307 0x80000001 0x005F6B 0
172.16.111.0 4.4.4.4 307 0x80000001 0x002044 0
192.168.44.0 4.4.4.4 307 0x80000001 0x00D723 0
192.168.55.0 4.4.4.4 307 0x80000001 0x005E91 0
No Type 5 LSAs exist in R13's OSPF LSDB:
R13#show ip ospf database
OSPF Router with ID (13.13.13.13) (Process ID 1)
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
2.2.2.2 2.2.2.2 548 0x80000002 0x007082 1
13.13.13.13 13.13.13.13 547 0x80000003 0x002E9C 2
Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
192.168.168.2 13.13.13.13 547 0x80000001 0x003F7B
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 2.2.2.2 600 0x80000001 0x0075C0
10.0.1.1 2.2.2.2 534 0x80000001 0x00BA26
10.0.1.2 2.2.2.2 506 0x80000001 0x00336C
10.0.1.3 2.2.2.2 534 0x80000001 0x002975
10.0.10.10 2.2.2.2 506 0x80000001 0x008904
10.0.11.11 2.2.2.2 534 0x80000001 0x007417
192.168.0.0 2.2.2.2 595 0x80000001 0x00F3D6
192.168.0.12 2.2.2.2 536 0x80000001 0x001664
192.168.0.16 2.2.2.2 536 0x80000001 0x00ED88
192.168.0.20 2.2.2.2 536 0x80000001 0x00C5AC
192.168.3.0 2.2.2.2 536 0x80000001 0x00D1A7
192.168.13.13 2.2.2.2 536 0x80000001 0x001692
192.168.33.33 2.2.2.2 536 0x80000001 0x0057DE
192.168.100.100 2.2.2.2 536 0x80000001 0x00EB0E
R13#
B. Totally stubby area (Type 5 LSA: NO, Type 3 LSA: NO)
General syntax: area area-id stub no-summary (on the ABR)
area area-id stub (on all other routers in the area)
Referring to the same areas and routers as above, we only have to add the "no-summary" keyword to the "area 1 stub" command on ABR R2:
R2(config)#router ospf 1
R2(config-router)#area 1 stub no-summary
R2#show run | s ospf
router ospf 1
router-id 2.2.2.2
log-adjacency-changes
area 1 stub no-summary
network 192.168.0.0 0.0.0.7 area 0
network 192.168.168.0 0.0.0.3 area 1
R13's configuration remains unchanged, because the "no-summary" keyword has to be added only on the ABR:
R13#show run | s ospf
router ospf 1
router-id 13.13.13.13
log-adjacency-changes
area 1 stub
network 192.168.168.0 0.0.0.3 area 1
network 192.168.169.0 0.0.0.255 area 1
As a result, R2 injects neither Type 3 nor Type 5 LSAs into Area 1.
Before configuring totally stubby:
R13#show ip route
...
Gateway of last resort is 192.168.168.1 to network 0.0.0.0
192.168.13.0/32 is subnetted, 1 subnets
O IA 192.168.13.13 [110/21] via 192.168.168.1, 00:02:36, FastEthernet0/0
10.0.0.0/32 is subnetted, 5 subnets
O IA 10.0.1.3 [110/148] via 192.168.168.1, 00:02:36, FastEthernet0/0
O IA 10.0.1.2 [110/148] via 192.168.168.1, 00:02:09, FastEthernet0/0
O IA 10.0.11.11 [110/149] via 192.168.168.1, 00:02:36, FastEthernet0/0
O IA 10.0.10.10 [110/149] via 192.168.168.1, 00:02:09, FastEthernet0/0
O IA 10.0.1.1 [110/84] via 192.168.168.1, 00:02:38, FastEthernet0/0
192.168.0.0/24 is variably subnetted, 4 subnets, 2 masks
O IA 192.168.0.12/30 [110/84] via 192.168.168.1, 00:02:38, FastEthernet0/0
O IA 192.168.0.0/29 [110/20] via 192.168.168.1, 00:02:48, FastEthernet0/0
O IA 192.168.0.16/30 [110/84] via 192.168.168.1, 00:02:38, FastEthernet0/0
O IA 192.168.0.20/30 [110/84] via 192.168.168.1, 00:02:38, FastEthernet0/0
192.168.168.0/30 is subnetted, 1 subnets
C 192.168.168.0 is directly connected, FastEthernet0/0
192.168.100.0/32 is subnetted, 1 subnets
O IA 192.168.100.100 [110/21] via 192.168.168.1, 00:02:40, FastEthernet0/0
C 192.168.169.0/24 is directly connected, Loopback169
192.168.3.0/30 is subnetted, 1 subnets
O IA 192.168.3.0 [110/94] via 192.168.168.1, 00:02:40, FastEthernet0/0
192.168.33.0/32 is subnetted, 1 subnets
O IA 192.168.33.33 [110/95] via 192.168.168.1, 00:02:40, FastEthernet0/0
O*IA 0.0.0.0/0 [110/11] via 192.168.168.1, 00:02:50, FastEthernet0/0
R13#show ip ospf database
OSPF Router with ID (13.13.13.13) (Process ID 1)
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
2.2.2.2 2.2.2.2 99 0x80000002 0x007082 1
13.13.13.13 13.13.13.13 98 0x80000003 0x002E9C 2
Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
192.168.168.2 13.13.13.13 98 0x80000001 0x003F7B
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 2.2.2.2 150 0x80000001 0x0075C0
10.0.1.1 2.2.2.2 84 0x80000001 0x00BA26
10.0.1.2 2.2.2.2 57 0x80000001 0x00336C
10.0.1.3 2.2.2.2 84 0x80000001 0x002975
10.0.10.10 2.2.2.2 57 0x80000001 0x008904
10.0.11.11 2.2.2.2 84 0x80000001 0x007417
192.168.0.0 2.2.2.2 145 0x80000001 0x00F3D6
192.168.0.12 2.2.2.2 88 0x80000001 0x001664
192.168.0.16 2.2.2.2 88 0x80000001 0x00ED88
192.168.0.20 2.2.2.2 88 0x80000001 0x00C5AC
192.168.3.0 2.2.2.2 88 0x80000001 0x00D1A7
192.168.13.13 2.2.2.2 88 0x80000001 0x001692
192.168.33.33 2.2.2.2 88 0x80000001 0x0057DE
192.168.100.100 2.2.2.2 88 0x80000001 0x00EB0E
After turning Area 1 into a totally stubby area:
R13#show ip route
...
Gateway of last resort is 192.168.168.1 to network 0.0.0.0
192.168.168.0/30 is subnetted, 1 subnets
C 192.168.168.0 is directly connected, FastEthernet0/0
C 192.168.169.0/24 is directly connected, Loopback169
O*IA 0.0.0.0/0 [110/11] via 192.168.168.1, 00:03:48, FastEthernet0/0
R13#show ip ospf database
OSPF Router with ID (13.13.13.13) (Process ID 1)
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
2.2.2.2 2.2.2.2 1154 0x80000002 0x007082 1
13.13.13.13 13.13.13.13 1153 0x80000003 0x002E9C 2
Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
192.168.168.2 13.13.13.13 1153 0x80000001 0x003F7B
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 2.2.2.2 253 0x80000002 0x0073C1
Note: The only remaining Summary LSA on R13 is for the default route injected by the ABR.
C. Not-so-stubby area - NSSA (Type 5 LSA: NO, Type 3 LSA: YES, Type 7 LSA: YES)
The not-so-stubby area (NSSA) option for stubby areas overcomes the restriction on external routes.
The solution itself is simple:
Because stubby areas can have no Type 5 LSAs, later OSPF RFCs defined a newer LSA type (Type 7) that serves the same purpose as the Type 5 LSA, but only for external routes in stubby areas.
So, an NSSA area can act just like a stub area, except that routers can inject external routes into the area.
General syntax: area area-id nssa (configured on each router in the area)
We have Area 3 as a NSSA area (with R12 as an ASBR between RIP and OSPF and R9 as an ABR to the rest of the OSPF domain):
R9#show run | s ospf
ip ospf network point-to-multipoint non-broadcast
router ospf 1
router-id 9.9.9.9
log-adjacency-changes
area 3 nssa
network 10.0.1.0 0.0.0.255 area 2
network 192.168.0.20 0.0.0.3 area 0
network 192.168.3.0 0.0.0.3 area 3
neighbor 10.0.1.3
neighbor 10.0.1.2
R12#show run | s ospf
router ospf 1
router-id 12.12.12.12
log-adjacency-changes
area 3 nssa
network 192.168.3.0 0.0.0.3 area 3
network 192.168.33.0 0.0.0.255 area 3
After configuring redistribution from RIP into the OSPF domain, ASBR R12 creates a Type 7 LSA for the RIP network and sends it into Area 3 NSSA.
Also, being a NSSA area (not a totally NSSA) Type 3 LSAs are permitted into the area.
R12#show ip ospf database
OSPF Router with ID (12.12.12.12) (Process ID 1)
Router Link States (Area 3)
Link ID ADV Router Age Seq# Checksum Link count
9.9.9.9 9.9.9.9 199 0x80000002 0x00AB50 1
12.12.12.12 12.12.12.12 31 0x80000003 0x00180E 2
Net Link States (Area 3)
Link ID ADV Router Age Seq# Checksum
192.168.3.2 12.12.12.12 198 0x80000001 0x004004
Summary Net Link States (Area 3)
Link ID ADV Router Age Seq# Checksum
10.0.1.1 9.9.9.9 179 0x80000001 0x00887E
10.0.1.2 9.9.9.9 147 0x80000001 0x0001C4
10.0.1.3 9.9.9.9 179 0x80000001 0x00F6CD
10.0.10.10 9.9.9.9 147 0x80000001 0x00575C
10.0.11.11 9.9.9.9 179 0x80000001 0x00426F
192.168.0.0 9.9.9.9 225 0x80000001 0x002B3B
192.168.0.12 9.9.9.9 225 0x80000001 0x00E837
192.168.0.16 9.9.9.9 229 0x80000001 0x00C05B
192.168.0.20 9.9.9.9 239 0x80000001 0x001642
192.168.13.13 9.9.9.9 192 0x80000001 0x004DF6
192.168.100.100 9.9.9.9 229 0x80000001 0x00BEE0
192.168.168.0 9.9.9.9 192 0x80000001 0x006847
192.168.169.170 9.9.9.9 192 0x80000001 0x00CE31
Type-7 AS External Link States (Area 3)
Link ID ADV Router Age Seq# Checksum Tag
192.168.22.0 12.12.12.12 34 0x80000001 0x00F347 0
Router R9, being an ABR, receives the Type 7 LSA, installs it in its LSDB, converts it into a Type 5 LSA (with itself as the advertising router) and sends is to other areas it is connected to:
R9#show ip ospf database
OSPF Router with ID (9.9.9.9) (Process ID 1)
...
Router Link States (Area 3)
Link ID ADV Router Age Seq# Checksum Link count
9.9.9.9 9.9.9.9 436 0x80000002 0x00AB50 1
12.12.12.12 12.12.12.12 270 0x80000003 0x00180E 2
Net Link States (Area 3)
Link ID ADV Router Age Seq# Checksum
192.168.3.2 12.12.12.12 437 0x80000001 0x004004
Summary Net Link States (Area 3)
Link ID ADV Router Age Seq# Checksum
10.0.1.1 9.9.9.9 416 0x80000001 0x00887E
10.0.1.2 9.9.9.9 385 0x80000001 0x0001C4
10.0.1.3 9.9.9.9 416 0x80000001 0x00F6CD
10.0.10.10 9.9.9.9 387 0x80000001 0x00575C
10.0.11.11 9.9.9.9 419 0x80000001 0x00426F
192.168.0.0 9.9.9.9 465 0x80000001 0x002B3B
192.168.0.12 9.9.9.9 465 0x80000001 0x00E837
192.168.0.16 9.9.9.9 465 0x80000001 0x00C05B
192.168.0.20 9.9.9.9 476 0x80000001 0x001642
192.168.13.13 9.9.9.9 429 0x80000001 0x004DF6
192.168.100.100 9.9.9.9 465 0x80000001 0x00BEE0
192.168.168.0 9.9.9.9 429 0x80000001 0x006847
192.168.169.170 9.9.9.9 429 0x80000001 0x00CE31
Type-7 AS External Link States (Area 3)
Link ID ADV Router Age Seq# Checksum Tag
192.168.22.0 12.12.12.12 272 0x80000001 0x00F347 0
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
192.168.22.0 9.9.9.9 266 0x80000001 0x00E26E 0
Router R9 installs the route to the RIP network in its routing table, marking it as "O N2", meaning OSPF NSSA external type 2:
R9#show ip route
...
O N2 192.168.22.0/24 [110/20] via 192.168.3.2, 00:09:53, FastEthernet0/0
Routers inside other (not stubby) areas receive the Type 5 LSA created by R9 for networks in the RIP domain:
R10#show ip ospf database
OSPF Router with ID (10.10.10.10) (Process ID 1)
...
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
192.168.22.0 9.9.9.9 729 0x80000001 0x00E26E 0
R3#show ip ospf database
OSPF Router with ID (3.3.3.3) (Process ID 1)
...
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
192.168.22.0 9.9.9.9 763 0x80000001 0x00E26E 0
D. Totally Not-so-stubby area - Totally NSSA (Type 5 LSA: NO, Type 3 LSA: NO, Type 7 LSA: YES)
Keeping the configuration in the above example, we add the "no-summary" keyword to the "area 3 nssa" command on ABR R9, making Area 3 Totally NSSA.
In addition to the previous example, Area 3 will not accept any Type 3 LSAs.
R9(config)#router ospf 1
R9(config-router)#area 3 nssa no-summary
R9#show run | s ospf
ip ospf network point-to-multipoint non-broadcast
router ospf 1
router-id 9.9.9.9
log-adjacency-changes
area 3 nssa no-summary
network 10.0.1.0 0.0.0.255 area 2
network 192.168.0.20 0.0.0.3 area 0
network 192.168.3.0 0.0.0.3 area 3
neighbor 10.0.1.3
neighbor 10.0.1.2
Verifying R9's and R12's OSPF LSDBs, we shouldn't find any Type 3 LSAs for Area 3 (except for the default route created by ABR and advertised to the ASBR):
R9#show ip ospf database
OSPF Router with ID (9.9.9.9) (Process ID 1)
...
Router Link States (Area 3)
Link ID ADV Router Age Seq# Checksum Link count
9.9.9.9 9.9.9.9 1318 0x80000002 0x00AB50 1
12.12.12.12 12.12.12.12 1152 0x80000003 0x00180E 2
Net Link States (Area 3)
Link ID ADV Router Age Seq# Checksum
192.168.3.2 12.12.12.12 1319 0x80000001 0x004004
Summary Net Link States (Area 3)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 9.9.9.9 95 0x80000001 0x002AE7
Type-7 AS External Link States (Area 3)
Link ID ADV Router Age Seq# Checksum Tag
192.168.22.0 12.12.12.12 1151 0x80000001 0x00F347 0
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
192.168.22.0 9.9.9.9 1149 0x80000001 0x00E26E 0
R12#show ip ospf database
OSPF Router with ID (12.12.12.12) (Process ID 1)
Router Link States (Area 3)
Link ID ADV Router Age Seq# Checksum Link count
9.9.9.9 9.9.9.9 1432 0x80000002 0x00AB50 1
12.12.12.12 12.12.12.12 1264 0x80000003 0x00180E 2
Net Link States (Area 3)
Link ID ADV Router Age Seq# Checksum
192.168.3.2 12.12.12.12 1431 0x80000001 0x004004
Summary Net Link States (Area 3)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 9.9.9.9 209 0x80000001 0x002AE7
Type-7 AS External Link States (Area 3)
Link ID ADV Router Age Seq# Checksum Tag
192.168.22.0 12.12.12.12 1263 0x80000001 0x00F347 0
No comments:
Post a Comment