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OSPF Stub and Totally Stubby Areas

Table of Contents

What a stub area is for

Multiple areas and the ABR showed that splitting a network into areas keeps Router-LSAs (type 1) inside their own area. AS External-LSAs (type 5), however, cross area boundaries and flood the whole AS. In a network that redistributes internet routes, those type 5 LSAs end up being most of the LSDB.

OSPF therefore offers a way to stop type 5 LSAs at the edge of an area. That is a stub area. External routes are no longer visible inside it; instead the area’s ABRs inject a default route that says “leave the area this way”.

TermDescription
Stub areaAn area that type 4 and type 5 LSAs do not enter. Its ABRs inject 0.0.0.0/0 as a type 3 LSA
Totally stubby areaA stub area that also blocks type 3. Only intra-area routes and the default route are left inside
Default route injectionAn ABR advertising 0.0.0.0/0 as a Summary-LSA (type 3). RFC 2328 calls it DefaultDestination
default-costThe metric of the default route an ABR injects. The IOS XR default is 1

This article turns one of the three areas of a seven-router lab into a stub area and then into a totally stubby area, to see on real devices what disappears and what appears, how much the LSDB shrinks, and what happens when one router is left unconfigured. NSSA (not-so-stubby area) is covered in OSPF NSSA and totally NSSA.

The area types side by side

TypeType 1 / 2Type 3Type 4 / 5Default route
Normal areayesyesyesNot injected automatically
Stub areayesyesnoThe ABRs inject 0.0.0.0/0
Totally stubby areayesnonoThe ABRs inject 0.0.0.0/0
NSSAyesyesno (externals arrive as type 7)Depends on the configuration

RFC 2328 does not define anything under the name “totally stubby”. It does, however, state in section 12.4.3.1 that an ABR connected to a stub area may choose to originate only a subset of the summary-LSAs, possibly under configuration control, so the behaviour itself is within the standard. The name and the no-summary keyword are Cisco’s.

The restrictions on stub areas

Section 3.6 of RFC 2328 lists three restrictions.

RestrictionContentWhere it is shown
Every router in the area must agree“The OSPF protocol ensures that all routers belonging to an area agree on whether the area has been configured as a stub.” An adjacency does not come up when the E bit differsSTEP 5
A stub area cannot hold an ASBRA router that brings in external routes cannot sit inside a stub areaSTEP 4
The backbone cannot be a stubArea 0 is what distributes routes between areasBelow
Virtual links cannot cross a stub area—Not tested here (covered in a separate article)

IOS XR rejects the backbone case at configuration time. Trying it on R2 (the ABR between area 0 and area 1) makes the commit fail with a semantic error, and the change is rolled back automatically.

R2, trying to make the backbone a stub area
RP/0/RP0/CPU0:R2#configure terminal
Sun Sep  6 23:21:12.360 UTC
RP/0/RP0/CPU0:R2(config)#router ospf 1
RP/0/RP0/CPU0:R2(config-ospf)#area 0
RP/0/RP0/CPU0:R2(config-ospf-ar)#stub
RP/0/RP0/CPU0:R2(config-ospf-ar)#commit
Sun Sep  6 23:21:13.708 UTC

% Failed to commit one or more configuration items during a pseudo-atomic operation. All changes made have been reverted. Please issue 'show configuration failed [inheritance]' from this session to view the errors
RP/0/RP0/CPU0:R2(config-ospf-ar)#show configuration failed
Sun Sep  6 23:21:19.713 UTC
!! SEMANTIC ERRORS: This configuration was rejected by 
!! the system due to semantic errors. The individual 
!! errors with each failed configuration command can be 
!! found below.


router ospf 1
 area 0
  stub
!!% 'OSPF' detected the 'warning' condition 'Error: Backbone area cannot be configured as a stub or NSSA'
 !
!
end

RP/0/RP0/CPU0:R2(config-ospf-ar)#abort

The message says stub or NSSA, so NSSA is equally impossible on the backbone. After the attempt R2 still has all three adjacencies in FULL and no stub in its configuration (both are recorded in the second half of the attached file).

Lab topology

The same seven-router, three-area lab as Multiple areas and the ABR is reused, and area 2 becomes the stub area. Two ABRs (R3 and R4) face area 2, so which of them the default route uses can be observed as well. Area 1 stays a normal area, which gives a side-by-side contrast: the same LSAs reach area 1 but not area 2.

ItemContent
Area 1R1’s Loopback0 and the R1 - R2 link. R1 is the ASBR and redistributes 192.168.1.0/24
Area 0The Loopback0 of R2, R3 and R4, plus R2 - R3 and R2 - R4
Area 2The Loopback0 of R11, R12 and R13, plus R3 - R11, R4 - R12, R11 - R13 and R12 - R13. This is the area that becomes a stub
ABRR2 (area 1 to 0), R3 and R4 (area 0 to 2)
Observation pointR13, an internal router in area 2
Link costCost 1 everywhere by default, except the R4 - R12 link at cost 10 on both ends (so that R3 is the closer ABR from R13)
Network typenetwork point-to-point on every link. No DR/BDR is elected and no Network-LSA (type 2) has to be generated

The normal-area starting point (STEP 0)

First, the state before anything is made a stub. R13’s routing table holds both inter-area routes (O IA) and an external route (O E2).

STEP 0 R13 show route ospf (normal area)
RP/0/RP0/CPU0:R13#show route ospf
Sun Sep  6 22:13:51.197 UTC

O IA 1.1.1.1/32 [110/5] via 10.1.11.11, 00:04:29, GigabitEthernet0/0/0/0
O IA 2.2.2.2/32 [110/4] via 10.1.11.11, 00:04:29, GigabitEthernet0/0/0/0
O IA 3.3.3.3/32 [110/3] via 10.1.11.11, 00:04:29, GigabitEthernet0/0/0/0
O IA 4.4.4.4/32 [110/5] via 10.1.11.11, 00:04:29, GigabitEthernet0/0/0/0
O IA 10.0.12.0/24 [110/4] via 10.1.11.11, 00:04:29, GigabitEthernet0/0/0/0
O IA 10.0.23.0/24 [110/3] via 10.1.11.11, 00:04:29, GigabitEthernet0/0/0/0
O IA 10.0.24.0/24 [110/4] via 10.1.11.11, 00:04:29, GigabitEthernet0/0/0/0
O    11.11.11.11/32 [110/2] via 10.1.11.11, 00:04:29, GigabitEthernet0/0/0/0
O    12.12.12.12/32 [110/2] via 10.1.12.12, 00:04:34, GigabitEthernet0/0/0/1
O    172.16.3.0/24 [110/2] via 10.1.11.11, 00:04:29, GigabitEthernet0/0/0/0
O    172.16.4.0/24 [110/11] via 10.1.12.12, 00:04:34, GigabitEthernet0/0/0/1
O E2 192.168.1.0/24 [110/20] via 10.1.11.11, 00:04:29, GigabitEthernet0/0/0/0

Four kinds of LSA are in the LSDB: 5 Router-LSAs, 14 Summary-LSAs (type 3), 2 ASBR Summary-LSAs (type 4) and 1 AS External-LSA (type 5), 22 in total.

STEP 0 R13 show ospf database (normal area, 22 LSAs)
RP/0/RP0/CPU0:R13#show ospf database
Sun Sep  6 22:13:54.402 UTC


            OSPF Router with ID (13.13.13.13) (Process ID 1)

		Router Link States (Area 2)

Link ID         ADV Router      Age         Seq#       Checksum Link count
3.3.3.3         3.3.3.3         302         0x80000002 0x00cc8d 2
4.4.4.4         4.4.4.4         290         0x80000002 0x0084b4 2
11.11.11.11     11.11.11.11     276         0x80000004 0x00ca9d 5
12.12.12.12     12.12.12.12     279         0x80000004 0x006ed1 5
13.13.13.13     13.13.13.13     275         0x80000004 0x0091ef 5

		Summary Net Link States (Area 2)

Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.1         3.3.3.3         275         0x80000001 0x001f0b
1.1.1.1         4.4.4.4         275         0x80000001 0x000125
2.2.2.2         3.3.3.3         310         0x80000001 0x00e640
2.2.2.2         4.4.4.4         302         0x80000001 0x00c85a
3.3.3.3         3.3.3.3         311         0x80000001 0x00ae75
3.3.3.3         4.4.4.4         302         0x80000001 0x00a479
4.4.4.4         3.3.3.3         297         0x80000001 0x009489
4.4.4.4         4.4.4.4         309         0x80000001 0x0062b9
10.0.12.0       3.3.3.3         310         0x80000001 0x003cdc
10.0.12.0       4.4.4.4         302         0x80000001 0x001ef6
10.0.23.0       3.3.3.3         311         0x80000001 0x00b856
10.0.23.0       4.4.4.4         302         0x80000001 0x00a465
10.0.24.0       3.3.3.3         310         0x80000001 0x00b755
10.0.24.0       4.4.4.4         309         0x80000001 0x008f7a

		Summary ASB Link States (Area 2)

Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.1         3.3.3.3         275         0x80000001 0x000723
1.1.1.1         4.4.4.4         275         0x80000001 0x00e83d

		Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
192.168.1.0     1.1.1.1         290         0x80000001 0x000d25 0

show ospf border-routers lists the ASBR (1.1.1.1) alongside the two ABRs. Area 2 can reach the ASBR because the ABRs originate type 4 LSAs for it.

STEP 0 R13 show ospf border-routers
RP/0/RP0/CPU0:R13#show ospf border-routers
Sun Sep  6 22:13:59.261 UTC

OSPF 1 Internal Routing Table

Codes: i - Intra-area route, I - Inter-area route

I 1.1.1.1 [4] via 10.1.11.11, GigabitEthernet0/0/0/0, ASBR , Area 2, SPF 7
i 3.3.3.3 [2] via 10.1.11.11, GigabitEthernet0/0/0/0, ABR , Area 2, SPF 7
i 4.4.4.4 [11] via 10.1.12.12, GigabitEthernet0/0/0/1, ABR , Area 2, SPF 7

Making area 2 a stub area (STEP 1)

The stub setting goes on every router attached to the area — here the two ABRs (R3, R4) and the three internal routers (R11, R12, R13).

The configuration applied in STEP 1
(applied to all five: R3, R4, R11, R12 and R13)
router ospf 1
 area 2
  stub

From R13 the external route 192.168.1.0/24 is gone and 0.0.0.0/0 has appeared in its place. Reachability to 192.168.1.1 is unchanged and so is the path the traceroute takes: the same destination is still reached, through a default route instead of a specific one.

STEP 1 R13 show route, ping and traceroute
RP/0/RP0/CPU0:R13#show route 0.0.0.0/0
Sun Sep  6 22:16:17.583 UTC

Routing entry for 0.0.0.0/0
  Known via "ospf 1", distance 110, metric 3, candidate default path, type inter area
  Installed Sep  6 22:15:58.890 for 00:00:18
  Routing Descriptor Blocks
    10.1.11.11, from 3.3.3.3, via GigabitEthernet0/0/0/0
      Route metric is 3
  No advertising protos. 
RP/0/RP0/CPU0:R13#ping 192.168.1.1 source 13.13.13.13
Sun Sep  6 22:16:17.827 UTC
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.1 timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/13/16 ms
RP/0/RP0/CPU0:R13#traceroute 192.168.1.1 source 13.13.13.13
Sun Sep  6 22:16:18.697 UTC

Type escape sequence to abort.
Tracing the route to 192.168.1.1

 1  10.1.11.11 7 msec  5 msec  5 msec 
 2  172.16.3.3 9 msec  7 msec  8 msec 
 3  10.0.23.2 12 msec  10 msec  10 msec 
 4  10.0.12.1 17 msec  *  18 msec

Looking at the injected default in the LSDB, both R3 (3.3.3.3) and R4 (4.4.4.4) advertise 0.0.0.0 with metric 1 — the IOS XR default value of default-cost.

STEP 1 R13 show ospf database summary 0.0.0.0
RP/0/RP0/CPU0:R13#show ospf database summary 0.0.0.0
Sun Sep  6 22:20:52.664 UTC


            OSPF Router with ID (13.13.13.13) (Process ID 1)

		Summary Net Link States (Area 2)

  Routing Bit Set on this LSA
  LS age: 365
  Options: (No TOS-capability, DC)
  LS Type: Summary Links (Network)
  Link State ID: 0.0.0.0 (Summary Network Number)
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0x57da
  Length: 28
  Network Mask: /0
	TOS: 0 	Metric: 1 

  LS age: 351
  Options: (No TOS-capability, DC)
  LS Type: Summary Links (Network)
  Link State ID: 0.0.0.0 (Summary Network Number)
  Advertising Router: 4.4.4.4
  LS Seq Number: 80000001
  Checksum: 0x39f4
  Length: 28
  Network Mask: /0
	TOS: 0 	Metric: 1

Type 4 and type 5 LSAs disappear from the database entirely.

STEP 1 R13 show ospf database asbr-summary (now empty)
RP/0/RP0/CPU0:R13#show ospf database asbr-summary
Sun Sep  6 22:19:00.396 UTC


            OSPF Router with ID (13.13.13.13) (Process ID 1)
STEP 1 R13 show ospf database external (now empty)
RP/0/RP0/CPU0:R13#show ospf database external
Sun Sep  6 22:19:00.862 UTC


            OSPF Router with ID (13.13.13.13) (Process ID 1)

With no type 4 LSA, the ASBR line also disappears from show ospf border-routers; only the two ABRs remain.

STEP 1 R13 show ospf border-routers (the ASBR line is gone)
RP/0/RP0/CPU0:R13#show ospf border-routers
Sun Sep  6 22:19:01.801 UTC

OSPF 1 Internal Routing Table

Codes: i - Intra-area route, I - Inter-area route

i 3.3.3.3 [2] via 10.1.11.11, GigabitEthernet0/0/0/0, ABR , Area 2, SPF 10
i 4.4.4.4 [11] via 10.1.12.12, GigabitEthernet0/0/0/1, ABR , Area 2, SPF 10

Changing the area type also changes the E bit in the Options field of the Hello packets. Listing the Options of the Hellos captured inside area 2 catches the moment it goes from 0x12 to 0x10: R13 is still sending 0x12 at No.11 while R11 has already moved to 0x10 at No.12, and during that window the two disagree. The E bit itself is covered in the OSPF Options field.

Options of the Hellos in the capture (tshark filtered on ospf.msg == 1)
1	10.1.11.13	0x12
2	10.1.11.11	0x12
3	10.1.11.13	0x12
4	10.1.11.11	0x12
5	10.1.11.13	0x12
6	10.1.11.11	0x12
7	10.1.11.13	0x12
8	10.1.11.11	0x12
9	10.1.11.13	0x12
10	10.1.11.11	0x12
11	10.1.11.13	0x12
12	10.1.11.11	0x10
13	10.1.11.13	0x12
14	10.1.11.11	0x10
15	10.1.11.13	0x12
16	10.1.11.11	0x10
17	10.1.11.13	0x10
18	10.1.11.11	0x10
40	10.1.11.13	0x10
41	10.1.11.11	0x10
42	10.1.11.13	0x10
43	10.1.11.11	0x10
44	10.1.11.13	0x10
45	10.1.11.11	0x10
46	10.1.11.13	0x10
47	10.1.11.11	0x10
48	10.1.11.13	0x10
49	10.1.11.11	0x10
50	10.1.11.13	0x10
51	10.1.11.11	0x10
52	10.1.11.13	0x10
53	10.1.11.11	0x10
54	10.1.11.13	0x10
55	10.1.11.11	0x10
56	10.1.11.13	0x10
57	10.1.11.11	0x10
58	10.1.11.13	0x10
59	10.1.11.11	0x10
60	10.1.11.13	0x10
61	10.1.11.11	0x10
62	10.1.11.13	0x10
63	10.1.11.11	0x10
64	10.1.11.13	0x10
65	10.1.11.11	0x10
66	10.1.11.13	0x10
67	10.1.11.11	0x10
68	10.1.11.13	0x10
69	10.1.11.11	0x10
70	10.1.11.13	0x10
71	10.1.11.11	0x10
72	10.1.11.13	0x10
73	10.1.11.11	0x10
74	10.1.11.13	0x10
75	10.1.11.11	0x10
76	10.1.11.13	0x10
77	10.1.11.11	0x10
78	10.1.11.13	0x10
79	10.1.11.11	0x10
80	10.1.11.13	0x10
81	10.1.11.11	0x10
82	10.1.11.13	0x10

Changing the cost of the default route (STEP 2)

When two ABRs advertise the default route, cost decides which one is used. Making R3’s default more expensive shows this.

The configuration applied in STEP 2
(R3)
router ospf 1
 area 2
  default-cost 50

The ABR’s own show ospf reports the cost of the default route it injects into the area.

STEP 2 R3 show ospf (the area 2 part)
RP/0/RP0/CPU0:R3#show ospf
Sun Sep  6 22:22:35.797 UTC

 Routing Process "ospf 1" with ID 3.3.3.3
 Role: Primary Active
 NSR (Non-stop routing) is Enabled
 Supports only single TOS(TOS0) routes
 Supports opaque LSA
 It is an area border router
 Maximum number of non self-generated LSA allowed 500000
    Current number of non self-generated LSA 25
    Threshold for warning message 75%
    Ignore-time 5 minutes, reset-time 10 minutes
    Ignore-count allowed 5, current ignore-count 0
 Maximum number of external prefixes per router 50000 (suppress-neighbor)
  Warning threshold 75%
 Maximum Exchange time 10 Hold time 5 Max Recovery count 300
 Router is not originating router-LSAs with maximum metric
 Initial SPF schedule delay 50 msecs
 Minimum hold time between two consecutive SPFs 200 msecs
 Maximum wait time between two consecutive SPFs 5000 msecs
 Initial LSA throttle delay 50 msecs
 Minimum hold time for LSA throttle 200 msecs
 Maximum wait time for LSA throttle 5000 msecs
 Minimum LSA interval 200 msecs. Minimum LSA arrival 100 msecs
 LSA refresh interval 1800 seconds
 Flood pacing interval 33 msecs. Retransmission pacing interval 66 msecs
 Adjacency stagger enabled; initial (per area): 2, maximum: 64
    Number of neighbors forming: 0, 2 full
 Maximum number of configured interfaces 1024
 Number of external LSA 1. Checksum Sum 0x00250d
 Number of opaque AS LSA 0. Checksum Sum 00000000
 Number of DCbitless external and opaque AS LSA 0
 Number of DoNotAge external and opaque AS LSA 0
 Number of areas in this router is 2. 1 normal 1 stub 0 nssa
 External flood list length 0
 SNMP trap is enabled
 LSD not connected, revision 0
 Segment Routing Global Block default (16000-23999), not allocated
 Segment Routing Local Block, unknown
 Segment Routing prefix-sid-map, Advertise local: Disabled, Receive remote: Disabled
 Strict-SPF capability is enabled
    Area BACKBONE(0)
	Number of interfaces in this area is 2
	SPF algorithm executed 9 times
	Number of LSA 20.  Checksum Sum 0x0bc1b4
	Number of opaque link LSA 0.  Checksum Sum 00000000
	Number of DCbitless LSA 0
	Number of indication LSA 0
	Number of DoNotAge LSA 0
	Flood list length 0
	Number of LFA enabled interfaces 0, LFA revision 0
	Number of Per Prefix LFA enabled interfaces 0
	Number of neighbors forming in staggered mode 0, 1 full
    Area 2
	Number of interfaces in this area is 1
	It is a stub area
	  generates stub default route with cost 50

R13’s default route moves from R3 (2 plus 50 = 52) to R4 (11 plus 1 = 12). The LSDB now holds R3’s advertisement with metric 50 next to R4’s with metric 1. Section 12.4.3.1 of RFC 2328 states that StubDefaultCost “need not be configured identically in all of the stub area’s area border routers”, and the two values indeed coexist.

STEP 2 R13 show route, show ospf database summary and traceroute
RP/0/RP0/CPU0:R13#show route 0.0.0.0/0
Sun Sep  6 22:21:31.240 UTC

Routing entry for 0.0.0.0/0
  Known via "ospf 1", distance 110, metric 12, candidate default path, type inter area
  Installed Sep  6 22:21:19.306 for 00:00:12
  Routing Descriptor Blocks
    10.1.12.12, from 4.4.4.4, via GigabitEthernet0/0/0/1
      Route metric is 12
  No advertising protos. 
RP/0/RP0/CPU0:R13#show ospf database summary 0.0.0.0
Sun Sep  6 22:21:31.437 UTC


            OSPF Router with ID (13.13.13.13) (Process ID 1)

		Summary Net Link States (Area 2)

  LS age: 14
  Options: (No TOS-capability, DC)
  LS Type: Summary Links (Network)
  Link State ID: 0.0.0.0 (Summary Network Number)
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000002
  Checksum: 0x41be
  Length: 28
  Network Mask: /0
	TOS: 0 	Metric: 50 

  Routing Bit Set on this LSA
  LS age: 390
  Options: (No TOS-capability, DC)
  LS Type: Summary Links (Network)
  Link State ID: 0.0.0.0 (Summary Network Number)
  Advertising Router: 4.4.4.4
  LS Seq Number: 80000001
  Checksum: 0x39f4
  Length: 28
  Network Mask: /0
	TOS: 0 	Metric: 1 

RP/0/RP0/CPU0:R13#traceroute 192.168.1.1 source 13.13.13.13
Sun Sep  6 22:21:32.154 UTC

Type escape sequence to abort.
Tracing the route to 192.168.1.1

 1  10.1.12.12 7 msec  4 msec  6 msec 
 2  172.16.4.4 10 msec  8 msec  10 msec 
 3  10.0.24.2 12 msec  10 msec  10 msec 
 4  10.0.12.1 14 msec  *  16 msec

No.5 of the attached capture is the LS Update that carried this into area 2: a single 0.0.0.0 Summary-LSA whose Advertising Router is R3 (3.3.3.3) and whose Metric is 50.

No.5 LS Update (a type 3 for 0.0.0.0, metric 50) tshark -V
Open Shortest Path First
    OSPF Header
        Version: 2
        Message Type: LS Update (4)
        Packet Length: 56
        Source OSPF Router: 11.11.11.11
        Area ID: 0.0.0.2
        Checksum: 0xff90 [correct]
        Instance ID: Base IPv4 Unicast Instance (0)
        Auth Type: Null (0)
        Auth Data (none): 0000000000000000
    LS Update Packet
        Number of LSAs: 1
        LSA-type 3 (Summary-LSA (IP network)), len 28
            .000 0000 0000 0010 = LS Age (seconds): 2
            0... .... .... .... = Do Not Age Flag: 0
            Options: 0x20, (DC) Demand Circuits
                0... .... = DN: Not set
                .0.. .... = (O) Opaque: Not set
                ..1. .... = (DC) Demand Circuits: Supported
                ...0 .... = (L) LLS Data block: Not Present
                .... 0... = (N) NSSA: Not supported
                .... .0.. = (MC) Multicast: Not capable
                .... ..0. = (E) External Routing: Not capable
                .... ...0 = (MT) Multi-Topology Routing: No
            LS Type: Summary-LSA (IP network) (3)
            Link State ID: 0.0.0.0
            Advertising Router: 3.3.3.3
            Sequence Number: 0x80000002
            Checksum: 0x41be
            Length: 28
            Netmask: 0.0.0.0
            TOS: 0
            Metric: 50
Download the pcap of the packet in the tshark output above (No.5, a type 3 LSU)

Making it a totally stubby area (STEP 3)

After putting default-cost back to its default, no-summary is added on the two ABRs only. The internal routers (R11, R12, R13) keep plain stub.

The configuration applied in STEP 3
(R3 and R4 only; on R3 the default-cost from STEP 2 is removed as well)
router ospf 1
 area 2
  no default-cost 50
  stub no-summary

The ABR’s show ospf now reads It is a stub area, no summary LSA in this area.

STEP 3 R3 show ospf (the area 2 part)
RP/0/RP0/CPU0:R3#show ospf
Sun Sep  6 22:27:01.248 UTC

 Routing Process "ospf 1" with ID 3.3.3.3
 Role: Primary Active
 NSR (Non-stop routing) is Enabled
 Supports only single TOS(TOS0) routes
 Supports opaque LSA
 It is an area border router
 Maximum number of non self-generated LSA allowed 500000
    Current number of non self-generated LSA 18
    Threshold for warning message 75%
    Ignore-time 5 minutes, reset-time 10 minutes
    Ignore-count allowed 5, current ignore-count 0
 Maximum number of external prefixes per router 50000 (suppress-neighbor)
  Warning threshold 75%
 Maximum Exchange time 10 Hold time 5 Max Recovery count 300
 Router is not originating router-LSAs with maximum metric
 Initial SPF schedule delay 50 msecs
 Minimum hold time between two consecutive SPFs 200 msecs
 Maximum wait time between two consecutive SPFs 5000 msecs
 Initial LSA throttle delay 50 msecs
 Minimum hold time for LSA throttle 200 msecs
 Maximum wait time for LSA throttle 5000 msecs
 Minimum LSA interval 200 msecs. Minimum LSA arrival 100 msecs
 LSA refresh interval 1800 seconds
 Flood pacing interval 33 msecs. Retransmission pacing interval 66 msecs
 Adjacency stagger enabled; initial (per area): 2, maximum: 64
    Number of neighbors forming: 0, 2 full
 Maximum number of configured interfaces 1024
 Number of external LSA 1. Checksum Sum 0x00250d
 Number of opaque AS LSA 0. Checksum Sum 00000000
 Number of DCbitless external and opaque AS LSA 0
 Number of DoNotAge external and opaque AS LSA 0
 Number of areas in this router is 2. 1 normal 1 stub 0 nssa
 External flood list length 0
 SNMP trap is enabled
 LSD not connected, revision 0
 Segment Routing Global Block default (16000-23999), not allocated
 Segment Routing Local Block, unknown
 Segment Routing prefix-sid-map, Advertise local: Disabled, Receive remote: Disabled
 Strict-SPF capability is enabled
    Area BACKBONE(0)
	Number of interfaces in this area is 2
	SPF algorithm executed 10 times
	Number of LSA 20.  Checksum Sum 0x0bc1b4
	Number of opaque link LSA 0.  Checksum Sum 00000000
	Number of DCbitless LSA 0
	Number of indication LSA 0
	Number of DoNotAge LSA 0
	Flood list length 0
	Number of LFA enabled interfaces 0, LFA revision 0
	Number of Per Prefix LFA enabled interfaces 0
	Number of neighbors forming in staggered mode 0, 1 full
    Area 2
	Number of interfaces in this area is 1
	It is a stub area, no summary LSA in this area
	  generates stub default route with cost 1

The inter-area routes (O IA) are gone from R13’s routing table, leaving only intra-area routes and the default route. The LSDB is down to 7 LSAs: five Router-LSAs and two type 3 LSAs for 0.0.0.0.

STEP 3 R13 show route ospf, show ospf database and traceroute
RP/0/RP0/CPU0:R13#show route ospf
Sun Sep  6 22:25:54.007 UTC

O*IA 0.0.0.0/0 [110/3] via 10.1.11.11, 00:00:18, GigabitEthernet0/0/0/0
O    11.11.11.11/32 [110/2] via 10.1.11.11, 00:09:55, GigabitEthernet0/0/0/0
O    12.12.12.12/32 [110/2] via 10.1.12.12, 00:09:55, GigabitEthernet0/0/0/1
O    172.16.3.0/24 [110/2] via 10.1.11.11, 00:09:55, GigabitEthernet0/0/0/0
O    172.16.4.0/24 [110/11] via 10.1.12.12, 00:09:55, GigabitEthernet0/0/0/1
RP/0/RP0/CPU0:R13#show route 0.0.0.0/0
Sun Sep  6 22:25:54.193 UTC

Routing entry for 0.0.0.0/0
  Known via "ospf 1", distance 110, metric 3, candidate default path, type inter area
  Installed Sep  6 22:25:35.835 for 00:00:18
  Routing Descriptor Blocks
    10.1.11.11, from 3.3.3.3, via GigabitEthernet0/0/0/0
      Route metric is 3
  No advertising protos. 
RP/0/RP0/CPU0:R13#show ospf database
Sun Sep  6 22:25:54.352 UTC


            OSPF Router with ID (13.13.13.13) (Process ID 1)

		Router Link States (Area 2)

Link ID         ADV Router      Age         Seq#       Checksum Link count
3.3.3.3         3.3.3.3         25          0x80000006 0x00e275 2
4.4.4.4         4.4.4.4         14          0x80000005 0x00e262 1
11.11.11.11     11.11.11.11     25          0x80000009 0x00de86 5
12.12.12.12     12.12.12.12     4           0x80000009 0x0082ba 5
13.13.13.13     13.13.13.13     599         0x80000006 0x00abd5 5

		Summary Net Link States (Area 2)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         3.3.3.3         30          0x80000003 0x0053dc
0.0.0.0         4.4.4.4         14          0x80000002 0x0037f5
RP/0/RP0/CPU0:R13#traceroute 192.168.1.1 source 13.13.13.13
Sun Sep  6 22:25:54.991 UTC

Type escape sequence to abort.
Tracing the route to 192.168.1.1

 1  10.1.11.11 6 msec  4 msec  27 msec 
 2  172.16.3.3 8 msec  7 msec  7 msec 
 3  10.0.23.2 10 msec  10 msec  10 msec 
 4  10.0.12.1 45 msec  *  15 msec

The moment no-summary is applied, each ABR withdraws the type 3 LSAs it had been injecting by reflooding them with LS Age 3600 (MaxAge). No.4 of the attached capture is R3 pulling eight type 3 LSAs at once.

The LS Updates in the capture (tshark filtered on ospf.msg == 4)
    4  11.539631   10.1.11.11 → 224.0.0.5    OSPF 286 LS Update
    7  15.274140   10.1.11.11 → 224.0.0.5    OSPF 134 LS Update
    8  15.814386   10.1.11.11 → 224.0.0.5    OSPF 126 LS Update
    9  15.865280   10.1.11.11 → 224.0.0.5    OSPF 146 LS Update
   10  15.898546   10.1.11.11 → 224.0.0.5    OSPF 110 LS Update
   13  20.801426   10.1.11.11 → 10.1.11.13   OSPF 110 LS Update
   16  26.958238   10.1.11.13 → 224.0.0.5    OSPF 286 LS Update
   20  33.575569   10.1.11.13 → 224.0.0.5    OSPF 134 LS Update
   22  35.879174   10.1.11.13 → 224.0.0.5    OSPF 126 LS Update
   23  35.929703   10.1.11.13 → 224.0.0.5    OSPF 146 LS Update
   26  40.752365   10.1.11.13 → 224.0.0.5    OSPF 110 LS Update
Download the pcap of No.4 above (the LSU that withdraws eight type 3 LSAs)

The striking thing about a totally stubby area is that destinations that are not in the routing table are still reachable. R13 has no route to 1.1.1.1/32 (R1’s Loopback0), yet the ping succeeds, because the default route absorbs everything.

Reaching an inter-area destination from inside a totally stubby area (R13)
RP/0/RP0/CPU0:R13#show route 1.1.1.1/32
Sun Sep  6 23:09:13.422 UTC

% Network not in table

RP/0/RP0/CPU0:R13#ping 1.1.1.1 source 13.13.13.13
Sun Sep  6 23:09:13.686 UTC
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1 timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 11/13/16 ms
RP/0/RP0/CPU0:R13#traceroute 1.1.1.1 source 13.13.13.13
Sun Sep  6 23:09:14.518 UTC

Type escape sequence to abort.
Tracing the route to 1.1.1.1

 1  10.1.11.11 7 msec  5 msec  5 msec 
 2  172.16.3.3 10 msec  10 msec  8 msec 
 3  10.0.23.2 12 msec  10 msec  10 msec 
 4  10.0.12.1 16 msec  *  17 msec

How much the LSDB shrinks

Counting the LSAs of area 2 as R13 sees them, in the three states:

StateRouter (type 1)Summary (type 3)ASBR summary (type 4)External (type 5)TotalOSPF routes
Normal area (STEP 0)514212212
Stub area (STEP 1)516 (14 plus 2 defaults)002112
Totally stubby (STEP 3)52 (the defaults only)0075

Making an area a stub barely reduces the LSA count on its own. This lab has just one external route, so the three type 4 and type 5 LSAs go away while two default routes arrive — a net saving of one. In a real network with tens of thousands of external routes that is exactly where the win is, but when the goal is to lighten the routers inside the area, a totally stubby area shows it far more clearly: 22 LSAs become 7.

Trying to redistribute inside a stub area (STEP 4)

Section 3.6 of RFC 2328 states that AS boundary routers cannot be placed internal to stub areas. So what happens if the configuration is entered anyway? R13 redistributes a static route.

The configuration applied in STEP 4
(R13)
router static
 address-family ipv4 unicast
  192.168.13.0/24 Null0
!
router ospf 1
 redistribute static

The configuration itself is accepted without an error. show ospf even reports that static routes are being redistributed and that one prefix has been redistributed. Yet show ospf database external stays empty: not one type 5 LSA is created.

STEP 4 R13 show ospf database external and show ospf
RP/0/RP0/CPU0:R13#show ospf database external
Sun Sep  6 22:30:29.465 UTC


            OSPF Router with ID (13.13.13.13) (Process ID 1)
RP/0/RP0/CPU0:R13#show ospf
Sun Sep  6 22:30:30.091 UTC

 Routing Process "ospf 1" with ID 13.13.13.13
 Role: Primary Active
 NSR (Non-stop routing) is Enabled
 Supports only single TOS(TOS0) routes
 Supports opaque LSA
 It is an autonomous system boundary router
 Maximum number of non self-generated LSA allowed 500000
    Current number of non self-generated LSA 6
    Threshold for warning message 75%
    Ignore-time 5 minutes, reset-time 10 minutes
    Ignore-count allowed 5, current ignore-count 0
 Redistributing External Routes from,
    static
 Maximum number of redistributed prefixes 10240
    Threshold for warning message 75%
    Current number of redistributed prefixes 1
 Maximum number of external prefixes per router 50000 (suppress-neighbor)
  Warning threshold 75%
 Maximum Exchange time 10 Hold time 5 Max Recovery count 300
 Router is not originating router-LSAs with maximum metric
 Initial SPF schedule delay 50 msecs
 Minimum hold time between two consecutive SPFs 200 msecs
 Maximum wait time between two consecutive SPFs 5000 msecs
 Initial LSA throttle delay 50 msecs
 Minimum hold time for LSA throttle 200 msecs
 Maximum wait time for LSA throttle 5000 msecs
 Minimum LSA interval 200 msecs. Minimum LSA arrival 100 msecs
 LSA refresh interval 1800 seconds
 Flood pacing interval 33 msecs. Retransmission pacing interval 66 msecs
 Adjacency stagger enabled; initial (per area): 2, maximum: 64
    Number of neighbors forming: 0, 2 full
 Maximum number of configured interfaces 1024
 Number of external LSA 0. Checksum Sum 00000000
 Number of opaque AS LSA 0. Checksum Sum 00000000
 Number of DCbitless external and opaque AS LSA 0
 Number of DoNotAge external and opaque AS LSA 0
 Number of areas in this router is 1. 0 normal 1 stub 0 nssa
 External flood list length 0
 SNMP trap is enabled
 LSD not connected, revision 0
 Segment Routing Global Block default (16000-23999), not allocated
 Segment Routing Local Block, unknown
 Segment Routing prefix-sid-map, Advertise local: Disabled, Receive remote: Disabled
 Strict-SPF capability is enabled
    Area 2
	Number of interfaces in this area is 3
	It is a stub area
	SPF algorithm executed 17 times
	Number of LSA 7.  Checksum Sum 0x04fb11
	Number of opaque link LSA 0.  Checksum Sum 00000000
	Number of DCbitless LSA 0
	Number of indication LSA 0
	Number of DoNotAge LSA 0
	Flood list length 0
	Number of LFA enabled interfaces 0, LFA revision 0
	Number of Per Prefix LFA enabled interfaces 0
	Number of neighbors forming in staggered mode 0, 2 full

192.168.13.0/24 never appears on the neighbouring R11 either. The RFC restriction shows up as a configuration that commits but produces no route. To bring external routes into an area like this, use an NSSA.

Reproducing a missed configuration (STEP 5)

Every router in the area has to agree on the stub setting. Removing stub from R13 alone reproduces the classic operational mistake.

The configuration applied in STEP 5
(R13)
router ospf 1
 area 2
  no stub

Both of R13’s adjacencies go DOWN and every OSPF route disappears. The Nbrs F/C column of show ospf interface brief reads 0/1 — one neighbor configured, none Full.

STEP 5 R13 show ospf neighbor, show route ospf and show ospf interface brief
RP/0/RP0/CPU0:R13#show ospf neighbor
Sun Sep  6 22:34:03.132 UTC

* Indicates MADJ interface
# Indicates Neighbor awaiting BFD session up

Neighbors for OSPF 1

Neighbor ID     Pri   State           Dead Time   Address         Interface
11.11.11.11     1     DOWN/  -           -        10.1.11.11      GigabitEthernet0/0/0/0
12.12.12.12     1     DOWN/  -           -        10.1.12.12      GigabitEthernet0/0/0/1

Total neighbor count: 2
RP/0/RP0/CPU0:R13#show route ospf
Sun Sep  6 22:34:04.046 UTC

% No matching routes found

RP/0/RP0/CPU0:R13#show ospf interface brief
Sun Sep  6 22:34:04.165 UTC

* Indicates MADJ interface, (P) Indicates fast detect hold down state

Interfaces for OSPF 1

Interface          PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Lo0                1     2               13.13.13.13/32     1     LOOP  0/0
Gi0/0/0/0          1     2               10.1.11.13/24      1     P2P   0/1
Gi0/0/0/1          1     2               10.1.12.13/24      1     P2P   0/1

The syslog on the neighbouring R11 records the adjacency going down on the dead timer: the Hellos still arrive, but the mismatched E bit makes each side ignore them.

STEP 5 R11 show logging (this STEP only)
RP/0/RP0/CPU0:R11#show logging start Sep 6 22:32:47
Sun Sep  6 22:36:09.027 UTC
Time Zone UTC, DST disabled
Syslog logging: enabled (0 messages dropped, 0 flushes, 0 overruns)
    Console logging: Disabled
    Monitor logging: level debugging, 0 messages logged
    Trap logging: level informational, 0 messages logged
    Buffer logging: level debugging, 146 messages logged

Log Buffer (2097152 bytes):

RP/0/RP0/CPU0:Sep  6 22:32:48.091 UTC: logger[66500]: %OS-SYSLOG-6-LOG_INFO : informational STEP5-BEGIN 
RP/0/RP0/CPU0:Sep  6 22:32:48.410 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[9109]: Received disconnect from 10.100.3.1 port 51054:11: disconnected by user 
RP/0/RP0/CPU0:Sep  6 22:32:48.410 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[9109]: Disconnected from user cisco 10.100.3.1 port 51054 
RP/0/RP0/CPU0:Sep  6 22:34:22.025 UTC: ospf[1035]: %ROUTING-OSPF-5-ADJCHG : Process 1, Nbr 13.13.13.13 on GigabitEthernet0/0/0/1 in area 2 from FULL to DOWN, Neighbor Down: dead timer expired, vrf default vrfid 0x60000000 
RP/0/RP0/CPU0:Sep  6 22:35:53.406 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[9408]: Accepted authentication/pam for cisco from 10.100.3.1 port 51172 ssh2 
RP/0/RP0/CPU0:Sep  6 22:36:06.688 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[9413]: Received disconnect from 10.100.3.1 port 51172:11: disconnected by user 
RP/0/RP0/CPU0:Sep  6 22:36:06.688 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[9413]: Disconnected from user cisco 10.100.3.1 port 51172 
RP/0/RP0/CPU0:Sep  6 22:36:07.539 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[9614]: Accepted authentication/pam for cisco from 10.100.3.1 port 51177 ssh2

Restoring the setting (STEP 6)

Putting stub back on R13 re-establishes the adjacencies and brings the default route back.

STEP 6 R13 show ospf neighbor, show route ospf and ping (after the restore)
RP/0/RP0/CPU0:R13#show ospf neighbor
Sun Sep  6 22:38:36.429 UTC

* Indicates MADJ interface
# Indicates Neighbor awaiting BFD session up

Neighbors for OSPF 1

Neighbor ID     Pri   State           Dead Time   Address         Interface
11.11.11.11     1     FULL/  -        00:00:31    10.1.11.11      GigabitEthernet0/0/0/0
    Neighbor is up for 00:01:14
12.12.12.12     1     FULL/  -        00:00:38    10.1.12.12      GigabitEthernet0/0/0/1
    Neighbor is up for 00:01:08

Total neighbor count: 2
RP/0/RP0/CPU0:R13#show route ospf
Sun Sep  6 22:38:36.940 UTC

O*IA 0.0.0.0/0 [110/3] via 10.1.11.11, 00:01:14, GigabitEthernet0/0/0/0
O    11.11.11.11/32 [110/2] via 10.1.11.11, 00:01:14, GigabitEthernet0/0/0/0
O    12.12.12.12/32 [110/2] via 10.1.12.12, 00:01:08, GigabitEthernet0/0/0/1
O    172.16.3.0/24 [110/2] via 10.1.11.11, 00:01:14, GigabitEthernet0/0/0/0
O    172.16.4.0/24 [110/11] via 10.1.12.12, 00:01:08, GigabitEthernet0/0/0/1
RP/0/RP0/CPU0:R13#ping 192.168.1.1 source 13.13.13.13
Sun Sep  6 22:38:37.127 UTC
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.1 timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/12/15 ms

The IOS XR commands

CommandWhere it goesMeaning
stub under area <ID>Every router attached to the areaMakes the area a stub. Missing it on one router breaks the adjacency
stub no-summary under area <ID>The ABRs onlyBlocks type 3 as well (totally stubby). Internal routers keep plain stub
default-cost <value> under area <ID>The ABRs onlyThe metric of the injected default route. The default is 1, and the ABRs may use different values
Example (ABR R3: a totally stubby area with a non-default cost)
router ospf 1
 area 2
  stub no-summary
  default-cost 50
 !
!

On IOS XE the equivalents are area <ID> stub, area <ID> stub no-summary and area <ID> default-cost <value> under router ospf <process>. Configuration examples are covered in a separate article.

Verification config and show output

For each STEP the following three files were collected from all seven routers, split per router. The verification config is the ..._run.txt file (the final state is the one from STEP 6).

FileContent
..._show.txtshow version / show interface description / show route / show route ospf / show ospf / show ospf interface / show ospf interface brief / show ospf neighbor / show ospf neighbor detail / show ospf database / show ospf database router / show ospf database network / show ospf statistics interface / show ospf database summary / show ospf database asbr-summary / show ospf database external / show ospf border-routers / show ospf routes / show route 0.0.0.0/0 / show route 1.1.1.1/32 / show route 192.168.1.0/24 / show ospf trace events
..._log.txtshow logging narrowed to that STEP. A marker was written with logmsg at the start of each STEP and its timestamp passed to show logging start
..._run.txtshow running-config at that point in time (that is, the verification config for that STEP)

The final state has area 2 as a totally stubby area (no-summary on the ABRs, stub on the internal routers).

STEP 0: a normal area (before anything is made a stub) — both O IA and O E2 are present and there are 22 LSAs

Routershow outputsyslogrunning-config
R1showlogrun
R2showlogrun
R3showlogrun
R4showlogrun
R11showlogrun
R12showlogrun
R13showlogrun

This STEP also has a ping and a traceroute taken from R13.

STEP 1: stub on the five routers of area 2 — type 4 and type 5 disappear and 0.0.0.0/0 arrives as a type 3 (21 LSAs)

Routershow outputsyslogrunning-config
R1showlogrun
R2showlogrun
R3showlogrun
R4showlogrun
R11showlogrun
R12showlogrun
R13showlogrun

This STEP also has show route, ping and traceroute and show ospf database summary 0.0.0.0 taken from R13.

STEP 2: default-cost 50 on R3 — the default route moves to the path via R4 (cost 12)

Routershow outputsyslogrunning-config
R1showlogrun
R2showlogrun
R3showlogrun
R4showlogrun
R11showlogrun
R12showlogrun
R13showlogrun

This STEP also has show route, show ospf database summary 0.0.0.0 and traceroute taken from R13.

STEP 3: stub no-summary on the two ABRs (totally stubby) — type 3 disappears too, leaving 7 LSAs and 5 OSPF routes

Routershow outputsyslogrunning-config
R1showlogrun
R2showlogrun
R3showlogrun
R4showlogrun
R11showlogrun
R12showlogrun
R13showlogrun

This STEP also has show route ospf, show ospf database and traceroute and show ospf database summary 0.0.0.0 taken from R13.

STEP 4: R13 redistributes a static route — the configuration commits but no type 5 LSA is generated

Routershow outputsyslogrunning-config
R1showlogrun
R2showlogrun
R3showlogrun
R4showlogrun
R11showlogrun
R12showlogrun
R13showlogrun

This STEP also has show ospf database external and show ospf taken from R13.

STEP 5: stub removed from R13 alone (a missed configuration) — the E bit mismatch takes the adjacencies down and every OSPF route with them

Routershow outputsyslogrunning-config
R1showlogrun
R2showlogrun
R3showlogrun
R4showlogrun
R11showlogrun
R12showlogrun
R13showlogrun

This STEP also has show ospf neighbor, show route ospf and show ospf interface brief taken from R13.

STEP 6: stub put back on R13 (final state) — the adjacencies and the default route come back

Routershow outputsyslogrunning-config
R1showlogrun
R2showlogrun
R3showlogrun
R4showlogrun
R11showlogrun
R12showlogrun
R13showlogrun

This STEP also has show ospf neighbor, show route ospf and ping and show route 1.1.1.1/32, ping and traceroute taken from R13.

The attempt to make the backbone a stub area is recorded in R2’s terminal log.

The captures were taken inside area 2 (between R11 and R13) in three runs, all filtered with ip proto 89.

Download the capture around the switch to a stub area (STEP 1)

Download the capture of the default-cost change (STEP 2)

Download the capture of the switch to a totally stubby area (STEP 3)

References

RFCTitleSummary
RFC 2328OSPF Version 2The definition of stub areas and their restrictions (section 3.6); originating summary-LSAs into a stub area, the default route, StubDefaultCost and the rule that type 4 LSAs are never originated into a stub area (section 12.4.3.1).

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