OSPF NSSA External-LSA (Type 7)

What an NSSA external-LSA is

An NSSA external-LSA (type 7) is the LSA an ASBR inside an NSSA uses to carry external routes. An NSSA refuses type 5 LSAs, so an ASBR inside it originates a type 7 and the ABR at the exit rebuilds it as a type 5.

Where an AS external-LSA (type 5) is never rebuilt at an area boundary, a type 7 always is. This article works out on real hardware what that rebuild rewrites and what it carries over.

How it differs from a type 5

Section 2.3 of RFC 3101 lists six points under “Type-7 LSAs are identical to Type-5 LSAs except for the following”. The layout is the same; the handling differs.

#	Difference
1	The LS type in the header is 7
2	It is flooded only within the originating NSSA (the same scope as types 1 and 2)
3	It lives in the area data structure; a type 5 lives in the AS data structure
4	The NSSA border routers choose which type 7 LSAs become type 5 LSAs
5	The header options carry a P (propagate) bit
6	A type 7 that is to be translated must have a non-zero forwarding address

Points 4 and 6 are the heart of this article. The NSSA area type itself (the N bit, electing the translator, totally NSSA) is explained in OSPF NSSA and totally NSSA.

Packet format

A 20-byte header plus a 16-byte body: 36 bytes, the same as a type 5 (RFC 3101 Appendix C, which refers to RFC 2328 Appendix A.4.5 for the field definitions).

Field	Size	Content
Network Mask	4 bytes	The mask of the external destination
E	1 bit	0 means E1, 1 means E2. E2 is the default
metric	3 bytes	The external cost
Forwarding Address	4 bytes	Mandatory (non-zero) on a type 7 with the P bit set. A 0.0.0.0 is never translated
External Route Tag	4 bytes	An arbitrary 32-bit value

Test topology

Area 2 is the NSSA, with R13 and R11 as ASBRs inside it. R3 and R4 are the ABRs and R4, with the higher router ID, is the translator. R1 in area 1 is an ordinary ASBR whose plain type 5 serves as a control. R11’s 192.168.11.0/24 is left alone through every STEP.

The tests at a glance

The verification is split across two labs. The first is the topology above, ten STEPs through the properties of a type 7. The second is a dedicated setup for the rule that the less preferred of two equivalent type 7 LSAs is withdrawn (RFC 3101 Section 2.4); it needs two routers to agree on a forwarding address, so it was built separately (see “Breaking the three conditions one at a time”).

Lab 1 (the three-area topology above, STEP 0-9)

STEP	Change	What it confirms	Which property of type 7
0	Right after boot	A type 7 and its translated type 5 read side by side	The fields translation carries over, the forwarding address, the P bit
1	R11 redistributes the same `192.168.13.0/24` as R13	Two inside the NSSA, one in area 0, and not even an LS update	The translator only translates the one it installed
2	R11’s redistribution metric set to 5	The translated type 5 now carries R11’s contents	When the winner changes, the same single type 5 is rewritten
3	Metric restored	Back to R13’s contents	Same as above
4	R2 redistributes the same `192.168.1.0/24` as R1 (type 5)	Both type 5 LSAs stay in area 0	The contrast with type 5, which is never rebuilt and so survives in as many copies as were originated
5	R2 stops redistributing	Back to one	Same as above
6	R13’s Loopback0 removed from area 2	The forwarding address is re-selected as `10.1.11.13`	How the forwarding address is chosen and re-chosen (Section 2.3)
7	Loopback0 restored	Back to `13.13.13.13`	Same as above
8	R13’s `192.168.13.0/24` deleted	Both the type 7 and the translated type 5 are withdrawn; 165 ms to propagate	How withdrawal works and the delay translation adds
9	Route restored	Both restart at `0x80000001`	A withdrawn LSA comes back as a new one

Lab 2 (the setup with matching forwarding addresses, STEP 0-7)

STEP	Change	What it confirms	Which property of type 7
0	Only RB redistributes `192.168.99.0/24 10.2.0.254`	The forwarding address is the next hop `10.2.0.254`	A next hop inside the NSSA is copied into the forwarding address (Section 2.3); R2 is the translator, not R3 with the highest router ID
1	RA redistributes it with the same next hop	Destination, cost and forwarding address all match. RA originates nothing	The less preferred of two equivalent type 7 LSAs is not advertised (Section 2.4)
2	RA’s redistribution metric set to 30	Only the cost differs. Both stay	“Equivalent” includes the cost
3	Metric restored	Equivalent again. RA withdraws with MaxAge	The “flush its LSA” of Section 2.4
4	RA’s next hop changed to `10.2.0.13`	Only the forwarding address differs. Both stay	“Equivalent” includes the forwarding address
5	Next hop restored	Withdrawn again	Same as above
6	`nssa default-information-originate` on R2	The ABR’s default type 7 appears with the P bit clear and FA `0.0.0.0`	A type 7 with the P bit clear, and what its forwarding address is
7	Everything restored	Back to the STEP 0 state

The sections that follow are organised by property of the type 7, not in STEP order.

The same route in two guises

Inside the NSSA first.

R13 STEP 0: the type 7 LSAs inside the NSSA
RP/0/RP0/CPU0:R13#show ospf database nssa-external
Wed Sep  9 08:19:53.262 UTC


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

		Type-7 AS External Link States (Area 2)

  Routing Bit Set on this LSA
  LS age: 398
  Options: (No TOS-capability, Type 7/5 translation, DC)
  LS Type: AS External Link
  Link State ID: 192.168.11.0 (External Network Number)
  Advertising Router: 11.11.11.11
  LS Seq Number: 80000002
  Checksum: 0x3c5
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 11.11.11.11
	External Route Tag: 0

  LS age: 380
  Options: (No TOS-capability, Type 7/5 translation, DC)
  LS Type: AS External Link
  Link State ID: 192.168.13.0 (External Network Number)
  Advertising Router: 13.13.13.13
  LS Seq Number: 80000002
  Checksum: 0x15a1
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 13.13.13.13
	External Route Tag: 0

Two type 7 LSAs, from R11 and R13, each with its own loopback as the Forward Address and Type 7/5 translation in Options (how the CLI renders the P bit). Area 0 at the same moment:

R4 STEP 0: the type 5 LSAs in area 0
RP/0/RP0/CPU0:R4#show ospf database external
Wed Sep  9 08:18:43.539 UTC


            OSPF Router with ID (4.4.4.4) (Process ID 1)

		Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 316
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 192.168.1.0 (External Network Number)
  Advertising Router: 1.1.1.1
  LS Seq Number: 80000001
  Checksum: 0xd25
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 0.0.0.0
	External Route Tag: 0

  LS age: 258
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 192.168.11.0 (External Network Number)
  Advertising Router: 4.4.4.4
  LS Seq Number: 80000001
  Checksum: 0x6c83
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 11.11.11.11
	External Route Tag: 0

  LS age: 296
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 192.168.13.0 (External Network Number)
  Advertising Router: 4.4.4.4
  LS Seq Number: 80000001
  Checksum: 0xba2b
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 13.13.13.13
	External Route Tag: 0

Of the three type 5 LSAs, the first differs in kind from the other two.

Link State ID	Advertising Router	Forward Address	Where it came from
192.168.1.0	1.1.1.1	`0.0.0.0`	A plain type 5 originated by R1
192.168.11.0	4.4.4.4	`11.11.11.11`	R11’s type 7, translated by R4
192.168.13.0	4.4.4.4	`13.13.13.13`	R13’s type 7, translated by R4

The bottom two have R4, the translating ABR, as advertising router, and the originating ASBR’s loopback survives in the forwarding address, as Section 2.3 of RFC 3101 says:

Type-5 LSAs that are translations of Type-7 LSAs copy the Type-7 LSAs’ non-zero forwarding addresses.

A zero forwarding address tells you the LSA is a native type 5; a non-zero one tells you it was translated.

Which router is the translator

show ospf names it in the area stanza.

R4 STEP 0: the area 2 stanza of show ospf
    Area 2
	Number of interfaces in this area is 1
	It is a NSSA area
        Perform type-7/type-5 LSA translation
	SPF algorithm executed 10 times

R3, the other ABR:

R3 STEP 0: the same stanza has no translation line
    Area 2
	Number of interfaces in this area is 1
	It is a NSSA area
	SPF algorithm executed 8 times

Perform type-7/type-5 LSA translation appears only on R4, because in the election of RFC 3101 Section 3.1 the NSSA border router with the highest router ID wins (an ABR configured to always translate takes precedence).

When two routers advertise the same route

Advertise one destination from two ASBRs, and type 7 and type 5 look different from the outside.

With a type 5, two LSAs with different advertising routers sit in the LSDB and a receiver picks one under RFC 2328 Section 16.4.1. With a type 7, the translating ABR follows Section 3.2 of RFC 3101 and translates only the type 7 it has installed, so exactly one type 5 leaves the NSSA; change the winner and that single LSA’s contents are swapped.

Section 2.4 adds a rule that clears the duplicate inside the NSSA as well:

If two NSSA routers, both reachable from one another over the NSSA, originate functionally equivalent Type-7 LSAs (i.e., same destination, cost and non-zero forwarding address), then the router having the least preferred LSA should flush its LSA.

The less preferred one should withdraw its own, preference going to the P bit and then to the higher router ID. “Equivalent” means the destination, the cost and the non-zero forwarding address all match; the same phrase in Section 3.2 (several translators translating one type 7, whose forwarding addresses necessarily coincide) shows that same governs all three.

Separate ASBRs normally put their own address in the forwarding address, so the rule only bites where several ASBRs on one segment point at the same external gateway. This lab does not meet that condition, so both stay; a second lab that does meet it comes later.

Two inside the NSSA, one outside

Have R11 redistribute the same 192.168.13.0/24 as R13 (STEP 1).

R11 STEP 1: redistribute the same route as R13
router static
 address-family ipv4 unicast
  192.168.13.0/24 Null0

Inside the NSSA there are two type 7 LSAs.

R11 STEP 1: two LSAs for the same 192.168.13.0
RP/0/RP0/CPU0:R11#show ospf database nssa-external
Wed Sep  9 08:24:31.558 UTC


            OSPF Router with ID (11.11.11.11) (Process ID 1)

		Type-7 AS External Link States (Area 2)

  LS age: 676
  Options: (No TOS-capability, Type 7/5 translation, DC)
  LS Type: AS External Link
  Link State ID: 192.168.11.0 (External Network Number)
  Advertising Router: 11.11.11.11
  LS Seq Number: 80000002
  Checksum: 0x3c5
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 11.11.11.11
	External Route Tag: 0

  LS age: 249
  Options: (No TOS-capability, Type 7/5 translation, DC)
  LS Type: AS External Link
  Link State ID: 192.168.13.0 (External Network Number)
  Advertising Router: 11.11.11.11
  LS Seq Number: 80000001
  Checksum: 0xeed8
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 11.11.11.11
	External Route Tag: 0

  Routing Bit Set on this LSA
  LS age: 659
  Options: (No TOS-capability, Type 7/5 translation, DC)
  LS Type: AS External Link
  Link State ID: 192.168.13.0 (External Network Number)
  Advertising Router: 13.13.13.13
  LS Seq Number: 80000002
  Checksum: 0x15a1
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 13.13.13.13
	External Route Tag: 0

Yet area 0 still shows only one.

R2 STEP 1: the type 5 count in area 0 has not changed
RP/0/RP0/CPU0:R2#show ospf database external
Wed Sep  9 08:23:22.544 UTC


            OSPF Router with ID (2.2.2.2) (Process ID 1)

		Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 594
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 192.168.1.0 (External Network Number)
  Advertising Router: 1.1.1.1
  LS Seq Number: 80000001
  Checksum: 0xd25
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 0.0.0.0
	External Route Tag: 0

  Routing Bit Set on this LSA
  LS age: 538
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 192.168.11.0 (External Network Number)
  Advertising Router: 4.4.4.4
  LS Seq Number: 80000001
  Checksum: 0x6c83
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 11.11.11.11
	External Route Tag: 0

  Routing Bit Set on this LSA
  LS age: 576
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 192.168.13.0 (External Network Number)
  Advertising Router: 4.4.4.4
  LS Seq Number: 80000001
  Checksum: 0xba2b
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 13.13.13.13
	External Route Tag: 0

A single type 5 for 192.168.13.0, its forwarding address still 13.13.13.13. R4 had installed R13’s type 7, so R11’s was not a candidate. The capture shows R11’s type 7 inside area 2:

Area 2 No.51 LS update (the type 7 from R11), tshark -V
Open Shortest Path First
    OSPF Header
        Version: 2
        Message Type: LS Update (4)
        Packet Length: 64
        Source OSPF Router: 11.11.11.11
        Area ID: 0.0.0.2
        Checksum: 0xd7b1 [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 7 (NSSA AS-External-LSA), len 36
            .000 0000 0000 0001 = LS Age (seconds): 1
            0... .... .... .... = Do Not Age Flag: 0
            Options: 0x28, (DC) Demand Circuits, (P) Propagate
                0... .... = DN: Not set
                .0.. .... = (O) Opaque: Not set
                ..1. .... = (DC) Demand Circuits: Supported
                ...0 .... = (L) LLS Data block: Not Present
                .... 1... = (P) Propagate: Set
                .... .0.. = (MC) Multicast: Not capable
                .... ..0. = (E) External Routing: Not capable
                .... ...0 = (MT) Multi-Topology Routing: No
            LS Type: NSSA AS-External-LSA (7)
            Link State ID: 192.168.13.0
            Advertising Router: 11.11.11.11
            Sequence Number: 0x80000001
            Checksum: 0xeed8
            Length: 36
            Netmask: 255.255.255.0
            1... .... = External Type: Type 2 (metric is larger than any other link state path)
            .000 0000 = TOS: 0
            Metric: 20
            Forwarding Address: 11.11.11.11
            External Route Tag: 0

Download the pcap of the packet in the tshark output above (No.51 LS update)

Around that timestamp the area 0 capture contains no LS update at all. As long as the translator’s output is unchanged, one more LSA inside the NSSA produces silence outside.

In a normal area both survive

The same thing in a normal area: R2 redistributes the same 192.168.1.0/24 as R1 (STEP 4).

R3 STEP 4: two type 5 LSAs for the same 192.168.1.0
RP/0/RP0/CPU0:R3#show ospf database external
Wed Sep  9 08:36:09.504 UTC


            OSPF Router with ID (3.3.3.3) (Process ID 1)

		Type-5 AS External Link States

  LS age: 1362
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 192.168.1.0 (External Network Number)
  Advertising Router: 1.1.1.1
  LS Seq Number: 80000001
  Checksum: 0xd25
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 0.0.0.0
	External Route Tag: 0

  Routing Bit Set on this LSA
  LS age: 125
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 192.168.1.0 (External Network Number)
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000001
  Checksum: 0xee3f
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 0.0.0.0
	External Route Tag: 0

  Routing Bit Set on this LSA
  LS age: 375
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 192.168.11.0 (External Network Number)
  Advertising Router: 4.4.4.4
  LS Seq Number: 80000003
  Checksum: 0x6885
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 11.11.11.11
	External Route Tag: 0

  Routing Bit Set on this LSA
  LS age: 375
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 192.168.13.0 (External Network Number)
  Advertising Router: 4.4.4.4
  LS Seq Number: 80000003
  Checksum: 0xb62d
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 13.13.13.13
	External Route Tag: 0

Both type 5 LSAs for 192.168.1.0 are in the LSDB and neither goes away; a receiver picks one under RFC 2328 Section 16.4.1.

Two routers advertising	LSAs inside	LSAs outside
Type 7 (R11 and R13)	two	one
Type 5 (R1 and R2)	—	two

As many type 5 LSAs survive as were originated; of the type 7 LSAs, only the one the ABR picks leaves.

The translator changes its mind

Lower only R11’s redistribution metric to 5 so that R11’s type 7 becomes the preferred one (STEP 2).

R11 STEP 2: give R11 the smaller metric
router ospf 1
 redistribute static metric 5

R2 STEP 2: the translated type 5 was swapped
RP/0/RP0/CPU0:R2#show ospf database external
Wed Sep  9 08:27:30.809 UTC


            OSPF Router with ID (2.2.2.2) (Process ID 1)

		Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 842
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 192.168.1.0 (External Network Number)
  Advertising Router: 1.1.1.1
  LS Seq Number: 80000001
  Checksum: 0xd25
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 0.0.0.0
	External Route Tag: 0

  Routing Bit Set on this LSA
  LS age: 101
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 192.168.11.0 (External Network Number)
  Advertising Router: 4.4.4.4
  LS Seq Number: 80000002
  Checksum: 0xd32a
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 5 
	Forward Address: 11.11.11.11
	External Route Tag: 0

  Routing Bit Set on this LSA
  LS age: 101
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 192.168.13.0 (External Network Number)
  Advertising Router: 4.4.4.4
  LS Seq Number: 80000002
  Checksum: 0xbd3e
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 5 
	Forward Address: 11.11.11.11
	External Route Tag: 0

The count is still one; only the contents changed.

Item	STEP 1 (R13 wins)	STEP 2 (R11 wins)	STEP 3 (back again)
`LS Seq Number`	`80000001`	`80000002`	`80000003`
`Metric`	20	5	20
`Forward Address`	13.13.13.13	11.11.11.11	13.13.13.13
`Advertising Router`	4.4.4.4	4.4.4.4	4.4.4.4

The advertising router is always R4. From the outside it merely looks as though R4 changed its LSA; nothing reveals that the ASBR inside the NSSA was swapped.

This lab does not meet the withdrawal condition

R11’s type 7 has Forward Address 11.11.11.11 and R13’s has 13.13.13.13 — they do not match. LSAs that send traffic to different places cannot stand in for one another, so ninety seconds later both were still there. The next section builds a lab where all three do match.

Breaking the three conditions one at a time

Making two routers agree on a forwarding address needs an external gateway on a segment inside the NSSA, with both ASBRs pointing at it as their next hop, because Section 2.3 of RFC 3101 then makes the next hop the forwarding address:

If this address belongs to a network connected to the NSSA ASBR via one of its NSSAs’ active interfaces, then the NSSA ASBR copies this next hop address into the forwarding address field

RA and RB both redistribute 192.168.99.0/24 10.2.0.254; the next hop sits on the NSSA’s LAN, so both forwarding addresses come out as 10.2.0.254. From there the three conditions are broken one at a time.

Condition	Destination	Cost	Forwarding address	Type 7 LSAs in the NSSA
All three match	same	20 / 20	`10.2.0.254` / `10.2.0.254`	one
Cost differs	same	30 / 20	`10.2.0.254` / `10.2.0.254`	two
Forwarding address differs	same	20 / 20	`10.2.0.13` / `10.2.0.254`	two

Only when all three match does the count drop to one.

With all three matching, RA never originates

Immediately after RA starts redistributing the same route:

RA: the LSDB with all three conditions matching
RP/0/RP0/CPU0:RA#show ospf database nssa-external
Wed Sep  9 10:32:08.718 UTC


            OSPF Router with ID (11.11.11.11) (Process ID 1)

		Type-7 AS External Link States (Area 2)

  Routing Bit Set on this LSA
  LS age: 1566
  Options: (No TOS-capability, Type 7/5 translation, DC)
  LS Type: AS External Link
  Link State ID: 192.168.99.0 (External Network Number)
  Advertising Router: 13.13.13.13
  LS Seq Number: 80000001
  Checksum: 0x4d3d
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 10.2.0.254
	External Route Tag: 0

Only RB’s type 7 is there; RA has none of its own. The self-originated view is empty:

RA: no self-originated type 7
RP/0/RP0/CPU0:RA#show ospf database nssa-external self-originate
Wed Sep  9 10:32:10.680 UTC


            OSPF Router with ID (11.11.11.11) (Process ID 1)

The static route is in the routing table and RA’s router-LSA carries AS Boundary Router, yet no type 7 is generated and none appears in the capture. With an equivalent, preferred LSA already present, origination itself is suppressed.

Shift the cost and it appears

Change only RA’s redistribution metric to 30.

RA: change only the metric so the LSAs are no longer equivalent
router ospf 1
 redistribute static metric 30

RA: a different cost leaves two LSAs
RP/0/RP0/CPU0:RA#show ospf database nssa-external
Wed Sep  9 10:37:37.942 UTC


            OSPF Router with ID (11.11.11.11) (Process ID 1)

		Type-7 AS External Link States (Area 2)

  LS age: 157
  Options: (No TOS-capability, Type 7/5 translation, DC)
  LS Type: AS External Link
  Link State ID: 192.168.99.0 (External Network Number)
  Advertising Router: 11.11.11.11
  LS Seq Number: 80000001
  Checksum: 0xed9a
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 30 
	Forward Address: 10.2.0.254
	External Route Tag: 0

  Routing Bit Set on this LSA
  LS age: 1895
  Options: (No TOS-capability, Type 7/5 translation, DC)
  LS Type: AS External Link
  Link State ID: 192.168.99.0 (External Network Number)
  Advertising Router: 13.13.13.13
  LS Seq Number: 80000001
  Checksum: 0x4d3d
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 10.2.0.254
	External Route Tag: 0

RA now originates its own type 7: a different cost is not “equivalent”.

Put it back and the LSA is withdrawn

Restore the metric so the two are equivalent again, and RA withdraws the LSA it is currently advertising.

Area 2 No.168 LS update (RA withdrawing its type 7), tshark -V
Open Shortest Path First
    OSPF Header
        Version: 2
        Message Type: LS Update (4)
        Packet Length: 64
        Source OSPF Router: 11.11.11.11
        Area ID: 0.0.0.2
        Checksum: 0xadbe [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 7 (NSSA AS-External-LSA), len 36
            .000 1110 0001 0000 = LS Age (seconds): 3600
            0... .... .... .... = Do Not Age Flag: 0
            Options: 0x28, (DC) Demand Circuits, (P) Propagate
                0... .... = DN: Not set
                .0.. .... = (O) Opaque: Not set
                ..1. .... = (DC) Demand Circuits: Supported
                ...0 .... = (L) LLS Data block: Not Present
                .... 1... = (P) Propagate: Set
                .... .0.. = (MC) Multicast: Not capable
                .... ..0. = (E) External Routing: Not capable
                .... ...0 = (MT) Multi-Topology Routing: No
            LS Type: NSSA AS-External-LSA (7)
            Link State ID: 192.168.99.0
            Advertising Router: 11.11.11.11
            Sequence Number: 0x80000002
            Checksum: 0xbee6
            Length: 36
            Netmask: 255.255.255.0
            1... .... = External Type: Type 2 (metric is larger than any other link state path)
            .000 0000 = TOS: 0
            Metric: 16777215
            Forwarding Address: 10.2.0.254
            External Route Tag: 0

Download the pcap of the packet in the tshark output above (No.168 LS update)

LS Age 3600 (MaxAge) and Metric 16777215 (LSInfinity): this is what RFC 3101 means by “flush its LSA”. XRd behaves in one of two ways:

Situation	Behaviour
An equivalent LSA already exists when redistribution starts	nothing is generated (nothing in the capture either)
The state becomes equivalent after it has originated	withdrawn with MaxAge

Shift only the forwarding address and it appears too

Point the next hop at RB itself (10.2.0.13) so that only the forwarding address changes.

RA: a different forwarding address leaves two LSAs
RP/0/RP0/CPU0:RA#show ospf database nssa-external
Wed Sep  9 10:45:24.078 UTC


            OSPF Router with ID (11.11.11.11) (Process ID 1)

		Type-7 AS External Link States (Area 2)

  LS age: 123
  Options: (No TOS-capability, Type 7/5 translation, DC)
  LS Type: AS External Link
  Link State ID: 192.168.99.0 (External Network Number)
  Advertising Router: 11.11.11.11
  LS Seq Number: 80000001
  Checksum: 0x4e36
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 10.2.0.13
	External Route Tag: 0

  Routing Bit Set on this LSA
  LS age: 323
  Options: (No TOS-capability, Type 7/5 translation, DC)
  LS Type: AS External Link
  Link State ID: 192.168.99.0 (External Network Number)
  Advertising Router: 13.13.13.13
  LS Seq Number: 80000002
  Checksum: 0x4b3e
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 10.2.0.254
	External Route Tag: 0

Both survive. A different forwarding address means not equivalent, even with the same destination and cost. Throughout all of this, area 0 held exactly one type 5.

What translation rewrites

The type 7 in area 2 and the type 5 in area 0, captured right after the route was recreated in STEP 9.

Area 2 No.511 LS update (the type 7 from R13), tshark -V
Open Shortest Path First
    OSPF Header
        Version: 2
        Message Type: LS Update (4)
        Packet Length: 64
        Source OSPF Router: 13.13.13.13
        Area ID: 0.0.0.2
        Checksum: 0xa2de [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 7 (NSSA AS-External-LSA), len 36
            .000 0000 0000 0001 = LS Age (seconds): 1
            0... .... .... .... = Do Not Age Flag: 0
            Options: 0x28, (DC) Demand Circuits, (P) Propagate
                0... .... = DN: Not set
                .0.. .... = (O) Opaque: Not set
                ..1. .... = (DC) Demand Circuits: Supported
                ...0 .... = (L) LLS Data block: Not Present
                .... 1... = (P) Propagate: Set
                .... .0.. = (MC) Multicast: Not capable
                .... ..0. = (E) External Routing: Not capable
                .... ...0 = (MT) Multi-Topology Routing: No
            LS Type: NSSA AS-External-LSA (7)
            Link State ID: 192.168.13.0
            Advertising Router: 13.13.13.13
            Sequence Number: 0x80000001
            Checksum: 0x17a0
            Length: 36
            Netmask: 255.255.255.0
            1... .... = External Type: Type 2 (metric is larger than any other link state path)
            .000 0000 = TOS: 0
            Metric: 20
            Forwarding Address: 13.13.13.13
            External Route Tag: 0

Download the pcap of the packet in the tshark output above (No.511 LS update)

Area 0:

Area 0 No.507 LS update (the type 5 translated by R4), tshark -V
Open Shortest Path First
    OSPF Header
        Version: 2
        Message Type: LS Update (4)
        Packet Length: 64
        Source OSPF Router: 4.4.4.4
        Area ID: 0.0.0.0 (Backbone)
        Checksum: 0x2c7b [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 5 (AS-External-LSA (ASBR)), len 36
            .000 0000 0000 0001 = LS Age (seconds): 1
            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: AS-External-LSA (ASBR) (5)
            Link State ID: 192.168.13.0
            Advertising Router: 4.4.4.4
            Sequence Number: 0x80000001
            Checksum: 0xba2b
            Length: 36
            Netmask: 255.255.255.0
            1... .... = External Type: Type 2 (metric is larger than any other link state path)
            .000 0000 = TOS: 0
            Metric: 20
            Forwarding Address: 13.13.13.13
            External Route Tag: 0

Download the pcap of the packet in the tshark output above (No.507 LS update)

Exactly as Section 3.2 of RFC 3101 specifies:

The newly originated Type-5 LSA will describe the same network and have the same network mask, path type, metric, forwarding address and external route tag as the Type-7 LSA. The advertising router field will be the router ID of this NSSA border router.

Setting the LS type aside, only the advertising router and the options are rewritten.

Field	Type 7	Type 5
LS Type	7	5
`Options`	`0x28, (DC), (P) Propagate`	`0x20, (DC)`
Link State ID	192.168.13.0	192.168.13.0
Advertising Router	13.13.13.13	4.4.4.4
`Netmask` / `Metric` / External Type	255.255.255.0 / 20 / Type 2	the same
`Forwarding Address`	13.13.13.13	13.13.13.13

Options drops from 0x28 to 0x20 and the (P) Propagate: Set line is gone. The P bit means “you may translate this LSA”, so a translated type 5 has no use for it. The CLI cannot show this; only the bit-level expansion of tshark -V can.

Both sequence numbers reading 0x80000001 is a coincidence. The two LSAs are numbered independently, so one can advance without the other (the STEP 2 and 3 table).

Why the forwarding address has to be non-zero

Section 2.3 of RFC 3101 puts it plainly:

If the P-bit is set, the forwarding address must be non-zero; otherwise it may be 0.0.0.0.

A type 7 with the P bit set must carry a non-zero forwarding address; anything else may leave it at 0.0.0.0. Section 2.3 explains the first half:

Non-zero forwarding addresses produce efficient inter-area routing to an NSSA’s AS external destinations when it has multiple border routers.

The translated type 5’s advertising router is the translating ABR, so the forwarding address keeps the original ASBR’s address and says where the real exit is.

When it is 0.0.0.0

otherwise it may be 0.0.0.0 applies to a type 7 with the P bit clear, typically the default route an NSSA border router injects as a type 7. Section 2.4 of RFC 3101 requires it:

The Type-7 default LSA originated by an NSSA border router must have the P-bit clear.

The reason, in the same section, is “so that it isn’t translated into a Type-5 LSA by another NSSA border router”: otherwise two ABRs would translate each other’s default and leak it into the backbone. Never translated, it keeps a 0.0.0.0 forwarding address. Have the ABR originate a default in the previous lab and both kinds sit in one output.

One with the P bit and one without
RP/0/RP0/CPU0:RA#show ospf database nssa-external
Wed Sep  9 10:59:05.078 UTC


            OSPF Router with ID (11.11.11.11) (Process ID 1)

		Type-7 AS External Link States (Area 2)

  Routing Bit Set on this LSA
  LS age: 466
  Options: (No TOS-capability, No Type 7/5 translation, DC)
  LS Type: AS External Link
  Link State ID: 0.0.0.0 (External Network Number)
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000001
  Checksum: 0xd0d8
  Length: 36
  Network Mask: /0
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 1 
	Forward Address: 0.0.0.0
	External Route Tag: 0

  Routing Bit Set on this LSA
  LS age: 1144
  Options: (No TOS-capability, Type 7/5 translation, DC)
  LS Type: AS External Link
  Link State ID: 192.168.99.0 (External Network Number)
  Advertising Router: 13.13.13.13
  LS Seq Number: 80000002
  Checksum: 0x4b3e
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 10.2.0.254
	External Route Tag: 0

The 0.0.0.0 one is R2’s default (No Type 7/5 translation, Forward Address: 0.0.0.0); the ASBR’s below it has Type 7/5 translation and a non-zero address. A type 7’s forwarding address is not “always non-zero” but “non-zero if you want it to leave the NSSA”.

On IOS XR, default-information originate does not produce a type 7 default. It makes the router an ASBR with a type 5 default: area 0 received one, but nothing entered the NSSA (the LAN capture holds nothing but hellos). The type 7 default is a per-area setting:

Injecting a type 7 default into an NSSA
router ospf 1
 area 2
  nssa default-information-originate

How the non-zero address is chosen

Take R13’s Loopback0 out of area 2 and watch (STEP 6).

R13 STEP 6: remove the loopback from area 2
router ospf 1
 area 2
  no interface Loopback0

R13 STEP 6: the forwarding address was re-selected
RP/0/RP0/CPU0:R13#show ospf database nssa-external
Wed Sep  9 08:45:58.635 UTC


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

		Type-7 AS External Link States (Area 2)

  Routing Bit Set on this LSA
  LS age: 963
  Options: (No TOS-capability, Type 7/5 translation, DC)
  LS Type: AS External Link
  Link State ID: 192.168.11.0 (External Network Number)
  Advertising Router: 11.11.11.11
  LS Seq Number: 80000004
  Checksum: 0xfec7
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 11.11.11.11
	External Route Tag: 0

  LS age: 228
  Options: (No TOS-capability, Type 7/5 translation, DC)
  LS Type: AS External Link
  Link State ID: 192.168.13.0 (External Network Number)
  Advertising Router: 13.13.13.13
  LS Seq Number: 80000003
  Checksum: 0x477f
  Length: 36
  Network Mask: /24
	Metric Type: 2 (Larger than any link state path)
	TOS: 0 
	Metric: 20 
	Forward Address: 10.1.11.13
	External Route Tag: 0

Forward Address moved from 13.13.13.13 (Loopback0) to 10.1.11.13 (the physical interface facing R11) and the sequence number advanced, as Section 2.3 of RFC 3101 requires:

When the interface whose IP address is the LSA’s forwarding address transitions to a Down state (see [OSPF] Section 9.3), the router must select a new forwarding address for the LSA and then re-originate it. If one is not available the LSA should be flushed.

The same section gives the order of preference:

When a router is forced to pick a forwarding address for a Type-7 LSA, preference should be given first to the router’s internal addresses (provided internal addressing is supported). If internal addresses are not available, preference should be given to the router’s active OSPF stub network addresses.

First an internal address, then stub network addresses, meaning loopbacks; with the loopback gone it fell back to a transit interface. The translated type 5 inherited 10.1.11.13 too, and restoring the loopback brings 13.13.13.13 back (STEP 7).

An address on an interface that is not running OSPF is never chosen. Where the external-facing interface is kept out of OSPF, removing the loopback can leave the router unable to originate a type 7.

Withdrawal and recovery

Deleting R13’s static (STEP 8) withdraws both the type 7 and the translated type 5.

Area 2 No.461 LS update (withdrawing the type 7), tshark -V
Open Shortest Path First
    OSPF Header
        Version: 2
        Message Type: LS Update (4)
        Packet Length: 64
        Source OSPF Router: 13.13.13.13
        Area ID: 0.0.0.2
        Checksum: 0x66fd [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 7 (NSSA AS-External-LSA), len 36
            .000 1110 0001 0000 = LS Age (seconds): 3600
            0... .... .... .... = Do Not Age Flag: 0
            Options: 0x28, (DC) Demand Circuits, (P) Propagate
                0... .... = DN: Not set
                .0.. .... = (O) Opaque: Not set
                ..1. .... = (DC) Demand Circuits: Supported
                ...0 .... = (L) LLS Data block: Not Present
                .... 1... = (P) Propagate: Set
                .... .0.. = (MC) Multicast: Not capable
                .... ..0. = (E) External Routing: Not capable
                .... ...0 = (MT) Multi-Topology Routing: No
            LS Type: NSSA AS-External-LSA (7)
            Link State ID: 192.168.13.0
            Advertising Router: 13.13.13.13
            Sequence Number: 0x80000006
            Checksum: 0x4482
            Length: 36
            Netmask: 255.255.255.0
            1... .... = External Type: Type 2 (metric is larger than any other link state path)
            .000 0000 = TOS: 0
            Metric: 16777215
            Forwarding Address: 13.13.13.13
            External Route Tag: 0

Download the pcap of the packet in the tshark output above (No.461 LS update)

Area 0 No.457 LS update (withdrawing the translated type 5), tshark -V
Open Shortest Path First
    OSPF Header
        Version: 2
        Message Type: LS Update (4)
        Packet Length: 64
        Source OSPF Router: 4.4.4.4
        Area ID: 0.0.0.0 (Backbone)
        Checksum: 0xf297 [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 5 (AS-External-LSA (ASBR)), len 36
            .000 1110 0001 0000 = LS Age (seconds): 3600
            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: AS-External-LSA (ASBR) (5)
            Link State ID: 192.168.13.0
            Advertising Router: 4.4.4.4
            Sequence Number: 0x80000007
            Checksum: 0xe50e
            Length: 36
            Netmask: 255.255.255.0
            1... .... = External Type: Type 2 (metric is larger than any other link state path)
            .000 0000 = TOS: 0
            Metric: 16777215
            Forwarding Address: 13.13.13.13
            External Route Tag: 0

Download the pcap of the packet in the tshark output above (No.457 LS update)

Both carry LS Age 3600 (MaxAge) and Metric 16777215 (LSInfinity), the same as a summary-LSA or an AS external-LSA. Restore the route (STEP 9) and both start again at 0x80000001: a withdrawn LSA comes back as a new one.

Propagation time: translation costs you

An ABR forwards a type 5 untouched, but translating a type 7 makes it generate an LSA, and IOS XR throttles LSA generation. Here is the gap between the type 7 appearing in area 2 and the translated type 5 appearing in area 0.

STEP	Change	Gap
2	R11’s metric down to 5	153.5 ms
3	Metric back again	126.9 ms
6	Loopback out of area 2	173.0 ms
7	Loopback back again	125.2 ms
8	Route deleted	164.6 ms
9	Route restored	11.7 ms

Alongside the other LSA types:

LSA type	Delay across the boundary	What happens at the boundary
Type 3	190 ms	The ABR rebuilds it
Type 4	126 ms	The ABR rebuilds it
Type 5	5-10 ms	It is forwarded as is
Type 7 to type 5	12-173 ms	The ABR rebuilds it

R4’s show ospf carries the throttle values.

R4 STEP 0: the LSA generation throttle values in show ospf
 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

The 200 ms hold time matches the 126-173 ms measured. STEP 9’s 11.7 ms is most likely the withdrawn LSA being generated as a new one, paying only the 50 ms initial delay.

Summary

The format matches a type 5. What differs is where it flows and that it is translated
Translation rewrites only the advertising router and the P bit; everything else is inherited
Advertise one route from two routers and the NSSA holds two LSAs while one leaves. With a type 5 both survive
The forwarding address is the only thing in a translated type 5 that points at the ASBR, which is why a type 7 with the P bit set must carry one; with the P bit clear it may stay 0.0.0.0
Translation is LSA generation, so it is slower than forwarding a type 5

When an NSSA misbehaves, comparing the LSDB inside the NSSA with the one on the backbone is the shortest path to the answer.

References

Document	Section	Content
RFC 3101	Section 2.3	The definition of type 7: six differences from a type 5, choosing and re-selecting the forwarding address
RFC 3101	Section 2.4	Preference between two equivalent type 7 LSAs, and the rule that the loser flushes its own
RFC 3101	Section 2.5	Calculating type 7 AS external routes
RFC 3101	Section 3.2	Translating a type 7 into a type 5: which fields are inherited and which are rewritten
RFC 3101	Section 3.1	Electing the type 7 translator
RFC 3101	Appendix A	The N/P bit in the options field
RFC 3101	Appendix C	The type 7 packet format; a 0.0.0.0 forwarding address means traffic goes to the originator
RFC 2328	Appendix A.4.5	The type 5 packet format (a type 7 is laid out the same way)
RFC 2328	Section 16.4.1	Choosing between several external LSAs for one destination

Test setup

A three-area lab of seven XRd routers (IOS XR 26.1.1) on CML. Area 2 is the NSSA, holding the ASBRs R13 and R11, with R3 and R4 as its border routers. Captures ran on area 2 (R11 - R13) and area 0 (R2 - R4) at the same time, so that the translation delay could be measured.

Download the area 2 capture (R11 - R13)

Download the area 0 capture (R2 - R4)

The setup that makes the forwarding addresses match

The section “Breaking the three conditions one at a time” uses a second lab, because making two routers agree on a forwarding address needs an external gateway on a segment inside the NSSA. Six XRd routers: R3, R1 and R2 in area 0, and RA, RB and GW in the NSSA (area 2).

RA and RB share a LAN inside the NSSA (10.2.0.0/24) and both redistribute 192.168.99.0/24 10.2.0.254. GW does not run OSPF. R2 holds the higher router ID of the two NSSA border routers, so it is the translator.

Download the NSSA LAN capture (RB - SW)

Download the area 0 capture (R2 - R3)

Test configs and show output

At every STEP all seven routers were captured into these three files each. The test config is the ..._run.txt (the final state is the STEP 9 one).

File	Content
`..._show.txt`	`show 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 nssa-external` / `show ospf database external` / `show ospf database database-summary`
`..._log.txt`	`show logging` narrowed to that STEP
`..._run.txt`	`show running-config` at that STEP (the test config for it)

show ospf database nssa-external returns content only on routers attached to an NSSA; on R1 in area 1 it comes back empty. That output is itself a demonstration that type 7 LSAs live in the area data structure.

STEP 0: right after boot

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
R4	show	log	run
R11	show	log	run
R12	show	log	run
R13	show	log	run

STEP 1: R11 redistributes 192.168.13.0/24 as well

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
R4	show	log	run
R11	show	log	run
R12	show	log	run
R13	show	log	run

STEP 2: R11’s redistribution metric set to 5

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
R4	show	log	run
R11	show	log	run
R12	show	log	run
R13	show	log	run

STEP 3: R11’s redistribution restored

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
R4	show	log	run
R11	show	log	run
R12	show	log	run
R13	show	log	run

STEP 4: R2 redistributes 192.168.1.0/24 as well

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
R4	show	log	run
R11	show	log	run
R12	show	log	run
R13	show	log	run

STEP 5: R2 stops redistributing

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
R4	show	log	run
R11	show	log	run
R12	show	log	run
R13	show	log	run

STEP 6: R13’s Loopback0 removed from area 2

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
R4	show	log	run
R11	show	log	run
R12	show	log	run
R13	show	log	run

STEP 7: R13’s Loopback0 restored

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
R4	show	log	run
R11	show	log	run
R12	show	log	run
R13	show	log	run

STEP 8: R13’s 192.168.13.0/24 deleted

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
R4	show	log	run
R11	show	log	run
R12	show	log	run
R13	show	log	run

STEP 9: everything restored (final state)

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
R4	show	log	run
R11	show	log	run
R12	show	log	run
R13	show	log	run

Test output from the forwarding-address lab

The captures from the lab used in “Breaking the three conditions one at a time”. The test config is the ..._run.txt (the final state is the STEP 7 one).

STEP	Change
0	only RB redistributes `192.168.99.0/24 10.2.0.254`
1	RA redistributes it with the same next hop (all three match)
2	RA’s redistribution metric set to 30
3	the metric restored
4	RA’s next hop changed to `10.2.0.13`
5	the next hop restored
6	`nssa default-information-originate` added on R2
7	everything restored (final state)

STEP 0

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
RA	show	log	run
RB	show	log	run
GW	show	log	run

STEP 1

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
RA	show	log	run
RB	show	log	run
GW	show	log	run

STEP 2

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
RA	show	log	run
RB	show	log	run
GW	show	log	run

STEP 3

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
RA	show	log	run
RB	show	log	run
GW	show	log	run

STEP 4

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
RA	show	log	run
RB	show	log	run
GW	show	log	run

STEP 5

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
RA	show	log	run
RB	show	log	run
GW	show	log	run

STEP 6

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
RA	show	log	run
RB	show	log	run
GW	show	log	run

STEP 7

Router	show output	syslog	running-config
R1	show	log	run
R2	show	log	run
R3	show	log	run
RA	show	log	run
RB	show	log	run
GW	show	log	run