OSPF Router BIT set

gongya001 · ‎06-07-2018

I have the above topology.

On R3, I have the following confguration

R3

int lo0

ip add 3.3.3.3 255.255.255.255

ip ospf 1 area 10

interface Loopback1

ip address 33.33.33.33 255.255.255.255

int e0/0

ip add 10.10.10.3 255.255.255.0

ip ospf 1 area 10

no shut

int e0/1

ip add 172.16.3.3 255.255.255.224

ip ospf 1 area 10

no shut

Router ospf 1

router-id 3.3.3.3

passive-interface lo0

red eigrp 10

redistribute connected subnets

Router eigrp 10

network 10.10.10.0 255.255.255.0

On R4

Int lo0

ip add 4.4.4.4 255.255.255.255

ip ospf 1 area 20

Int e0/2

ip add 172.16.4.4 255.255.255.224

ip ospf 1 area 20

no shut

Int e0/1

ip add 10.10.10.4 255.255.255.0

no shut

Router ospf 1

router-id 4.4.4.4

passive-interface lo0

My questions are while 33.33.33.33 has Routing-bit set, but 6.6.6.6 does not

R4#show ip ospf data external 6.6.6.6

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

Type-5 AS External Link States

LS age: 19

Options: (No TOS-capability, DC, Upward)

LS Type: AS External Link

Link State ID: 6.6.6.6 (External Network Number )

Advertising Router: 3.3.3.3

LS Seq Number: 8000000A

Checksum: 0x242B

Length: 36

Network Mask: /32

Metric Type: 2 (Larger than any link state path)

MTID: 0

Metric: 20

Forward Address: 10.10.10.6

External Route Tag: 0

R4#show ip ospf data external 33.33.33.33

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

Type-5 AS External Link States

Routing Bit Set on this LSA in topology Base with MTID 0

LS age: 1085

Options: (No TOS-capability, DC, Upward)

LS Type: AS External Link

Link State ID: 33.33.33.33 (External Network Number )

Advertising Router: 3.3.3.3

LS Seq Number: 80000003

Checksum: 0x9578

Length: 36

Network Mask: /32

Metric Type: 2 (Larger than any link state path)

MTID: 0

Metric: 20

Forward Address: 0.0.0.0

External Route Tag: 0

I know how to fix it. After I added ip ospf 1 area 20 on e0/1 on R4, routing bit will be set.

I do not know why 6.6.6.6 does not have routing bit set without ip ospf 1 area 20 on e0/1 on R4, but 33.33.33.33 has.

thanks !!

Georg Pauwen · ‎06-07-2018

Hello,

usually, no routing bit set means that (R4 in your case) is not the preferred path for the network in question. Can you reach 6.6.6.6 from R4, is there a route in the routing table ?

gongya001 · ‎06-07-2018

R4 could not reach 6.6.6.6.

If I add ip ospf 1 area 20 under e0/1 on R4, R4 could reach 6.6.6.6. Also Routing Bit Set appears.

What I don't understand is why Routing Bit Set is not there without configuring ip ospf 1 area 20 under e0/1 for R4. But 33.33.33.33 has Routing Bit Set. Both are redistributed into ospf on R3.

R4 can reach advertising router 3.3.3.3.

thanks !!

gongya001 · ‎06-07-2018

I do not understand why it is not preferred.

thanks !!

Georg Pauwen · ‎06-07-2018

Hello,

not preferred or no route essentially cause the same thing, which is the routing bit not being present.

The routing bit being present simply means that the network is being routed by this router. When you look at the routing table of R4, it probably looks like this:

R4#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
a - application route
+ - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is not set

1.0.0.0/32 is subnetted, 1 subnets
O IA 1.1.1.1 [110/3] via 172.16.4.1, 00:34:37, GigabitEthernet0/2
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/3] via 172.16.4.1, 00:33:48, GigabitEthernet0/2
4.0.0.0/32 is subnetted, 1 subnets
C 4.4.4.4 is directly connected, Loopback0
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.10.10.0/24 is directly connected, GigabitEthernet0/1
L 10.10.10.4/32 is directly connected, GigabitEthernet0/1
33.0.0.0/32 is subnetted, 1 subnets
O E2 33.33.33.33 [110/20] via 172.16.4.1, 00:34:37, GigabitEthernet0/2
140.143.0.0/30 is subnetted, 1 subnets
O IA 140.143.1.0 [110/2] via 172.16.4.1, 00:34:37, GigabitEthernet0/2
172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks
O IA 172.16.3.0/27 [110/2] via 172.16.4.1, 00:34:37, GigabitEthernet0/2
C 172.16.4.0/27 is directly connected, GigabitEthernet0/2
L 172.16.4.4/32 is directly connected, GigabitEthernet0/2

There is no route for 6.6.6.6.

Adding the interface to the OSPF area makes 6.6.6.6 reachable:

R4#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
a - application route
+ - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is not set

1.0.0.0/32 is subnetted, 1 subnets
O IA 1.1.1.1 [110/3] via 172.16.4.1, 00:38:41, GigabitEthernet0/2
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/3] via 172.16.4.1, 00:37:52, GigabitEthernet0/2
4.0.0.0/32 is subnetted, 1 subnets
C 4.4.4.4 is directly connected, Loopback0
6.0.0.0/32 is subnetted, 1 subnets
O E2 6.6.6.6 [110/20] via 10.10.10.6, 00:00:10, GigabitEthernet0/1
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.10.10.0/24 is directly connected, GigabitEthernet0/1
L 10.10.10.4/32 is directly connected, GigabitEthernet0/1
33.0.0.0/32 is subnetted, 1 subnets
O E2 33.33.33.33 [110/20] via 172.16.4.1, 00:38:41, GigabitEthernet0/2
140.143.0.0/30 is subnetted, 1 subnets
O IA 140.143.1.0 [110/2] via 172.16.4.1, 00:38:41, GigabitEthernet0/2
172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks
O IA 172.16.3.0/27 [110/2] via 172.16.4.1, 00:38:41, GigabitEthernet0/2
C 172.16.4.0/27 is directly connected, GigabitEthernet0/2
L 172.16.4.4/32 is directly connected, GigabitEthernet0/2

gongya001 · ‎06-07-2018

Yes, if I add enabling ospf on e0/1 for R4, it works.

without enabling ospf on e0/1 for R4, Routing Bit Set disappears based on what ?

I searched the internet, most mentions either the advertising router is not reachable or some route is more preferred than ospf.

But in this scenarios, the advertising route is reachable, and no other route exists for the prefix 6.6.6.6.

That is where I am lost.

thanks !!