IT Certification

Note:

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Follow the article: Part1:Why Are Some OSPF Routes in the Database but Not in the Routing Table? (CCIE-Lab)

Reason 4: Broken PVC in Fully Meshed Frame Relay Environment
Consider this network diagram as an example:

R9-2500
interface Loopback0
ip address 50.50.50.50 255.255.255.255
!
interface Serial0
ip address 10.10.10.5 255.255.255.0
encapsulation frame-relay
ip ospf network broadcast
frame-relay map ip 10.10.10.6 102 broadcast
frame-relay map ip 10.10.10.7 101 broadcast

router ospf 10
network 10.10.10.0 0.0.0.255 area 0
network 50.50.50.0 0.0.0.255 area 0

R4-4K
interface Loopback0
ip address 70.70.70.70 255.255.255.255
!
interface Serial0
ip address 10.10.10.7 255.255.255.0
encapsulation frame-relay
ip ospf network broadcast
frame-relay map ip 10.10.10.5 101 broadcast
frame-relay map ip 10.10.10.6 100 broadcast

router ospf 10
network 10.10.10.0 0.0.0.255 area 0
network 70.70.70.0 0.0.0.255 area 0

R3-4K
interface Loopback0
ip address 60.60.60.60 255.255.255.255
!
interface Serial0
no ip address
encapsulation frame-relay
!
interface Serial0.1 multipoint
ip address 10.10.10.6 255.255.255.0
ip ospf network broadcast
frame-relay map ip 10.10.10.5 102 broadcast
frame-relay map ip 10.10.10.7 100 broadcast
!
router ospf 10
network 10.10.10.0 0.0.0.255 area 0
network 60.60.60.0 0.0.0.255 area 0

The broadcast model over Frame Relay works properly as long as the Frame Relay cloud is fully meshed. If any permanent virtual circuits (PVCs) are broken, it can create problems in the OSPF database, which in turn produces the Adv router not reachable message.

In this example, the PVC between R9-2500 and R4-4K is broken, and R9-2500 link to the designated router (DR) is broken. As a result, R9-2500 declares all LSAs from R3-4K (which is not a DR), as unreachable. As you can see, R9-2500 isn’t generating a transit link for the serial interface attached to R3-4K; it is generating a stub link instead because as far as R9-2500 is concerned there is no DR on this link.

R9-2500(3)# show ip ospf database router

OSPF Router with ID (50.50.50.50) (Process ID 10)
Router Link States (Area 0)
LS age: 148
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 50.50.50.50
Advertising Router: 50.50.50.50
LS Seq Number: 8000000B
Checksum: 0x55A
Length: 48
Number of Links: 2

Link connected to: a Stub Network
(Link ID) Network/subnet number: 10.10.10.0
(Link Data) Network Mask: 255.255.255.0
Number of TOS metrics: 0
TOS 0 Metrics: 64

Link connected to: a Stub Network
(Link ID) Network/subnet number: 50.50.50.50
(Link Data) Network Mask: 255.255.255.255
Number of TOS metrics: 0
TOS 0 Metrics: 1

Adv Router is not-reachable
LS age: 1081
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 60.60.60.60
Advertising Router: 60.60.60.60
LS Seq Number: 80000006
Checksum: 0x4F72
Length: 48
Number of Links: 2

Link connected to: a Stub Network
(Link ID) Network/subnet number: 60.60.60.60
(Link Data) Network Mask: 255.255.255.255
Number of TOS metrics: 0
TOS 0 Metrics: 1

Link connected to: a Transit Network
(Link ID) Designated Router address: 10.10.10.7
(Link Data) Router Interface address: 10.10.10.6
Number of TOS metrics: 0
TOS 0 Metrics: 64

Adv Router is not-reachable
LS age: 306
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 70.70.70.70
Advertising Router: 70.70.70.70
LS Seq Number: 80000007
Checksum: 0xC185
Length: 48
Number of Links: 2

Link connected to: a Stub Network
(Link ID) Network/subnet number: 70.70.70.70
(Link Data) Network Mask: 255.255.255.255
Number of TOS metrics: 0
TOS 0 Metrics: 1

Link connected to: a Transit Network
(Link ID) Designated Router address: 10.10.10.7
(Link Data) Router Interface address: 10.10.10.7
Number of TOS metrics: 0
TOS 0 Metrics: 64

Refer to Problems with Running OSPF in NBMA Mode over Frame Relay for more detailed information about this problem.

Reason 5: Forwarding Address Known via an External Route
Consider this network diagram as an example:

R2507
interface Serial0
ip address 1.1.1.1 255.255.255.0

interface Serial1
ip address 7.7.7.1 255.255.255.0

router ospf 1
network 1.1.1.1 0.0.0.0 area 0
default- information originate metric 20

ip route 0.0.0.0 0.0.0.0 Serial1

R2504
interface Serial0
ip address 1.1.1.2 255.255.255.0

interface TokenRing0
ip address 3.3.4.2 255.255.255.0

router ospf 1
network 1.1.1.0 0.0.0.255 area 0
network 3.0.0.0 0.255.255.255 area 1
area 1 range 3.0.0.0 255.0.0.0
R2515
interface Serial1
ip address 4.4.4.3 255.255.255.0

interface TokenRing0
ip address 3.3.4.3 255.255.255.0

interface ethernet 0
ip address 3.44.66.3 255.255.255.0

interface ethernet 1
ip address 3.22.88.3 255.255.255.0

router ospf 1
redistribute rip metric 20 subnets
network 0.0.0.0 255.255.255.255 area 1

router rip
network 3.0.0.0

R2513
interface TokenRing0
ip address 3.3.4.4 255.255.255.0

interface ethernet 0
ip address 200.1.1.4 255.255.255.0

router rip
network 3.0.0.0
network 200.1.1.0

R2507# show ip ospf data external 200.1.1.0
OSPF Router with ID (7.7.7.1) (Process ID 1)
Type- 5 AS External Link States
LS age: 72
Options: (No TOS- capability, DC)
LS Type: AS External Link
Link State ID: 200.1.1.0 (External Network Number )
Advertising Router: 3.44.66.3
LS Seq Number: 80000001
Checksum: 0xF161
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 3.3.4.4
External Route Tag: 0

R2507 has 200.1.1.0/24 in its database but it hasn’t installed it in the routing table because 3.3.4.4 is learned via an OSPF external route.

R2507# show ip route 3.3.4.4
Routing entry for 3.3.4.0/ 24
Known via “ospf 1″, distance 110, metric 20,type extern 2, forward metric 70
Redistributing via ospf 1
Last update from 1.1.1.2 on Serial0, 00: 00: 40 ago
Routing Descriptor Blocks:
* 1.1.1.2, from 3.44.66.3, 00: 00: 40 ago, via Serial0
Route metric is 20, traffic share count is 1

Note: With the fix of Cisco bug ID CSCdp72526 ( registered customers only) , OSPF does not generate a type-5 link-state advertisement (LSA) of an overlapped external network; therefore, R2507 will only have a summary intra-area route of 3.0.0.0/8. Then, R2507 will install 200.1.1.0/24 as the forwarding address and it will be reachable via intra-area route 3.0.0.0/8, thus in compliance with RFC 2328 .

After the fix of above mentioned bug, output will look like the following:

R2507# show ip route 3.3.4.4
Routing entry for 3.0.0.0/8
Known via “ospf 1″, distance 110, metric 74, type inter area
Last update from 1.1.1.2 on Serial0, 00:19:20 ago
Routing Descriptor Blocks:
* 1.1.1.2, from 3.3.4.2, 00:19:20 ago, via Serial0

R2507# show ip route
Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2
i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
ia – IS-IS inter area, * – candidate default, U – per-user static route
o – ODR, P – periodic downloaded static route

Gateway of last resort is not set

1.0.0.0/24 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Serial0
O IA 3.0.0.0/8 [110/74] via 1.1.1.2, 00:30:18, Serial0
O E2 200.1.1.0/24 [110/20] via 1.1.1.2, 00:22:58, Serial0
Route metric is 74, traffic share count is 1

R2507#

If the forwarding address is also known via an external route, OSPF doesn’t install that route in the routing table. For more detailed information about this problem, see Common Routing Problem with OSPF Forwarding Address.

Reason 6: Distribute List Is Blocking the Routes
Let’s use the following network diagram as an example:

R4-4K
interface Loopback0
ip address 172.16.33.1 255.255.255.255

interface Serial2
ip address 172.16.32.1 255.255.255.0

router ospf 20
network 172.16.0.0 0.0.255.255 area 0

R1-7010
interface Loopback0
ip address 172.16.30.1 255.255.255.255
!
interface Serial1/0
ip address 172.16.32.2 255.255.255.0
clockrate 64000

router ospf 20
network 172.16.0.0 0.0.255.255 area 0
distribute-list 1 in
!
access-list 1 permit 172.16.32.0. 0.0.0.255

As you can see above, R1-7010 has the distribute-list command configured and it’s only allowing the 172.16.32.0/24 address range to be installed in the routing table. In link-state protocols you can not really filter an LSA with the distribute-list command. The LSA will still be in the database; however the LSA will not be installed in the routing table.

R1-7010(5)# show ip ospf database router 172.16.33.1

LS age: 357
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 172.16.33.1
Advertising Router: 172.16.33.1
LS Seq Number: 8000000A
Checksum: 0xD4AA
Length: 48
Number of Links: 3

Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 172.16.32.2
(Link Data) Router Interface address: 172.16.32.1
Number of TOS metrics: 0
TOS 0 Metrics: 64

The distribute-list configuration command on R1-7010 is filtering the 172.16.33.1/32 network from being installed in the routing table.

R1-7010(5)# show ip route
172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks
C 172.16.32.0/24 is directly connected, Serial1/0
C 172.16.30.1/32 is directly connected, Loopback0

Solution
To solve this problem, configure R1-7010 and allow 172.16.33.0/24 in the access control list (ACL) so this network gets installed in the routing table.

R1-7010(5)# configure terminal
R1-7010(5)(config)# access-list 1 permit 172.16.33.0 0.0.0.255
R1-7010(5)(config)# end

R1-7010(5)# show ip access-list 1
Standard IP access list 1
permit 172.16.32.0, wildcard bits 0.0.0.255
permit 172.16.33.0, wildcard bits 0.0.0.255

R1-7010(5)# show ip route
172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks
C 172.16.32.0/24 is directly connected, Serial1/0
O 172.16.33.1/32 [110/65] via 172.16.32.1, 00:00:08, Serial1/0
C 172.16.30.1/32 is directly connected, Loopback0