ORIGINATION OF TYPE 3 LSA

Introduction

Type 3 LSA generated by an ABR to inform the routers in one area about the networks in another area.


Prerequisites

Basic Understanding of OSPF protocol.

Network Diagram

 

In the above topology R1, R2, R3 & R4 are ABR and router R1 and R2 are summarizing the area 1 address into area0.

 

R1#

router ospf 1
router-id 1.1.1.1
log-adjacency-changes
area 1 range 155.55.0.0 255.255.0.0
network 155.55.55.1 0.0.0.0 area 1
network 192.168.12.1 0.0.0.0 area 1
network 192.168.15.1 0.0.0.0 area 0

R2#

router ospf 1
router-id 2.2.2.2
log-adjacency-changes
area 1 range 155.55.0.0 255.255.0.0 cost 10
network 155.55.55.2 0.0.0.0 area 1
network 192.168.12.2 0.0.0.0 area 1
network 192.168.25.2 0.0.0.0 area 0

Now R3 and R4 both will receive the respective route from R5 via Area0 and will also originate a type 3 LSA for area 2. So R3 will receive the type3 LSA originated by R4 and R4 should receive the type 3 LSA originated by R3.

R3#sh ip ospf database

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

Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#                  Checksum
155.55.0.0      1.1.1.1               207         0x80000003    0x00F36F
155.55.0.0      2.2.2.2             1861        0x80000002   0x003225

Summary Net Link States (Area 2)

Link ID         ADV Router      Age         Seq#                  Checksum
155.55.0.0      3.3.3.3             98          0x80000001     0x00C01C
155.55.0.0      4.4.4.4             99          0x80000001     0x00A236

R4#sh ip ospf database

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

Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#             Checksum
155.55.0.0      1.1.1.1         305         0x80000003    0x00F36F
155.55.0.0      2.2.2.2         1958     0x80000002 0x003225

Summary Net Link States (Area 2)

Link ID         ADV Router      Age         Seq#            Checksum
155.55.0.0      3.3.3.3         170         0x80000001   0x00C01C
155.55.0.0      4.4.4.4         170         0x80000001   0x00A236

OSPF works on LSDB and now R3 should advertise summary LSA received from R4 in Area 2 to Area 0 and R4 should advertise summary LSA received from R3 in Area 2 to Area 0.

However this is not correct, OSPF behaves like a distance routing protocol here and only advertise routes which are installed in RIB via ospf & this is per RFC 2328 clause 12.4.3. This is the reason why we do not see in above output LSA originated in Area 0 for 155.55.0.0 by R3 or R4 as in both routers route installed in RIB is via area0.

R3#sh ip route 155.55.0.0
Routing entry for 155.55.0.0/16
Known via “ospf 1”, distance 110, metric 129, type inter area
Last update from 192.168.35.5 on Serial0/0, 00:09:32 ago
Routing Descriptor Blocks:
* 192.168.35.5, from 1.1.1.1, 00:09:32 ago, via Serial0/0    >>>> serial 0/0 is in Area 0
Route metric is 129, traffic share count is 1

R4#sh ip route 155.55.0.0
Routing entry for 155.55.0.0/16
Known via “ospf 1”, distance 110, metric 129, type inter area
Last update from 192.168.45.5 on Serial0/0, 00:08:53 ago
Routing Descriptor Blocks:
* 192.168.45.5, from 1.1.1.1, 00:08:53 ago, via Serial0/0   >>>> serial 0 is in area0.
Route metric is 129, traffic share count is 1

Now a distribute-list will be applied on R3 and will block the route to be installed in the RIB.

ip access-list standard DENY
deny   155.55.0.0 0.0.255.255
permit any
!

Router ospf 1

distribute-list DENY in Serial0/0

R3#sh ip route 155.55.0.0
% Network not in table

Though route is in ospf RIB:

R3#sh ip ospf rib 155.55.0.0

OSPF local RIB for Process 1
Codes: * – Best, > – Installed in global RIB

*   155.55.0.0/16, Inter, cost 129, area 0
flags 0x0, SPF Instance 48, age 00:21:33
via 192.168.35.5, Serial0/0
flags 0x0, LSA: 3/155.55.0.0/1.1.1.1

R3#sh ip ospf database

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

Summary Net Link States (Area 2)

Link ID         ADV Router      Age         Seq#       Checksum
155.55.0.0      4.4.4.4         1097        0x80000001 0x00A236

We see R3 stopped injecting the route in area 2. though R4 is still injecting it because it has the route installed into its RIB.

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