OSPF Area Range – Active vs Passive Advertisment

I was troubleshooting an OSPF area range summarization and came upon something I haven’t seen before called Passive Advertisement. There weren’t too many Cisco documents that explained it so I decided to post a really quick description explaining it in little detail and where you could see it . This could be useful for the CCIE troubleshooting section, when dealing with OSPF area summarization problems.

I will use R3 to demonstrate. This router is connected to area 0 and area 1 which makes it the only ABR connecting the two areas. R3 should be sending a summary route 4.4.0.0/16 for the two component routes 4.4.4.0/24 and 4.4.5.0/24. Looks pretty simple. To verify, I check the output of show ip ospf to make sure the area 0 range 4.4.0.0 255.255.0.0 command is configured:

R3#sh ip ospf
Routing Process "ospf 1" with ID 10.3.3.3
Start time: 00:00:23.404, Time elapsed: 00:01:06.080
Supports only single TOS(TOS0) routes
Supports opaque LSA
Supports Link-local Signaling (LLS)
Supports area transit capability
It is an area border and autonomous system boundary router
Redistributing External Routes from,
rip
Router is not originating router-LSAs with maximum metric
Initial SPF schedule delay 5000 msecs
Minimum hold time between two consecutive SPFs 10000 msecs
Maximum wait time between two consecutive SPFs 10000 msecs
Incremental-SPF disabled
Minimum LSA interval 5 secs
Minimum LSA arrival 1000 msecs
LSA group pacing timer 240 secs
Interface flood pacing timer 33 msecs
Retransmission pacing timer 66 msecs
Number of external LSA 3. Checksum Sum 0x022C12
Number of opaque AS LSA 0. Checksum Sum 0x000000
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. 2 normal 0 stub 0 nssa
Number of areas transit capable is 0
External flood list length 0
Area BACKBONE(0)
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm last executed 00:00:47.192 ago
SPF algorithm executed 3 times
Area ranges are
4.4.0.0/16 Passive Advertise +--------------------------Passive
Number of LSA 7. Checksum Sum 0x02B97F
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
Area 1
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm last executed 00:00:47.200 ago
SPF algorithm executed 3 times
Area ranges are
Number of LSA 6. Checksum Sum 0x0262B7
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0

As you can see area 0 range is configured, but it has passive advertise term right next to it and no summary route present in area 0. Next, I check to make sure my component routes are in the routing table being learned from OSPF (4.4.4.0/24 and 4.4.5.0/24):
R3#sh ip route ospf
<..>
4.0.0.0/24 is subnetted, 2 subnets
O IA 4.4.4.0 [110/11] via 10.1.34.4, 00:00:37, FastEthernet0/1
O IA 4.4.5.0 [110/11] via 10.1.34.4, 00:00:37, FastEthernet0/1


Both routes are there, but here is the problem, they are all Inter Area LSA Type 3 (IA) routes. The ABR can only summarize Intra Area (LSA Type 1 and 2) routes, so this is the problem. The source of these component routes is advertising them from the wrong area, area 1. Once I make the correction to advertise them into area 0 instead of area 1, you can see that area 0 range has the words Active Advertise next it.


R3#sh ip ospf
Routing Process "ospf 1" with ID 10.3.3.3
Start time: 00:00:23.404, Time elapsed: 00:21:05.884
Supports only single TOS(TOS0) routes
Supports opaque LSA
Supports Link-local Signaling (LLS)
Supports area transit capability
It is an area border and autonomous system boundary router
Redistributing External Routes from,
rip
Router is not originating router-LSAs with maximum metric
Initial SPF schedule delay 5000 msecs
Minimum hold time between two consecutive SPFs 10000 msecs
Maximum wait time between two consecutive SPFs 10000 msecs
Incremental-SPF disabled
Minimum LSA interval 5 secs
Minimum LSA arrival 1000 msecs
LSA group pacing timer 240 secs
Interface flood pacing timer 33 msecs
Retransmission pacing timer 66 msecs
Number of external LSA 3. Checksum Sum 0x022C12
Number of opaque AS LSA 0. Checksum Sum 0x000000
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. 2 normal 0 stub 0 nssa
Number of areas transit capable is 0
External flood list length 0
Area BACKBONE(0)
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm last executed 00:00:12.320 ago
SPF algorithm executed 9 times
Area ranges are
4.4.0.0/16 Active(11) Advertise +--------------------------Active
Number of LSA 5. Checksum Sum 0x01C00A
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
Area 1
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm last executed 00:00:12.332 ago
SPF algorithm executed 7 times
Area ranges are
Number of LSA 5. Checksum Sum 0x01D43C
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0

This is a good quick trick to know, what Passive Advertise and Active Advertise means in regards to area range summary routes. The passive advertise term will be added anytime there is a problem with the component routes, whether they are not in the routing table or don’t have the correct type (intra area vs inter area). Active advertise term, means it is actively advertising the area range and everything is working as expected.