OSPF Troubleshooting – Default Route Not Advertised to All Areas

ciscoospfpacket-tracer

I am trying to find the reason why R1 doesn't have the default-route, propagated and generated by my Internet-PAT-Router, in his routing-table. The weird thing is, that R2 actually has an entry for the default-route!

I am sorry if it is a really weird simple error which is easy to find, but I just couldn't find it (even after long googling and comparing R1 and R2).

I am able to ping like this:

PC -----> R1
R1 -----> ABR
PC --!--> ABR
R1 --!--> Internet (I have simulated the Internet with a simple webserver)
R2 -----> Internet 
Router in the area down below the area 45 -----> Internet

Gigabit-Interfaces are for the LAN-Networks which are configured as passive-interface.

Here's the overview over my network:

Configuration of R1:

en
 conf t
 no ip domain-lookup
 ipv6 unicast-routing

 int loopback 0
  ip address 1.1.1.1 255.255.255.255
  exit

 ipv6 router ospf 1
  router-id 1.1.1.1
  auto-cost reference-bandwidth 100000
  passive-interface g0/0
  area 45 range 2001:db8:18:4565::/56
  exit

 router ospf 1
  auto-cost reference-bandwidth 100000
  passive-interface g0/0
  area 45 range 10.45.65.0 255.255.255.0
  exit

 int s0/0/0
  ip address 10.45.0.6 255.255.255.252
  ipv6 address 2001:db8:18:4502::2/64
  ipv6 ospf 1 area 45
  ip ospf 1 area 45
  ip ospf hello-interval 3
  ipv6 ospf hello-interval 3
  no sh

 int g0/0
  ip address 10.45.65.1 255.255.255.0
  ipv6 add 2001:db8:18:4565::1/64
  no sh

  exit

 access-list 100 deny ip any host 224.0.0.5
 access-list 100 deny ip any host 244.0.0.6
 access-list 100 permit ip any any

 int g0/0
  ip access-group 100 in
 exit

Configuration of R2:

en
 conf t
 no ip domain-lookup
 ipv6 unicast-routing

 int loopback 0
  ip address 1.1.1.1 255.255.255.255
  exit

 ipv6 router ospf 1
  router-id 1.1.1.1
  auto-cost reference-bandwidth 100000
  exit

 router ospf 1
  auto-cost reference-bandwidth 100000
  network 10.45.0.0 0.0.0.3 area 45
  exit

 int s0/0/0
  ip address 10.45.0.2 255.255.255.252
  ipv6 address 2001:db8:18:4501::2/64
  ipv6 ospf 1 area 45
  ip ospf hello-interval 3
  ipv6 ospf hello-interval 3
  no sh

 int g0/0
  ip address 10.45.64.1 255.255.255.0
  ipv6 add 2001:db8:18:4564::1/64
  ipv6 ospf 1 area 45
  ip ospf hello-interval 3
  ipv6 ospf hello-interval 3
  no sh

  exit
  exit

Configuration of the ABR:

en
 conf t
 no ip domain-lookup
 ipv6 unicast-routing

 int loopback 0
  ip address 2.2.1.1 255.255.255.255
  exit

 ipv6 router ospf 1
  area 45 range 2001:db8:18:4500::/56
  auto-cost reference-bandwidth 100000
  router-id 2.2.1.1
  exit

 router ospf 1
  area 45 range 10.45.0.0 255.255.0.0
  area 0 range 10.0.0.0 255.255.0.0
  auto-cost reference-bandwidth 100000
  network 10.45.0.0 0.0.0.255 area 45
  network 10.0.0.0 0.0.0.255 area 0

 int s0/1/0
  ip add 10.45.0.1 255.255.255.252
  ipv6 add 2001:db8:18:4501::1/64
  ipv6 ospf 1 area 45
  ip ospf hello-interval 3
  ipv6 ospf hello-interval 3
  no sh

 int s0/1/1
  ip add 10.45.0.5 255.255.255.252
  ipv6 add 2001:db8:18:4502::1/64
  ipv6 ospf 1 area 45
  ip ospf hello-interval 3
  ipv6 ospf hello-interval 3
  no sh

 int s0/0/0
  ip add 10.0.0.2 255.255.255.252
  ipv6 add 2001:db8:18:0001::2/64
  ipv6 ospf 1 area 0
  ip ospf hello-interval 3
  ipv6 ospf hello-interval 3
  no sh

 int s0/0/1
  ip add 10.0.0.6 255.255.255.252
  ipv6 add 2001:db8:18:0002::2/64
  ipv6 ospf 1 area 0
  ip ospf hello-interval 3
  ipv6 ospf hello-interval 3
  no sh

  exit

This is the "show ip ospf int"-command on the ABR

Serial0/1/0 is up, line protocol is up
  Internet address is 10.45.0.1/30, Area 45
  Process ID 1, Router ID 2.2.1.1, Network Type POINT-TO-POINT, Cost: 64
  Transmit Delay is 1 sec, State POINT-TO-POINT, Priority 0
  No designated router on this network
  No backup designated router on this network
  Timer intervals configured, Hello 3, Dead 40, Wait 40, Retransmit 5
    Hello due in 00:00:02
  Index 1/1, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 1 , Adjacent neighbor count is 1
    Adjacent with neighbor 1.1.1.1
  Suppress hello for 0 neighbor(s)
Serial0/1/1 is up, line protocol is up
  Internet address is 10.45.0.5/30, Area 45
  Process ID 1, Router ID 2.2.1.1, Network Type POINT-TO-POINT, Cost: 64
  Transmit Delay is 1 sec, State POINT-TO-POINT, Priority 0
  No designated router on this network
  No backup designated router on this network
  Timer intervals configured, Hello 3, Dead 40, Wait 40, Retransmit 5
    Hello due in 00:00:02
  Index 2/2, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 0 msec, maximum is 0 msec
  Suppress hello for 0 neighbor(s)
Serial0/0/0 is up, line protocol is up
  Internet address is 10.0.0.2/30, Area 0
  Process ID 1, Router ID 2.2.1.1, Network Type POINT-TO-POINT, Cost: 64
  Transmit Delay is 1 sec, State POINT-TO-POINT, Priority 0
  No designated router on this network
  No backup designated router on this network
  Timer intervals configured, Hello 3, Dead 40, Wait 40, Retransmit 5
    Hello due in 00:00:02
  Index 3/3, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 1 , Adjacent neighbor count is 1
    Adjacent with neighbor 3.3.3.3
  Suppress hello for 0 neighbor(s)
Serial0/0/1 is up, line protocol is up
  Internet address is 10.0.0.6/30, Area 0
  Process ID 1, Router ID 2.2.1.1, Network Type POINT-TO-POINT, Cost: 64
  Transmit Delay is 1 sec, State POINT-TO-POINT, Priority 0
  No designated router on this network
  No backup designated router on this network
  Timer intervals configured, Hello 3, Dead 40, Wait 40, Retransmit 5
    Hello due in 00:00:02
  Index 4/4, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 1 , Adjacent neighbor count is 1
    Adjacent with neighbor 4.4.4.4
  Suppress hello for 0 neighbor(s)

And the command on R1 doesn't show anything.

Best Answer

The first issue is that you configured same Router-ID (RID) router-id 1.1.1.1 for R1 and R2 in area 45.

All OSPF router should have different RIDs across the OSPF network domain.

And the reason PC1 cannot reach ABR (and other OSPF routers in the network also cannot reach the IPv4 and IPv6 addresses of R1's g0/0 interface) is that you did not enable either OSPFv2 or OSPFv3 on interface g0/0 of R1, thus the IPv4 subnet 10.45.65.0/24 and IPv6 network 2001:DB8:18:4565::/64 are unknown to other OSPF routers in the network:

 #On R1#
 !
 ipv6 router ospf 1
  router-id 1.1.1.1
  auto-cost reference-bandwidth 100000
  passive-interface g0/0
  area 45 range 2001:db8:18:4565::/56
  exit
 !
 router ospf 1
  auto-cost reference-bandwidth 100000
  passive-interface g0/0
  area 45 range 10.45.65.0 255.255.255.0
  exit
 !
 int g0/0
  ip address 10.45.65.1 255.255.255.0
  ipv6 add 2001:db8:18:4565::1/64
  no sh

The command area 45 range ... is to summarize the more specific routes/prefixes within this range, and it should be used only on ABRs.

To fix this issue, you can enable OSPFv2 and/or OSPFv3 on R1's interface g0/0 like you did on interface s0/0/0:

 int g0/0
  ...
  ...
  ipv6 ospf 1 area 45
  ip ospf 1 area 45
  no sh

Last but not least, you need to ensure that you run OSPFv2 and/or OSPFv3 consistently across your network (on necessary interfaces).

OSPF Backbone

Why is area 0 the backbone area in OSPF? Why must all other areas connect to it?

This is explained very well in RFC 3509, Section 1.2¹:

1.2 Motivation

In OSPF domains the area topology is restricted so that there must be a backbone area (area 0) and all other areas must have either physical or virtual connections to the backbone. The reason for this star-like topology is that OSPF inter-area routing uses the distance-vector approach and a strict area hierarchy permits avoidance of the "counting to infinity" problem. OSPF prevents inter-area routing loops by implementing a split-horizon mechanism, allowing ABRs to inject into the backbone only Summary-LSAs derived from the intra-area routes, and limiting ABRs' SPF calculation to consider only Summary-LSAs in the backbone area's link-state database.

OSPF is usually considered a link-state protocol. What some people miss is that OSPF uses both link-state protocol and distance-vector protocol algorithms.

Routes within the backbone, or a non-backbone area are computed as a link-state protocol does (ref Dijkstra's algorithm).
When OSPF must carry non-backbone routes through the backbone, it uses some distance-vector behavior (i.e. parts of the Bellman Ford algorithm) to propagate Type3 LSA metrics into non-backbone areas.

Simple example of OSPF's distance-vector behavior:

<-- Area 5 --><-- Area 0 --><--           Area 4           -->

R5-----------R1-----------R2------------R3---------------------R4
     Cost 3      Cost 5        Cost 7            Cost 12

               LSA-->          LSA-->
               Type3 LSA       Type3 LSA
               {From R1}       {From R2}
               R5 cost is 3    R5 cost is 8

Consider what happens to a /32 Loopback route for R5.

R5 sends a Type1 LSA containing the /32 Loopback
R1 (Area 5 ABR), is connected to Area 0; it translates the Type1 LSA into a Type3 LSA with a cost of 3.
R2 (Area 4 ABR) receives R1's Type3 LSA (metric 3) and changes the metric to R5's Loopback, based on R2's cost to R1. Now R2's Type3 LSA for R5 has a cost of 8. This is the distance-vector behavior I mentioned above.

Requiring all non-backbone routes to go through the backbone is a loop-prevention mechanism.

Connecting non-backbone OSPF areas at an ABR

If 2 areas aren't connected through area 0 (discontiguous), how does OSPF behaving as a link state protocol increase the possibility of routing loops?

As we saw above, OSPF uses distance-vector behavior to send routes through the Area 0 backbone. Distance-vector protocols have well-known limits, such as the count-to-infinity problem. OSPF would be vulnerable to the same issues, if we didn't have boundaries on its behavior.

¹_{RFC 3509 describes Cisco IOS's ABR behavior}

Best Answer

Related Solutions

Cisco – Summarized OSPF routes advertised

OSPF Routing – Why Must All Areas Connect to Area 0 Backbone?

OSPF Backbone

Connecting non-backbone OSPF areas at an ABR

Related Topic