OSPF UNABLE TO INSTALL EXTERNAL ROUTES

There have been situations wherein we see that in ospf external/nssa external routes are not able to install due to forwarding address is not in the routing table as intra O or inter-area OIA route. So we have to configure the network in a way that either we have the route via O or OIA to install that external route in the routing table.

However if in some corner cases customer cannot make changes to accommodate the above and may have the forwarding address from some other source. In this situation we can use a command local-rib-criteria forwarding-address which will only check the ospf rib and not global rib to install the route in the routing table.


 

Note: Ospf only adds the forwarding address if the next-hop of the route is in ospf and not as an external route else it will set the forwarding address to the interface on which peering is configured.

First look at the behavior without using the command.

R2#sh run | sec router ospf

router ospf 1

router-id 2.2.2.2

area 1 nssa no-summary

network 172.168.1.1 0.0.0.0 area 0

network 192.168.1.2 0.0.0.0 area 1

network 192.168.24.2 0.0.0.0 area 0

 

R2#sh ip ospf database nssa-external

OSPF Router with ID (2.2.2.2) (Process ID 1)

Type-7 AS External Link States (Area 1)

LS age: 320

Options: (No TOS-capability, Type 7/5 translation, DC, Upward)

LS Type: AS External Link

Link State ID: 155.55.55.87 (External Network Number )

Advertising Router: 1.1.1.1

LS Seq Number: 80000002

Checksum: 0xC855

Length: 36

Network Mask: /32

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

MTID: 0

Metric: 20

Forward Address: 10.1.1.6

External Route Tag: 0

 

R2#sh ip route 10.1.1.6

Routing entry for 10.1.1.6/32

Known via “static”, distance 1, metric 0

Routing Descriptor Blocks:

* 192.168.1.1

Route metric is 0, traffic share count is 1

 

R2#sh ip route 155.55.55.87

% Network not in table

 

Let us configure the local-rib command for forwarding address and see the behavior:

R2#sh run | sec router

router ospf 1

local-rib-criteria forwarding-address

 

R2#sh ip ospf database nssa-external

OSPF Router with ID (2.2.2.2) (Process ID 1)

Type-7 AS External Link States (Area 1)

 

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

LS age: 117

Options: (No TOS-capability, Type 7/5 translation, DC, Upward)

LS Type: AS External Link

Link State ID: 155.55.55.87 (External Network Number )

Advertising Router: 1.1.1.1

LS Seq Number: 80000002

Checksum: 0xC855

Length: 36

Network Mask: /32

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

MTID: 0

Metric: 20

Forward Address: 10.1.1.6

External Route Tag: 0

 

R2#sh ip route 10.1.1.6

Routing entry for 10.1.1.6/32

Known via “static”, distance 1, metric 0

Routing Descriptor Blocks:

* 192.168.1.1

Route metric is 0, traffic share count is 1

 

R2#sh ip route 155.55.55.87

Routing entry for 155.55.55.87/32

Known via “ospf 1”, distance 110, metric 20, type NSSA extern 2, forward metric 20

Last update from 192.168.1.1 on Ethernet0/0, 00:04:44 ago

Routing Descriptor Blocks:

* 192.168.1.1, from 1.1.1.1, 00:04:44 ago, via Ethernet0/0

Route metric is 20, traffic share count is 1

If we have the route in the routing table from ospf but as an external route then this command will not work as the condition to install the route has not met, this command is just changing the default global routing table lookup to occur on the ospf local rib. Rest all condition must match.

Please see the below link for more information on local-rib-criteria comaand:

http://www.cisco.com/en/US/docs/ios/12_4t/12_4t15/ht_osrib.html#wp1068050

link for more information on forwarding address:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009405a.shtml#topic1

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