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Default-Information Orginate <Always>

Posted on 2013-01-01
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Last Modified: 2013-01-15
I have been reading about the meaning of the command Default-Information originate, what I got from the explanation is:

When you configure a router under OSPF with default-information originate

then you are instructing the router to use its own default route. So it does not maintain a default learned from its neighbor.

when u add Always keyword:

then you are instructing the router to always and unconditionally use its own default route. So it does not maintain a default learned from its neighbor.

===But I am not sure in which case this command needs to be brought up

Any help ?

thanks
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Question by:jskfan
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by:gt2847c
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An OSPF router in a Normal OSPF Area will not generate (advertise) a default route.  If you want a default route to exist in a normal area, you must use the "default-information originate" command so that it will advertise a default route (if it has one in the route table).  

If the OSPF router advertising the default route depends on a dynamically learned default route (BGP, RIP, other), route flapping of the default route from the other protocol will cause excessive route changes in OSPF.  The "always" keyword is used to prevent this instability in the OSPF domain.  It forces the router to advertise the default whether it has it in the route table or not.

With Stub areas, the ABR will inject a default into the Stub rather than provide a full route table.

Here's two Cisco articles that may help...

How Does OSPF Generate Default Routes?

How OSPF Injects a Default Route into a Normal Area
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by:jskfan
ID: 38735626
I have configured OSPF routers in multiple areas and multiple area types with external areas like EIGRP and RIP, and I did not have to configure the  :Default-Information Orginate  with or without Always.
As you said:
<<With Stub areas, the ABR will inject a default into the Stub rather than provide a full route table>>
The same as with Totally Stubby area.

So in which case you configure :Default-Information Orginate , either with or without the Always keyword ??
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by:thpipfh
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by:gt2847c
ID: 38737422
If thpipfh's link above doesn't explain enough, let us know and we'll try something else.
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by:jskfan
ID: 38739359
I understand what Default-Information Originate <Always> does.


I did not run into a scenario where I had to use it even though I did Redistribution between different Routing Protocols and area types, and evrything worked fine.
.. this is why I wanted to know when it required to use Default-Information Originate <Always>
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gt2847c earned 333 total points
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To make sure we're covering things, I'll get a bit lengthy, but please bear with me as I hope this will answer your question.

Default-Information Originate does not affect the local routing table of the router this is configured on.  If a router has a static default route or receives a default route sent to it from a routing protocol, it will use that default route (when appropriate).  Default-Route Originate allows OSPF to take that default route and redistribute it into the OSPF route database.  That will allow other routers speaking OSPF that would not otherwise have a default route to receive a default route.  

Remember that a router can maintain several routing tables/databases, but only the active route table matters to the router itself.  All the other databases/routing information base (RIB) are used to either populate the local route table or to share with other routers speaking a common routing protocol.  The Default-Information Originate will place a default route from that router's active routing table into the OSPF database which will then cause it to share that default route to other OSPF neighbors when it does link state updates with them.  This will allow the other OSPF speakers to become aware of the default route known by the router with Default-Information Originate.

I mentioned above that a Stub Area Border Router will inject a default route into a stub area even if the ABR does not have a default route (and does not require the Default-Information Originate) as Stub areas do not receive external routes.  Totally Stubby areas do not even receive inter-area summary routes, just the default.  That's done for efficiency so as keep the number of routes in the Stub and Totally Stubby areas to a minimum.

Normal area OSPF routers will not create an OSPF database entry for a locally available default route unless you specify the Default-Information Originate.  If they happen to have one, they'll use it with their own active routing table, but they won't share.  You can verify this by comparing the OSPF database with the active route table.  If you do a "show ip ospf database" and look for the local router ID, you won't see the default route listed locally.  If you then configure the Default-Information Originate, then check the table, it should show up as advertised by that local router ID.


Here's an example from my lab router:
(Note Router "core" has a loopback of 10.255.1.1 which is the OSPF router ID)

Active Route Table:
core#sh 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 172.18.39.66 to network 0.0.0.0

     172.18.0.0/26 is subnetted, 1 subnets
C       172.18.39.64 is directly connected, FastEthernet1/0.900
     172.30.0.0/27 is subnetted, 1 subnets
C       172.30.255.0 is directly connected, FastEthernet0/0
     10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
O E1    10.0.0.0/8 [110/31] via 172.18.39.66, 19:09:30, FastEthernet1/0.900
C       10.255.1.1/32 is directly connected, Loopback0
     192.0.0.0/29 is subnetted, 1 subnets
O       192.0.0.16 [110/11] via 172.18.39.66, 19:09:30, FastEthernet1/0.900
     111.222.0.0/16 is variably subnetted, 2 subnets, 2 masks
C       111.222.20.160/28 is directly connected, FastEthernet1/0.950
O E1    111.222.0.0/16 
           [110/31] via 172.18.39.66, 19:09:30, FastEthernet1/0.900
S    192.168.1.0/24 [1/0] via 172.18.39.67
C    192.168.100.0/24 is directly connected, FastEthernet1/0.700
S*   0.0.0.0/0 [1/0] via 172.18.39.66
O E1 222.11.128.0/17 [110/31] via 172.18.39.66, 19:09:30, FastEthernet1/0.900
O E1 172.16.0.0/12 [110/31] via 172.18.39.66, 19:09:30, FastEthernet1/0.900         

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Notice the Gateway of last resort is listed (selected by the local active route table) and there is a static (S* route) listed in the route table showing where it came from.  This is the active route table that the router itself is using to forward packets.

OSPF Database - This is the list of routes known to OSPF.  The active route table will only include the single best route, but in the OSPF database there can be multiple routes for a single destination, all of the link state data is kept and shared so that each router can build its own model of the network and make selections for the best route.
core#sh ip ospf data

            OSPF Router with ID (10.255.1.1) (Process ID 2)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
10.255.1.1      10.255.1.1      597         0x80000B4B 0x008820 4
10.255.1.4      10.255.1.4      1362        0x800005A1 0x00B116 1
172.18.39.66    172.18.39.66    1001        0x80000619 0x005357 2
172.18.39.85    172.18.39.85    1008        0x80000BD3 0x004D22 1

                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
172.18.39.85    172.18.39.85    168         0x80000621 0x007EFA
192.0.0.18      10.255.1.4      1362        0x8000059E 0x008BF9

                Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
10.0.0.0        10.255.1.4      371         0x800005A1 0x00184A 0
111.222.0.0     10.255.1.4      371         0x800005A1 0x002635 0
172.16.0.0      10.255.1.4      371         0x800005A1 0x00D8E5 0
192.168.1.0     10.255.1.1      856         0x80000AEC 0x003952 0
222.11.128.0    10.255.1.4      371         0x800005A1 0x005909 0 

Open in new window


Notice that there isn't a 0.0.0.0 link ID listed above?

Now let's install the Default-Information Originate and recheck the database:
core#config t
core(config)#router ospf 2
core(config-router)#default-information originate 
core(config-router)#end
core#sh ip ospf data

            OSPF Router with ID (10.255.1.1) (Process ID 2)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
10.255.1.1      10.255.1.1      775         0x80000B4B 0x008820 4
10.255.1.4      10.255.1.4      1540        0x800005A1 0x00B116 1
172.18.39.66    172.18.39.66    1179        0x80000619 0x005357 2
172.18.39.85    172.18.39.85    1186        0x80000BD3 0x004D22 1

                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
172.18.39.85    172.18.39.85    346         0x80000621 0x007EFA
192.0.0.18      10.255.1.4      1540        0x8000059E 0x008BF9

                Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
0.0.0.0         10.255.1.1      3           0x80000001 0x00C6B6 2
10.0.0.0        10.255.1.4      548         0x800005A1 0x00184A 0
111.222.0.0     10.255.1.4      548         0x800005A1 0x002635 0
172.16.0.0      10.255.1.4      549         0x800005A1 0x00D8E5 0
192.168.1.0     10.255.1.1      1034        0x80000AEC 0x003952 0
222.11.128.0    10.255.1.4      549         0x800005A1 0x005909 0

Open in new window


Notice that the Link ID 0.0.0.0 (default route) is now there advertised by 10.255.1.1 (router "core")?  That was an "externally" available route (static in this case) which is why it's listed in the Type-5 section of the database. That means it is now in the OSPF route database and will get flooded to other OSPF speakers so that they will know about it.  That way other OSPF speakers know there is a default route and can make use of it.

Let me know if that clears it up any...
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Author Closing Comment

by:jskfan
ID: 38761836
Thank you
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