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Default-Information Orginate <Always>
Posted on 2013-01-01
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
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
[X]
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7 Comments
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
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
Active 1 day ago
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 ??
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 ??
Active 1 day ago
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>
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>
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:
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.
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:
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...
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
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
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
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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