default-information originate in different routing protocols

RIP

To generate a default route into Routing Information Protocol (RIP), use the default-informationoriginate command in router configuration mode. To disable this feature, use the no form of this command.

Example:

router rip 
 version 2
 network 172.17.16.0
 default-information originate route-map condition
!
route-map condition permit 10
 match ip address 10
 set interface s1/0
!
access-list 10 permit 172.17.16.0 0.0.255.255
setinterface Indicates where to forward packets that pass a match clause of a route map for policy routing.

OSPF:

Default routes injected into a normal area can be originated by any OSPF router. The OSPF router does not, by default, generate a default route into the OSPF domain. In order for OSPF to generate a default route, you must use the default-information originate command.

There are two ways to advertise a default route into a normal area. The first is to advertise 0.0.0.0 into the OSPF domain, provided the advertising router already has a default route. The second is to advertise 0.0.0.0 regardless of whether the advertising router already has a default route. The second method can be accomplished by adding the keyword always to the default-information originate command.

Example:

router ospf 2
 network 5.0.0.0 0.255.255.255 area 1
 network 6.0.0.0 0.255.255.255 area 0
 default-information originate
ip route 0.0.0.0 0.0.0.0 6.0.0.3

Because it has a default route, Router originates a type 5 LSA with a link ID of 0.0.0.0. This is the result of the default-information originate command in its OSPF configuration.You can also add the always keyword to the default-information originate command to make a router originate a 0.0.0.0 type 5 LSA even if the router does not have a default route in its routing table.

BGP:

refer to item 6 in

https://yingsnotebook.wordpress.com/2017/03/29/bgp-network-summary-ii/

The default-information originate, redistribution from a different source, and network 0.0.0.0 are all similar in the resulting effect: they will inject the default route into BGP RIB and it will be advertised to all BGP neighbors. The difference is in the origin of the default route that is injected into BGP:

  • default-information originate causes the default route to be artificially generated and injected into the BGP RIB, regardlessly of whether it is present in the routing table.
  • Redistribution and network 0.0.0.0 will inject the default route into BGP only if the default route is currently present in the routing table, and additionally in the case of redistribution, if learned by a specific source protocol we are redistributing from.

The neighbor X.X.X.X default-originate is different in that the default route will be advertised only to this specific BGP neighbor and not to all existing BGP neighbors as with the previous approaches. The default route will not be present in the BGP RIB of the router that is configured with the neighbor X.X.X.X default-originate command and so it won’t be generally advertised to all BGP neighbors. At the same time, this command is similar to the default-information originate in that the default route is artificially generated and does not need to be present in the routing table.

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