advertise default route

[Arjun Dabol]

just want to understand what is meaning of when we say : advertise default route in a routing protocol For ex....

"advertise default route in a routing protocol in OSPF" 

can someone help me understand the very basic concept of this. ?

and why we do this ?

Thanks

[Julio E. Moisa]

Hi

This term is used when you are going to redistribute a default route (like 0.0.0.0 0.0.0.0) into a routing protocol to be advertised to other peers in order provide a path to reach Internet or unknown prefixes.

For example an OSPF edge router is connected to the Internet provider, so an easy way to advertise the path to Internet is advertise this default route to the other OSPF peers dynamically in order to install an entry on their routing table.

Each routing protocol has methods to advertise a default route, example:

OPSF
Router ospf X
Default-information originate   <--- it requires a static default route configured on the device like: ip route 0.0.0.0 0.0.0.0 <IP next hop>

or 

Default-information originate always    <--- with 'always' command you will advertise a default route with or without a static default route.

Imagine the follow scenario:

ISP ---- R1 ---- R2

R1

interface FastEthernet0/0
description Point-to-Point-with-Internet-Provider
ip address <public ip and subnet mask>

interface FastEthernet0/1
ip address 10.12.0.1 255.255.255.0

Ip route 0.0.0.0 0.0.0.0 <IP next hop to ISP>

router ospf 100
network 10.12.0.1 0.0.0.0 area 0
default-information originate

R2

interface FastEthernet0/0
ip address 10.12.0.2 255.255.255.0


router ospf 100
network 10.12.0.2 0.0.0.0 area 0

R2#sh ip route
Codes: L - local, 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, H - NHRP, l - LISP
a - application route
+ - replicated route, % - next hop override

Gateway of last resort is 10.12.0.1 to network 0.0.0.0

O*E2 0.0.0.0/0 [110/1] via 10.12.0.1, 00:02:26, FastEthernet0/0
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.12.0.0/24 is directly connected, FastEthernet0/0
L 10.12.0.2/32 is directly connected, FastEthernet0/0

EIGRP
Interface G0/0
Ip summary-address Eigrp <ID Process> 0.0.0.0 0.0.0.0

Hope it is useful.

:-)

[Joseph W. Doherty · Accepted solution]

Are you familiar with injecting static routes into a dynamic routing protocol?

If so, injecting a default route is a similar and special case.  It allows a router or routers to advertise a default route to other routers using the same (both logically and physically) dynamic routing protocol.

Normally this is done so all routers within the same dynamic routing topology have a default route in their route table, without the need to go to each router and add a static like ip route 0.0.0.0 0.0.0.0 next-hop-IP.

Like other dynamic routes, it can be changed or withdrawn by the router injecting it, again without having to manually update all the other routers.