Where does the default route come from ?

[yangfrank]

Dear All 

As we know, the edge router in client side gets default route from ISP' edge router. Where does the default route come from in ISP? If the ISP edge router has full route table, it produces there. If not, it comes from its upstream router until full routes table(only BGP). Please correct me if I am wrong. Thank you. 

[Roberto Kippins]

Hi, with bgp the isp may have the full routing table of the internet but at the same time the isp can send just a default route to the customer or the full routing table, the customer decides what suits them best. The advertisement of a default route is done on a per neighbor basis with bgp or a bgp peer group.

[yangfrank]

Thanks for your reply. What I mean is the default route originate from the router which has full routes ( about 300 000)table in ISP if the client want to connect to internet, do you think so ? 

[Roberto Kippins · Accepted solution]

Yes it does originate from with the full routes, even though the isp router has the full routes, it can choose what routes to advertise to a neighbor (customer/client) router, this includes a default route.

[gnath.net78]

Hello.

While establishing BGP with ISP, there are three options for a client:

> Default route (used in general) - client router forwards every request (for destination route) to ISP router.

> Full route - ISP router's complete routing table would be received and maintained in client router.

> Partial route - Only specific route would be received from ISP (Head office - Branch office scenario)

ISP generate default route from their peering router that should have full routing table or knowledge of the router having it. A local ISP may generate default to their client and have one received from their upstream or global / larger ISP. Hope this helps.