BGP - how does IGP know the right route?

Question

Let's consider a host A in an autonomous system AS100. Suppose it wants to communicate with host B in another AS, say, AS200. In AS100 there are several routers. Within one AS, the rotuing protocols used are Internal gateway protocols such as OSPF or RIP.

AS100 needs to know the path through other AS-es to AS200, so the edge routers in AS100 are BGP routers, BGP is an external gateway protocol used for routing between autonomous systems.

If AS100 is connected to, say, two ASes (it has two eBGP routers), how does host A inside AS100 know which BGP router to talk to, in order to communicate with host B in AS200? Host A tells its router that it wants to communicate with host B. Now, that router would need to know the path to host B. But host B is outside AS100.

Do eBGP routers send their routing tables to all routers (both BGP and IGP) in one AS? I mean, 'normal' routers inside AS have to know a lot, they have to choose the appropriate border router so that host A can calk to host B.

If all routers know the same thing, then why would anyone need iBGP? Generally BGP routing tables are big, so it's probably a false statement, but I'm not sure how could host A know the right path if its neighbour routers didn't know all the routes the border routers have.

Answer 1

There are multiple ways to do this.

The other ASes could be sending a default route, or the router in AS100 could just have a default route configured.

The other ASes could just advertise their own routes through BGP.

The other ASes could advertise full BGP routes to AS100.

BGP has many factors which could play into the decision of which way to switch traffic destined to another AS. This is the subject of entire books, and it is beyond the scope of this site. It is far more complicated than IGP routing protocols, and it may involve many steps to determine the best path. Often is just boils down to how many AS hops away it is to get to the other AS.

You may be confused about the role of iBGP. The distinction of iBGP and eBGP is whether or not the neighbor is in the same AS. An AS will almost always have more routers than just the routers connecting to other ASes. The routers internal to the AS would have the same AS number as their neighbors, so they would use iBGP.

It would also be a huge discussion about how design within an AS. Again, you could have default routes, full or partial routing tables, a combination, etc., or a mix of IGP and iBGP (which is involved because you could use a full-mesh, route reflectors, confederations, etc.).

What BGP neighbors send each other can be controlled. It could be full routing tables, or it could be whatever the AS owner decides is appropriate. there is no one answer to this question.

Answer 2

Do eBGP routers send their routing tables to all routers (both BGP and IGP) in one AS?

No, that's the whole point of running a separate protocol. BGP speakers redistribute a very small number of routes into the IGP. If you only have one link outside your AS, then the BGP router just needs to redistribute a default route. If you have more than one exit point, you may redistribute a few summary routes to steer traffic in the right direction.

Often, only a default route is advertised into the IGP by the BGP speakers, and the BGP speakers will route the traffic outside the AS based on their routing information.

In other words, A doesn't need to know everything, it only needs to know how to get to the border routers, who can then route traffic correctly.

Answer 3

BGP protocol is the routing protocol of the INTERNET. IT´s the way that all the autonomus systems are interconnected. The BGP is conformed by 2 types of peers, internal and external peers(from the AS point of view).

Then, you, like a network administrator, need to know the networks that you want to announce to the world(once that you have been approved by some HIGHER organization like ARIN, RIPE, and so on). Well, you need to know the networks that you are propietary and you want to "announce". That´s what AS100 and AS200 does, announcing the networks that host A and B are connected to.

You have adoubt about how the known of that networks travels accross 2 other autonomous system. That is the way that BGP works: One autonomus system is on charge to announce its own network and resend(it is permitted or needed) the networks that learned from it eBGP peers.

On that way, is something like a HUGE mesh of BGP peers.

Then, host A can communicate to host B, on different autonomus system, cause,eg: the AS200 knows to reach network_of_host_A located on AS100, and it´s reached across some_number_of_AS(known as AS_PATH).

Answer 4

If AS100 is connected to, say, two ASes (it has two eBGP routers), how does host A inside AS100 know which BGP router to talk to, in order to communicate with host B in AS200? Host A tells its router that it wants to communicate with host B. Now, that router would need to know the path to host B. But host B is outside AS100.

AIUI there are three basic approaches.

1. Run iBGP for routers inside AS100, even if those routers have no eBGP connections.
2. Use default routes to direct the traffic towards a router that knows how to handle it (and take care to avoid routing loops).
3. Redistribute some BGP routes into the IGP.

Which approach is appropriate will depend on the situation.

You do NOT want to redistribute a full internet routing table into your IGP, but redistributing the odd route may be acceptable, say because you have a customer that speaks BGP, or because you want more efficient routing to a network you exchange a lot of traffic with.

Afaict what many ISPs do is divide their networks into "Core" and "Access" portions. In the core, routers are speaking BGP (both internal and external) and using that information to route external traffic to the correct destination, in the "Access" portion routers just send external traffic towards the core.