Routing Protocols - Again

Am I right in saying that routing protocols do not "Auto Discover" routes? I mean, the routes are always initially programmed/advertised by an engineer first on the actual routers that have the routes directly connected. From there other routers will learn of them.

We can't just have router A and B discover each other’s networks first, before actually advertising the routes with a protocol on each router?

Make Sense?

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CodeBlox

As far as I know, each router is to be configured to advertise it's directly connected networks. There's no auto discovery that I know about.

NetworkVeteran

control wrote: »

Am I right in saying that routing protocols do not "Auto Discover" routes? I mean, the routes are always initially programmed/advertised by an engineer first on the actual routers that have the routes directly connected. From there other routers will learn of them.

Yes and no. If my internal network is 172.16.x.x, and I tell my IGP (EIGRP, OSPF, RIP, etc) to include 172.16.0.0/16, as I bring up / address new interfaces on subnets of that network, they're auto-discovered and advertised by the routing protocol. You can take this a step further in OSPF with "network 0.0.0.0 255.255.255.255 area 0" (discover/advertise anything it's connected to).

(Tech Note-- Perhaps coupled with "passive-interface default", depending on whether you want the router to only advertise all connected networks/subnets, or to both advertise and attempt to form adjacencies on them.)

We can't just have router A and B discover each other’s networks first, before actually advertising the routes with a protocol on each router? Make Sense?

Again, the above configuration would allow router A to automatically discover/advertise all its directly connected networks/subnets. Router A has no means to automatically discover router's B networks/subnets without talking to it, and that's one thing a routing protocol, in fact, does. It allows those neighboring routers to speak together and share routing information.

(Tech Note-- There are alternatives to routing protocols for automatically discovering a neighboring router's directly connected networks/subnets. Notably, CDP-speakers can share this info if you configure ODR).

thedrama

control wrote: »

Am I right in saying that routing protocols do not "Auto Discover" routes? I mean, the routes are always initially programmed/advertised by an engineer first on the actual routers that have the routes directly connected. From there other routers will learn of them. We can't just have router A and B discover each other’s networks first, before actually advertising the routes with a protocol on each router? Make Sense?

In the internetwork, all routers have to have same routing table. They learn other networks from their neighbors(routers) by advertising their ownas updates.

NetworkVeteran

thedrama wrote: »

In the internetwork, all routers have to have same routing table.

Well, not the same, just a consistent routing policy across routing tables.

If a prefix is examined (show ip route prefix) on two routers in an internetwork, you'll almost certainly observe different route-types, metrics, next-hops, etc. And if you search for that prefix on a third router, it may not even be there, due to summarization.

example, R1:

C 10.1.102.0/29 is directly connected, Serial0/0

example, R2:

D 10.1.102.0/29 [90/2681856] via 10.1.203.2, 01:49:03, Serial0/1