Advertising Routes

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When configuring a routing protocol such as OSPF or EIGRP how do I know which networks I want to advertise?

For example, lets say that I have a router which has two serial interfaces connected and an ethernet also connected. Now suppose my ethernet interface is the only one that participates in the routing loop /area / protocol ( I forget what it's called!). Which networks would I advertise?

This is something that I've struggled with for a while now and with a practical looming I'm trying to figure it out.

 

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I dunno Simon, presumably the ones that you want to route packets through

Just a bump, good luck!

Whats wrong with RIP v 2?

 

a general rule is to list every connected network as a advertisable network unless you have anything clever going on

remember those serial networks may well have a /30 subnet but are still required for the routing process end to end, so make sure you tell people about them

 

Broadcasts. Too many, too often!

Imagine using RIP on an isdn link, RIP Updates every 30 seconds, and as such would be constantly bringing the isdn link up. Imagine the costs!

With OSPF it will only broadcast and update if something in the topology changes.

 

Thanks Phoenix!

Can you just expand on what you mean by something clever? It's probably not going to come up in my studies by just out of curiosity.

 

Ah I see, why do Microsoft bother with it then?

 

It's an Open Standard?

Despite it's issues in small networks it is actually good at what it does.

Waiting to be proven completely wrong!

 

In the submenu for the protocol you're using that you get to by typing router <protocol> protocol being RIP, IGRP, OSPF, EIGRP etc you then enter the networks that you want that protocol advertise. So for example if you're using OSPF and want to advertise the networks 10.0.0.0 and 11.0.0.0, from the initial login you'd follow these steps:

Enable
Conf t
Router ospf 15<--------------------this is the process id for OSPF not to be confused with an autonomous system number exam questions like to test this common mistake.
Network 10.0.0.0 0.255.255.255 0<------------------OSPF uses wildcard masks and area numbers
Network 11.0.0.0 0.255.255.255 0

So the 'network <ip address and subnet/wildcard mask (if applicable)>' command is how you advertise a network through your chosen protocol. To unadvertise a network just put a 'no' in front of the network advertisement command:

No network 10.0.0.0 0.255.255.255 0

I think what Pheonix means by anything clever going in is that it might not be appropriate for all your routes to be advertised if you wanted to keep a network that handled sensetive data separate from the other networks except maybe a chosen few. You'd have to configure routing for the networks that you wanted to be aware of said sensetive network by static routes or some other routing protocol. I may be wrong but I remember reading that when learning about route advertising. I'm sure access-lists can serve the same purpose just fine but an alternative to creating long fiddly access-lists is to just not advertise the routes you don't want advertised but let the networks you want know about them some other clever way which is beyond my knowledge!

Hope this helps!

 

Thanks! I'm sure that is what I was trying to ask originally. I think!
I only want a single port to participate in the routing protocol other connected ports are advertised through static routes.

Things are becoming clearer!

 

Are you thinking about routing between VLANs through the ethernet port?

 

Naa, this is for my final practical of the Cisco Net Acad. In semester 4 we don't even touch on VLANS, that was sem 3!

I'll try and scan and upload and example of what I'm expecting to have to set up.

 

Here's something simillar to what I've been told to expect.

It could be either Frame Relay or ISDN

I hope that it's understandable!

 

lol not by me Simon where is that child of five

 

I'd assign an address (if you haven't been given one to use) from the network you want to advertise to the ethernet interface. Then configure the protocol you want to use to advertise the network/subnet associated with the address on the ethernet port.

Nice drawing btw? What did you use? I used to think MS Visio was good but I want what you've got

 

I didn't draw it! It was the instructor on the Smart Board.

rather than preparing net diagrams before the session he likes to make them up when were there! Personally I think he's just lazy!

 

How are you getting on with this Simon?

 

I think that I've grasped what I needed to know thanks Pete.

I have my Practical at 5pm today in which I've got 2 hours to set up something which will be simillar to what I posted.

Fortunately I'm allowed to take in a single side of A4 as a crib sheet so I'm busy handwriting that out at work now.