Routing Basics questions.

Futura

Can somebody help me get an understanding of some routing basics and correct me if i'm wrong.

Ripv2, igrp and eigrp are classful by default?

Which means that you would always advertise the networks like so?

network 10.0.0.0
network 172.16.0.0 - 172.31.0.0
network 192.168.0.0 - 192.168.255.0

If any of the above private networks are subnetted and connected to different routers you need to put 'no auto summary' in the route config?

Are my statements correct? Many thanks.

Chris_

That's pretty much the case, don't worry about IGRP though; it's dead, gone, never to be seen again.

I would always turn auto-summary off as a matter of course, but that's up to you. You can then add your own classless, and more useful summarisations where needed.

chX

As Chris_ mentioned, you don't really need to worry about IGRP.

There's a few good things to note (and please feel free to correct me if I'm wrong too).

RIPv2/EIGRP/OSPF/IS-IS advertise subnet masks in their routing updates, whereas RIPv1 and IGRP do not.

If a router running RIPv1 receives a routing update from another router running RIPv1 (which will not include the subnet mask), the router has two options:

1. If the router already has a directly connected interface in the major network the route received is in, it will use the subnet mask configured on that interface.

For example, let's say a router receives a route for 10.3.0.0. It has an interface in the 10.1.0.0/16 network, which is part of the major 10.0.0.0/8 network. It'll assign the 10.3.0.0 route a subnet mask of /16 (255.255.0.0).

2. If the router does not have a directly connected interface in the same major network, it will use the classful subnet mask. In this case, it's going to apply a /8 to the 10.3.0.0 network.


You enter the network commands at classful boundaries with RIPv2 and EIGRP, but you can also specify a wildcard mask with EIGRP if you wish.

Auto-summary will occur when a routing update travels over a non-contiguous path.

For example:

If we have two routers with interfaces in the same major network, let's say:
(R1) 10.1.0.0/16; and
(R2) 10.2.0.0/16

They are both running EIGRP and they are connected by one link which falls into the 10.3.0.0/16 network.

When R1 advertises the 10.1.0.0/16 network to R2, it will not summarize it (even if auto-summary is turned on) because it passes over a contiguous network. The same is true for R2 advertising its route to R1.


Consider the same example, however this time the link between R1 and R2 falls in the 192.168.5.0/24 subnet.

If auto-summary is turned on, the routing updates between R1 and R2 will be summarized back to 10.0.0.0/8. This is because it's non-contiguous. Auto-summary can cause issues, so it's best to turn it off.



HTH.