RIP
We start out simply with defining the routing protocol and adding the network.
Taking a look at the wireshark capture we see only a request going out from
Default_Gateway_1_192_168_11_11 (192.168.123.1)
Why is this?
When we turn on debug on the router what do we see in addition?
We see that for some reason we are suppressing null updates.
Remember that RIPv1 (RIP version 1) only supports classful routing. Meaning that it
only understands 192.168.0.0 not our more specific route of 192.168.123.0/29.
Let us turn RIPv2 on and see what happens.
We see the same thing. The reason is that there is no need to include a particular route because the route is connected and shared between the two routers and there is no need to
advertise..
Now what happens when we remove version 2 and no auto-summary and add network
192.168.11.0?
As you can see we include the subnet of 192.168.11.0 in our broadcast out.
Other routers can see this new subnet as well.
Let us include the subnet 192.168.22.0 into the rip stanza.
Note: When you turn on auto-summary you will not see it displayed. Remember this is by default turned
on.
We now add the subnet 192.168.22.0 into the rip configuration.
We can immediately see that 192.168.123.2 sends out a broadcast with its network of
192.168.22.0.
We can see that both Default_Gateway_1_192_168_11_11 and
Default_Gateway_2_192_168_22_22 receive routes from the other one.
Output from Default_Gateway_2_192_168_22_22
Output from Default_Gateway_1_192_168_11_11
On Default_Gateway_3_192_168_33_33 what would you expect to see in the routing
table?
If you guessed both routes, you would be right!
Now take a look the packet that is travelling from one router to another. In the below
example we are sending from Default_Gateway_2_192_168_22_22 but we could also
just as easily send it from the other two routers.
We are not sending a tcp (connection-oriented packet) out, but rather a udp (best-effort)
packet to the broadcast address of 255.255.255.255.
Furthermore, we see that if sometime down the road we decide to block udp port 520 we
would be disabling the rip routing updates.
Also, by default we are sending out RIPv1 updates.
Auto-Summary and Version 2
Auto-summarization and version 1 is turned on by default when first running the router
rip command.
What does this mean?
First, RIP version 1 does not support Classless InterDomain Routing (CIDR) natively
which means that even if network 10.0.22.0/24 is valid on the router it will send out an
update packet that only contains 10.0.0.0/8 and not any more specific subnets.
Even though the real specific subnet is 10.0.11.0/24 to Default_Gateway_2 and
Default_Gateway_3 it appears as the entire Class A address 10.0.0.0/8. However, their
routing table already contains a more specific entry for the 10.0.0.0/8 subnet. So we have
a problem, as you can see.
The routes to get to 10.0.11.0/24 does not appear in Default_Gateway_2 or
Default_Gateway_3’s routing table.
A question to ask would be is the more specific route being advertised. As you can see
by the wireshark output, only the classful ip address 10.0.0.0/8 is being advertised.
This is where RIPv2 comes into play. RIPv2 understands CIDR which means it can
handle 10.0.X.0/24. However, it is important to enable this on both the sender and
receiver. Also, it is vital that auto-summarization is turned off. If it is on it will only
broadcast out the 10.0.0.0/8 subnet.
Two interesting things to note, RIPv2 sends a multi-cast to the ip address 224.0.0.9
instead of a broadcast and it starts to send the more specific route of 10.0.11.0 from
Default_Gateway_1.