Summarization Question

Answered Question
Aug 20th, 2009

I have deployed summarization on a boundary of my network . I am using EIGRP and i configured summarization on interfaces. On the routers when i write "show ip route" i can see the summarization on Null0. But i also see the other routes which belong to this summary address. Is that normal or not. If not what i must do ?

Thanks a lot

Moses

I have this problem too.
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Correct Answer by Peter Paluch about 7 years 3 months ago

The problem here is that with a summary route, you are declaring that all possible subnets that can be covered by this summary are present in your routing table. But this will not always be the case - the individual networks that are summarized may get disconnected or they might not be used yet. If such situation is coupled with a default route or with another route that is broad enough, a similar situation as described above may arise.

This is where the Null0 route comes into play. Whenever you configure a summary route, a discard route pointing to Null0 is automatically added to the routing table. Note how the route table looks like when every interface is workable and the summary route is configured:

200.20.2.0/30 via S0/0

192.168.0.0/24 via Fa0/0

192.168.1.0/24 via Fa0/1

192.168.0.0/23 via Null0

0.0.0.0/0 via S0/0

And now let's disconnect the Fa0/1:

200.20.2.0/30 via S0/0

192.168.0.0/24 via Fa0/0

192.168.0.0/23 via Null0

0.0.0.0/0 via S0/0

Note that the Fa0/1 network has disappeared from the routing table. Now, let's take again the packet going to 192.168.1.100. Your neighbor forwards it to you. You now examine your routing table and look for the longest match. The S0/0 network will not match, nor will the Fa0/0 network. But notice that the network 192.168.0.0/23 network pointing to Null0 will match the destination and it is a longer match than the default route. So the packet will be routed according to the Null0 route which means that it will simply be discarded - anything forwarded to the interface Null0 gets simply discarded.

And this is the idea of the Null0 route - to represent all those subnets of a summary route that do not presently exist. If there are any subnets of the summary route, their prefix will be longer which means they will be tested for a match first. Only if none of the presently known subnets matched the packet's destination, the entire summary route will eventually match but because it points to the Null0 interface, the packet will be dropped. The discard route therefore eliminates routing loops.

I hope this clarifies the discard route a bit.

Best regards,

Peter

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Jon Marshall Thu, 08/20/2009 - 01:43

Moses

On the router where you configured the summary address then yes you will see both the route to Null0 and the more specific routes within the summary route. This is normal and is in fact what you want to happen. Without the more specific routes anything sent to a destination network within the summary route would be routed to Null0 ie. dropped which is clearly not what you want.

So the router keeps both the more specific routes to actually route the packets and a route to Null0. This Null0 route is to prevent routing loops ie. you may be summarising a large block of networks not all of which the router knows about. If a packet arrives at the router for a network that the router hasn't got in it's routing table then the router will route to Null0 which is exactly what you want to happen.

All other downstream routers that receive the summary address should not have the more specific routes in the routing table. Note that the next-hop of the summary address is only Null0 on the actual router you configured it on. All downstream routers will have next-hops of router IP's.

Jon

Peter Paluch Thu, 08/20/2009 - 02:58

Jon,

A wonderful reply indeed. Let me just add that the route to Null0 is sometimes called the discard route in the documentation. It is added automatically by all routing protocols except the RIP. Unfortunately, the RIP seems to be badly underdeveloped in Cisco implementation.

Best regards,

Peter

Jon Marshall Thu, 08/20/2009 - 03:50

Peter

Thanks for the compliment.

As for RIP, well i have to admit i'm kind of glad Cisco haven't put that much effort into it :-).

The trouble with RIP is that not only routers but end devices can also run it. Coming from a Unix background i have seen situations where a unix server with 2 nics running RIP has actually ended up being the gateway for certain traffic between subnets. Not ideal :-)

Jon

Peter Paluch Thu, 08/20/2009 - 04:30

Jon,

You are welcome.

As for the RIP, well - yes, I agree that if workstations or servers are misconfigured, their unexpected presence in a routing protocol may have inconvenient results - no doubt about it. But that's not the problem of RIP alone. As far as I know, the Windows also support OSPF, for example, so this could happen with virtually any routing protocol.

What really bugs me is that the codebase in IOS already has all the functions and the code implemented that is missing in the RIP. If only somebody capable of making changes to the IOS code wanted to reuse it in the RIP code! There are many outspokenly silly deficiences of the RIP implementation on Cisco routers, such as:

1) Lack of the wildcard mask support in the "network" command

2) Inability to include a directly connected supernet into RIPv2 using the "network" command

3) Inability to manually summarize to a supernet

4) Inability to manually summarize to more than one subnet of a single major network on an interface

5) Inability to automatically install a discard route when summarizing networks

6) Inability to turn on or off the "next-hop-self" feature (an optimized usage of the next hop field in RIPv2 messages)

For all of this, an existing code is already routinely deployed in other IOS routing protocol implementations so this would merely an adaptation of the existing code, not inventing a completely new one. But well, many people think that RIP is dead... (I'm definitely not one of them).

Hmm, I seem to diverge from the OP topic. Sorry for that.

Best regards,

Peter

moses12315 Thu, 08/20/2009 - 20:38

Dear Jon,

thanks for your information, but to be honest i did not understand all of it. I understood about the route to Null0 but you also say that the router might not know about a specific route. How can this happen? Shouldn't the routers know about all the routes to the network(i am not talking about the stub routers)? Also you mentioned about the downstream routers that receive the summary address. I thought that summarization exists only on the router that you configure summarization. How the other routers know about this summary?

Thanks a lot for your time

Moses

Peter Paluch Fri, 08/21/2009 - 00:18

Hello Moses,

I hope that Jon will extend my answer here but let me try to give you a hint.

Imagine you have a router with three directly connected interfaces. The Fa0/0 is 192.168.0.1/24, the Fa0/1 is 192.168.1.1/24 and the S0/0 is, say, 200.20.2.1/30. On the S0/0 there is an EIGRP neighboring router connected and you would like to advertise your Ethernet networks as a summary route to it. So on S0/0 interface, you will configure a manual summary address with the command ip summary-address eigrp 1 192.168.0.0 255.255.254.0.

Imagine further that the S0/0 interface is also the default route out to the internet, so you configure your router with the command ip route 0.0.0.0 0.0.0.0 S0/0.

Now imagine that something goes wrong with the interface Fa0/1 and the network 192.168.1.0/24 gets disconnected. The manual summary route will still be advertised to your neighbor because the Fa0/0 network still exists and can be summarized using the preconfigured manual summary. In effect, your EIGRP neighbor does not see any change and it absolutely has no clue that the 192.168.1.0/24 network does not exist. It only knows what you have sent it - the summary network 192.168.0.0/23.

And now imagine that your EIGRP neighbor receives from somewhere a packet going to the 192.168.1.100. That destination falls into the network 192.168.0.0/23 that your neighbor knows about because you have sent it that route. So it will forward that packet to you. But when you receive the packet to 192.168.1.100, where will you route it? You need to find a match in your routing table. Note that after the Fa0/1 interface went down, the only networks you know about are 192.168.0.0/24 on Fa0/0 (does not match), 200.20.2.0/30 on S0/0 (does not match) - and default route which matches everything. Because the default route matched the destination 192.168.1.100, you will forward the packet according to the default route - back to your EIGRP neighbor that has sent you the packet in the first place. And now, the process repeats itself - your neighbor will examine its routing table, send the packet to you, you will find the only matching route to be the default route and you forward the packet back. You will be forwarding the packet back and forth between you and your neighbor until the TTL of the packet gets decremented to 0.

Continuation follows...

Correct Answer
Peter Paluch Fri, 08/21/2009 - 00:19

The problem here is that with a summary route, you are declaring that all possible subnets that can be covered by this summary are present in your routing table. But this will not always be the case - the individual networks that are summarized may get disconnected or they might not be used yet. If such situation is coupled with a default route or with another route that is broad enough, a similar situation as described above may arise.

This is where the Null0 route comes into play. Whenever you configure a summary route, a discard route pointing to Null0 is automatically added to the routing table. Note how the route table looks like when every interface is workable and the summary route is configured:

200.20.2.0/30 via S0/0

192.168.0.0/24 via Fa0/0

192.168.1.0/24 via Fa0/1

192.168.0.0/23 via Null0

0.0.0.0/0 via S0/0

And now let's disconnect the Fa0/1:

200.20.2.0/30 via S0/0

192.168.0.0/24 via Fa0/0

192.168.0.0/23 via Null0

0.0.0.0/0 via S0/0

Note that the Fa0/1 network has disappeared from the routing table. Now, let's take again the packet going to 192.168.1.100. Your neighbor forwards it to you. You now examine your routing table and look for the longest match. The S0/0 network will not match, nor will the Fa0/0 network. But notice that the network 192.168.0.0/23 network pointing to Null0 will match the destination and it is a longer match than the default route. So the packet will be routed according to the Null0 route which means that it will simply be discarded - anything forwarded to the interface Null0 gets simply discarded.

And this is the idea of the Null0 route - to represent all those subnets of a summary route that do not presently exist. If there are any subnets of the summary route, their prefix will be longer which means they will be tested for a match first. Only if none of the presently known subnets matched the packet's destination, the entire summary route will eventually match but because it points to the Null0 interface, the packet will be dropped. The discard route therefore eliminates routing loops.

I hope this clarifies the discard route a bit.

Best regards,

Peter

Jon Marshall Fri, 08/21/2009 - 01:27

Peter

"I hope that Jon will extend my answer here but let me try to give you a hint."

Not sure how i can improve on that :-). Great reply.

Jon

moses12315 Fri, 08/21/2009 - 08:29

Dear Jon and Peter,

thanks a lot. I think now i understood everything about this issue. You were both very clear and detailed . Now i know what to do , since i realised that i did wrong configuration. Thanks again for your time.

Moses.

Jon Marshall Fri, 08/21/2009 - 01:38

Moses

Hopefully Peter's excellent explanation has cleared up the confusion regarding the use of the Null0 route.

"I thought that summarization exists only on the router that you configure summarization. How the other routers know about this summary?"

A summary route is a kind of announcement to the network that you ie. the router, knows how to get to all the summarised networks.

If you apply the summary route to an interface any other routers that are peering with this router on that interface will receive the summary route via EIGRP updates. And they will then send the summary route to any of their neighbors etc..

You want this to happen otherwise you haven't really achieved anything by simply adding a summary route to one router.

Jon

Peter Paluch Fri, 08/21/2009 - 01:43

Jon,

A great point! I try to stress to my students that when I configure a summary address on my router, it is not for me - my router won't have a simpler routing table - rather it's for them. Their routing tables will be simpler. Making a summarization is basically doing a favor for my neighbors but not for myself.

Best regards,

Peter

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