Thanks Andrew! now i see it.
R2#sh ip route 172.16.0.0 255.255.254.0
Routing entry for 172.16.0.0/23
Known via "eigrp 1", <strong>distance 5</strong>, metric 28160, type internal
Redistributing via eigrp 1
Routing Descriptor Blocks:
* directly connected, via Null0
Route metric is 28160, traffic share count is 1
Total delay is 100 microseconds, minimum bandwidth is 100000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 0Hello Rene,
I did not understand why we used .254.0
is it possible to do another example please?
Thanks in advance.
R1(config)#interface fastEthernet 2/0
R1(config-if)#ip summary-address eigrp 1 172.16.0.0 255.255.254.0Hi ,
Great explanation . but have one doubt can you explain below subnetting
R1(config)#interface fastEthernet 2/0
R1(config-if)#ip summary-address eigrp 1 172.16.0.0 255.255.254.0
we used /24 on loopback so how did you get this mask /23 by subnetting. can you please explain
We have these two networks:
R2#show ip route eigrp
172.16.0.0/24 is subnetted, 2 subnets
D 172.16.0.0 [90/30720] via 192.168.12.1, 00:02:28, FastEthernet0/0
D 172.16.1.0 [90/30720] via 192.168.12.1, 00:02:28, FastEthernet0/0
The most optimal summary rounte for 172.16.0.0/24 and 172.16.1.0/24 is 172.16.0.0/23.
In binary, these two look like this:
* 10101100 00010000 00000000 00000000
* 10101100 00010000 00000001 00000000
Above you can see that the first 23 bits are the same, thatās why we use the /23 summary.
Rene
Sorry, I have solved it.
Why is the summarisation being applied to int ?fa0/2
Hi Andrew,
In EIGRP, the summary is sent OUTbound on the interface that you configure it on.
Rene
Thanks Rene. You have a great way of simplifying these concepts.
In the last page of this lesson you explain why R1 has an entry that points to null0. I understand the explanation but have always had trouble with this. Would you provide an example illustrating when the null0 interface would get used and how it protects against routing loops? Just having trouble āseeingā this. Thank you.
Hello Mike,
Here is a quick example. Instead of EIGRP, Iāll use RIP (since RIP doesnāt create a null0 route automatically).
We have two routers:
R1 has two loopback interfaces and sends the 172.16.0.0/16 summary to R2.
R2 is connected to the Internet and sends a default route to R1.
R1#show ip route rip
R* 0.0.0.0/0 [120/1] via 192.168.12.2, 00:00:28, GigabitEthernet0/1
And:
R2#show ip route rip
R 172.16.0.0/16 [120/1] via 192.168.12.1, 00:00:04, GigabitEthernet0/1
Now when you would send a packet to an IP address in the 172.16.0.0/16 range (besides 172.16.0.1 and 172.16.1.1) from R2 to R1, that packet will loop around. R2 sends it to R1 (because of the summary) and R1 sends it back to R2 (because of the default route).
Once you add a 172.16.0.0/16 null0 route on R1, this problem is solved. When R1 receives the packet destined for 172.16.3.3, it will match the 172.16.0.0/16 to null0 route and drops the packet, instead of using its default route.
Does this help? 
Rene
Yes, thank you very much!
Hi Laz,
please i want to know a few things
1- what is the role of Null0 ( does it mean that all the subnets not in the summarized address will be dropped?) when we configure IP summary address on an interface?
2- what is the role of adding AD in the IP summary address?
3- what is the effect of configuring Summary-metric when doing summarization, how does it influence the routing?
thank you very much for the support.
Oliver
Hello Olivier
Looking at R1, it has two directly connected networks 172.16.0.0/24 and 172.16.1.0/24. You have created a summary route using the 172.16.0.0/23 summary, and this summary route must be placed in the routing table. Where should it send such traffic? Since the directly connected routes will take care of any traffic destined for them, since their AD is 0, the 172.16.0.0/23 route must route somewhere. It is set to route to Null0, in other words drop any packets. In such a situation, packets will never match this route, so it would never be used.
However, what if you used 172.16.0.0/22 to summarize? It would still work, but it also contains additional subnets that are not directly connected such as 172.16.2.0/24 for example. Then this summarization will be communicated via EIGRP to neighbouring routers, and if those routers are sending traffic to 172.16.2.55 for example, they will send it to R1 since the summarization includes this subnet. But, the correct route may be elsewhere. This is why Null0 is placed as the exit interface, so all such packets will be dropped.
The administrative distance or AD measures the reliablility of the source of the routing information. Directly connected info has an AD of 0, statically configured routes have an AD of 1 while routing protocols have various ADs by default. You cannot change the AD of an IP summary address. If it is communicated via EIGRP, it will adopt the AD of EIGRP as is the case here. You can however adjust the metric of the summary address within EIGRP. You can read more about the AD here:
Changing the metric for the routing of any address, whether a summary address or otherwise, is done in order to affect routing in the way that you desire. Each situation is different and there is no one single reason to do it. You may have several routes via which you prefer traffic to go, or various summarizations that you want to be taken into account more than others on the network. If this is the case, you change the metric accordingly so you get the routing behaviour you desire.
Essentially, by changing the summary-metric value, you are changing the advertised metric value of that summary route. You are telling other EIGRP routers the administrative distance for that particular route, and as mentioned above, you do this in order to affect routing so you get the desired routing behaviour.
I hope this has been helpful!
Laz
Hi Lazaros,
The topology Rene has used there is only a single path FastEthernet 2/0 through R1 that summarised two networks, so what is the functionality of metric in this single path?
Would you please explain it as simple as possible.
Hello Muhammad
What R1 is doing is summarizing both networks into a single routing entry that is advertised to R2. Now each individual network may have a different metric, but since weāve summarized the two destinations into a single routing entry, only one metric can be used. The metric used is the lowest of all the summarized routes.
Now imagine that behind R1, there is more network infrastructure with, say, 10 networks such as 172.16.0.0/24 all the way to 172.16.9.0/24. Each of these has a different metric from the point of view of R1. You can summarize these with a route such as 172.16.0.0/20 and advertise this to R2, but this summary route can only have one metric associated with it. The best single metric among the 10 destinations represented by the summary route is used as the metric of this summarized.
I hope this has been helpful!
Laz
Thanks Lazaros,
Now it is clear for me very nice explanation.
Hi Rene,
I did below conf on R1
router eigrp 1
summary-metric 172.16.0.0/23 1000000 10 255 1 1600
network 192.16.0.0
network 192.16.1.0
network 192.168.12.0
But I didnāt get any change in metric in R2
Hello Sameh
It seems that your summary command is summarizing 172.16.0.0/23 while all of your networks (192.16.0.0, 192.16.1.0, and 192.16.12.0) are outside of this range.
I believe this is a typo. You needed to use the following configuration:
router eigrp 1
summary-metric 172.16.0.0/23 1000000 10 255 1 1600
network 172.16.0.0
network 172.16.1.0
network 192.168.12.0
I hope this has been helpful!
Laz
Thanks Laz, you correct itās a typo 