# Cisco IOS EIGRP Protocol #### Network Topology [![image.png](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/scaled-1680-/Synimage.png)](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/Synimage.png) #### Router0 Configuration Commands Router0>enable Router0#configure terminal Router0(config)#interface serial 0/0/0 Router0(config-if)#ip address 172.16.1.1 255.255.255.248 Router0(config-if)#no shutdown Router0(config-if)#interface gigabitEthernet 0/0 Router0(config-if)#ip address 192.168.10.1 255.255.255.0 Router0(config-if)#no shutdown Router0(config-if)#exit Router0(config)#router eigrp 100 Router0(config-router)#network 192.168.10.0 0.0.0.0 Router0(config-router)#network 172.16.1.0 0.0.0.0 Router0(config-router)#no auto-summary Router0(config-router)#end #### Router1 Configuration Commands Router1>enable Router1#configure terminal Router1(config)#interface serial 0/0/0 Router1(config-if)#ip address 172.16.1.2 255.255.255.248 Router1(config-if)#no shutdown Router1(config-if)#interface gigabitEthernet 0/0 Router1(config-if)#ip address 192.168.20.1 255.255.255.0 Router1(config-if)#no shutdown Router1(config-if)#exit Router1(config)#router eigrp 100 Router1(config-router)#network 192.168.20.0 0.0.0.0 Router1(config-router)#network 172.16.1.0 0.0.0.0 Router0(config-router)#no auto-summary Router1(config-router)#end #### Comments on a Couple of Configuration Items A couple of notes about the configurations. First, the network commands show a wildcard of 0.0.0.0, this denotes an exact match for the specific IP on the interfaces. I could have also made the command be for a subnet with network 192.168.10.0 0.0.0.255 It was simply a preference on my part for this configuration. Second, the command no auto-summary. EIGRP is a routing protocol that supports auto summarization. This isn't very intuitive to understand. Auto summarization in EIGRP means that it will automatically advertise the classful A, B, or C subnet if it is not told otherwise. Let's take a look at another network topology to demonstrate the auto-summary effect. [![image.png](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/scaled-1680-/Fo5image.png)](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/Fo5image.png) In the above example there are three routers configured for eigrp. In the initial setup Router0 and Router2 have auto-summary on for EIGRP. All three networks 192.168.2.0, 172.16.1.0, and 1.1.1.0 are configured in the EIGRP router for Router0. [![image.png](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/scaled-1680-/xc6image.png)](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/xc6image.png) Similarly, Router 2 has EIGRP configured and 1.3.3.0 and 192.168.20.0 are configured. Also, auto-summary has been explicitely turned on. [![image.png](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/scaled-1680-/kzLimage.png)](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/kzLimage.png) Router1 is pretty vanilla with EIGRP configured and advertising the 192.168.10.0 and 192.168.20.0 networks. No auto-summary is on for Router1. The interesting thing is taking a look at the routing tables of each of the routers to see what auto-summary does in this scenario. And how it will break the routing. Let's look at Router0 first. [![image.png](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/scaled-1680-/EwDimage.png)](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/EwDimage.png) The 1.1.1.0 /24 network has been summarized the classful A network of 1.0.0.0 /8. Similarly, the 172.16.1.0 /24 has been summarized to the classful B network of 172.16.0.0 /16. This isn't necessary bad when you just look at it from Router0's perspective but hold that thought until we take a closer look at Router1 in a moment. One last note for Router0, the network 192.168.10.0 /24 is as expected becasue it is already a class C network. No let's look at Router2 and then we will get to Router1. [![image.png](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/scaled-1680-/gbkimage.png)](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/gbkimage.png) Now, you can already start to see the problem, Router2 is advertising a summarized classful A for 1.0.0.0 /8 just as Router0 was. And what does that do to Router1, confuses the heck out of its routing table, that's what! Let's look. [![image.png](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/scaled-1680-/vgNimage.png)](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/vgNimage.png) So, now Router1 is advertising that 1.0.0.0 /8, which is the summarized subnet for both 1.1.1.0 and 1.3.3.0 is reachable through both the other routers. This is what unchecked auto-summary can do to EIGRP if you are not careful. The fix is to turn off auto-summary in Router0 and Router1. And problem resolved. Take a look at Router1, the one that was so confused before we turned off auto-summary, it has the correct routes now. [![image.png](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/scaled-1680-/bU1image.png)](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/bU1image.png) #### Back to the EIGRP Configuration Verify the configuration Router0#show ip route [![image.png](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/scaled-1680-/h94image.png)](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/h94image.png) Router0#show ip eigrp neighbors [![image.png](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/scaled-1680-/de0image.png)](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/de0image.png) Router0#show ip eigrp topology [![image.png](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/scaled-1680-/o87image.png)](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/o87image.png) Router0#show ip eigrp interfaces [![image.png](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/scaled-1680-/gz9image.png)](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/gz9image.png) Router0#show ip eigrp traffic [![image.png](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/scaled-1680-/XL4image.png)](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/XL4image.png) #### Ping and Tracert Tests [![image.png](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/scaled-1680-/VY6image.png)](https://bookstack.taylorhome.run/uploads/images/gallery/2023-01/VY6image.png) #### Cisco Packet Tracer Files [net16 eigrp.pkt](https://bookstack.taylorhome.run/attachments/11) [net17 eigrp auto-summary.pkt](https://bookstack.taylorhome.run/attachments/12)