CCNP ROUTE: 13.EIGRP Summarization

-EIGRP supports summarization at any router, unlike OSPF (only at ABR/ASBR).
-When EIGRP advertises a summary route, the advertising router considers the summary route to be up and working unless all subordinate routes fail.
-Summarization reduces Query scope: EIGRP Query stops at a router that has a summary route that includes the subnet listed in the Query but not the specific route listed in the Query.
-The summary has the metric of the best of the subnets being summarized.
-When summarizing, the router adds a route to its own routing table, for the summary prefix/prefix-length, with an outgoing interface of null0 (discard route).
-When the summarizing router receives a packet within the summary route’s range:
If the packet matches a more specific route than the summary route, the packet is forwarded based on that route.
When the packet does not match a more specific route, it matches the summary route and is discarded.
-When using auto-summary: When a router has multiple working interfaces, and those interfaces use IP addresses in different classful networks,
the router advertises a summary route for each classful network on interfaces attached to a different classful network. (EIGRP summarizes at classful boundaries)
-The "no auto-summary" command affects the behavior of the router on which it is configured only and tells that router to not advertise a summary route for the entire classful network.
-Instead, that router advertises all the subnets, as if the auto-summary feature did not exist.

General syntax (configured per interface): ip summary-address eigrp asn prefix subnet-mask

First we add some loopback interfaces to R4 and add them into the EIGRP process:
R4#show ip interface brief 
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0            unassigned      YES NVRAM  administratively down down  
Serial0/0                  172.16.0.2      YES NVRAM  up                    up    
FastEthernet0/1            unassigned      YES NVRAM  administratively down down  
Serial0/1                  192.168.0.14    YES NVRAM  up                    up    
Serial0/2                  172.16.0.13     YES NVRAM  up                    up    
Serial0/3                  unassigned      YES NVRAM  administratively down down  
Loopback44                 192.168.44.44   YES NVRAM  up                    up    
Loopback45                 192.168.45.45   YES manual up                    up    
Loopback46                 192.168.46.46   YES manual up                    up    
Loopback47                 192.168.47.47   YES manual up                    up    
Loopback48                 192.168.48.48   YES manual up                    up    
Loopback49                 192.168.49.49   YES manual up                    up    
Loopback50                 192.168.50.50   YES manual up                    up

R4#show run | s eigrp
router eigrp 1
 network 172.16.0.2 0.0.0.0
 network 172.16.0.12 0.0.0.3
 network 192.168.44.0
 network 192.168.45.0
 network 192.168.46.0
 network 192.168.47.0
 network 192.168.48.0
 network 192.168.49.0
 network 192.168.50.0
 no auto-summary

The newly added networks are advertised by R4 to all neighbors:
R6#show ip route
...
D    192.168.46.0/24 [90/2297856] via 172.16.0.2, 00:02:50, Serial0/0
D    192.168.47.0/24 [90/2297856] via 172.16.0.2, 00:02:47, Serial0/0
D    192.168.44.0/24 [90/2297856] via 172.16.0.2, 00:36:14, Serial0/0
D    192.168.45.0/24 [90/2297856] via 172.16.0.2, 00:02:56, Serial0/0
     172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
D       172.16.0.12/30 [90/2195456] via 172.16.0.6, 00:36:14, FastEthernet0/1
C       172.16.0.8/30 is directly connected, Serial0/1
C       172.16.0.4/30 is directly connected, FastEthernet0/1
C       172.16.0.0/30 is directly connected, Serial0/0
C       172.16.111.0/24 is directly connected, Loopback111
D    192.168.55.0/24 [90/409600] via 172.16.0.6, 00:36:15, FastEthernet0/1
D    192.168.50.0/24 [90/2297856] via 172.16.0.2, 00:02:37, Serial0/0
D    192.168.49.0/24 [90/2297856] via 172.16.0.2, 00:02:42, Serial0/0
D    192.168.48.0/24 [90/2297856] via 172.16.0.2, 00:02:46, Serial0/0

Now we can manually summarize these networks into a summary (applied on the interfaces to R4's neighbors) which can be advertised by R4 to its neighbors:

R4(config)#interface serial 0/0
R4(config-if)#ip summary-address eigrp 1 192.168.32.0 255.255.224.0
*Mar  1 03:22:52.387: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 172.16.0.1 (Serial0/0) is resync: summary configured
R4(config)#interface serial 0/2                                 
R4(config-if)#ip summary-address eigrp 1 192.168.32.0 255.255.224.0
*Mar  1 03:22:52.387: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 172.16.0.1 (Serial0/0) is resync: summary configured

Now verifying on R6 again:
*Mar  1 00:41:08.651: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 172.16.0.2 (Serial0/0) is resync: peer graceful-restart
R6#show ip route
...
     172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
D       172.16.0.12/30 [90/2195456] via 172.16.0.6, 00:43:50, FastEthernet0/1
C       172.16.0.8/30 is directly connected, Serial0/1
C       172.16.0.4/30 is directly connected, FastEthernet0/1
C       172.16.0.0/30 is directly connected, Serial0/0
C       172.16.111.0/24 is directly connected, Loopback111
D    192.168.55.0/24 [90/409600] via 172.16.0.6, 00:03:00, FastEthernet0/1
D    192.168.32.0/19 [90/2297856] via 172.16.0.2, 00:02:52, Serial0/0

R4 added the summary route to its routing table (also keeping the specific routes to each loopback), with an outgoing interface of Null0:
R4#show ip route
...
D    192.168.32.0/19 is a summary, 00:04:26, Null0