EIGRP Summarisation allows you to stream line the routing table making it more efficient. Fewer Routes listed results in less EIGRP updates being sent out and there fore less resources are consumed (Bandwidth, CPU ultilisation, load etc).
By default EIGRP performs auto-summarisation, that is to say that EIGRP will automatically summarise at a major class boundary during redistribution from EIGRP into a classful routing protocol (eg RIP).
EIGRP will also summarise at the major classful boundary when a route is advertised out of an interface that is on a different major class boundary.
In order to prevent a routing loop when summarisation is in effect (whether manual or automatic) a summary is automatically assigned to the NULL0 interface to prevent routing loops. If the router with a summary route received a packet for an unknown subnet that is part of a summarised range then the longest match ends up being the summary route itself (not a subnet of it) and so this is forwarded to the NULL0 interface and is dropped.
The idea is that the router is then prevented from forwarding the packet on to a default route and potentially creating a loop.
Benefits of summarising in this way include a smaller routing table leading to faster look ups, more specific routes will be hidden so if that specific route goes down the whole network does not need to recalculate the DUAL alogrithm, routing updates will be smaller and so limit the number of EGIRP Queries.
The down side of auto summarisation is that it won't factor in discontiguous networks. As a result you could have networks behind the same 172.16.0.0/16 network advertised out of 2 different interfaces to 2 different networks resulting in 50% of traffic arriving in the wrong place.
To address this issue you can disable automatic summarisation and use manual summarisation.
Disabling automatic summarisation is as simple as this:
R1(config)#router eigrp 1
Now the router will not perform summarisation and all available subnets will be advertised.
Manual summarisation is configured on a per-interface basis, when a summary route is applied a NULL0 summary route entry is immediately created in the routing table to help prevent loops.
To configure a manual summary route you do the following:
R1(config)#router eigrp 1
R1(config-if)#ip summary-address eigrp [AS] [IP] [Subnet Mask]
e.g) R1(config)#int s0/0/0
R1(config-if)#ip summary-address eigrp 1 172.16.0.0 255.255.224.0
With a manual summary route, the summary route is advertised only if a more specific entry of the summary is present in the routing table. Otherwise it won't appear in the routing table.