EIGRP Concepts

EIGRP Manual Route Summary  
Route summary command is configured on the interface which faces all the subordinate subnets, and only the summary route is broadcasted in all other interfaces. 
Whenever route summary configured, the summary route will be injected into local routing table with an outgoing interface of Null0, which is called “discard route”. Plus, its default administrative distance (AD) is 5 locally, but still 90 on other EIGRP neighbor routers. This summary route will automatically disappeared if there are no active subordinate subnet in routing table.
When one subordinate subnet becomes down, the summary router will send out EIGRP query to all its neighbors, the adjacent routers who only receive the summary route will reply this query with “destination unreachable” and does not extend the query any further because it does not have the specific problematic subordinate in its routing table. So, EIGRP Query stops on nodes who receive EIGRP summary route not on those who generate EIGRP summary route.
when route summary configured, the router immediate bring down and up again the eigrp neighborhood on that interface to stop advertising all subordinate subnets of the summary route and replace them with the summary route.
However, EIGRP Route Summary could possibly results in suboptimal routing or inefficient routing.
router(config-if)# ip summary-address eigrp asn subnet-id mask AD  //configured on outbound interface on which you do not want subordinate subnets to be advertised.  mask: 255.255.255.0

EIGRP Auto Summary
EIGRP auto-summary only summarize the subordinate subnets on the board of their classful major network in EIGRP domain, if no such boarder exist in EIGRP domain, auto summary will never happen. Furthermore, a static route with next hop to be Null0 is not generated.

EIGRP Stub
Stub feature does not prevent the stub subnet from being advertised to other eigrp neighbors as long as network command declared for that subnet.
Stub router indicate its stub attribute to neighbors in EIGRP Hello packet, thus the far-end peers will never send EIGRP Query to that stub router.
Stub router never play a transit role for traffic between 2 remote EIGRP learned subnet, and never advertise EIGRP-learned routes from one neighbor to another.
router(config-router)# eigrp stub [connected | summary | redistributed | receive-only]
receive-only: mutual exclusive with all other 3 options, as it prevent any routes from advertised out, thus nothing is shared with neighbors. use this when there is only one interface on the router.
connected: permit advertising the connected routes included by the network command, or connected routes redistributed into EIGRP. enabled implicitly by default
static: permit advertising the static routes redistributed into EIGRP.
summary: permit advertising the summary route manually or automatically summarized by local router. enabled implicitly by default.
NOTE: if only eigrp stub configured, then summary and connected are enabled implicitly. if any option configured, summary and connected options need manually explicit configuration to be activated.

EIGRP Passive Interface
Passive interface will disable all eigrp multicast and unicast message processing on both direction, but subnet on that interface can also be advertised by “network”command or “redistribute connected subnet”. (OSPF is no different while RIP still allows its passive interface to receive updates from neighbors)
Router(config-router)# passive-interface fa0/0

Inject default route into EIGRP
There are 2 options to inject default route into EIGRP, other EIGRP neighbors will flag that default route as “gateway of last resort”.
+ manually create default route and redistributed it into EIGRP.
   Router(config)# ip route 0.0.0.0 0.0.0.0 s0/0/0
  Router(config)# router eigrp 9
   Router(config-router)# network 0.0.0.0  or redistribute static
+ designate an existing EIGRP network as an default network, only classful network can be used in the command even though the real network is classless. Advertising router itself only mark this default network as a candidate default route while other neighbors will regard it as “gateway of last resort”. When a router receives several default networks from different EIGRP neighbors at the same time, it will select the one with lowest AD then lowest metric as “gateway of last resort”, and mark other default networks as candidate default route. (the classful network should exist in routing table before it can be advertised out, otherwise inject default route will fail unless manually create one with next hop set as Null0)
   Rourter(config)# ip default-network 192.31.7.0
+ use manually summary address to inject a default route, You can use the ip summary-address eigrp as-number 0.0.0.0 0.0.0.0 command to inject a default route to a neighbor, However, the automatically generated route to null0 may cause problems in some topologies.

Interface Bandwidth utilization for EIGRP
By default, EIGRP will limit itself to using no more than 50% of the interface bandwidth. The primary benefit of controlling EIGRP’s bandwidth usage is to avoid losing EIGRP packets, which could occur when EIGRP generates data faster than the interface line can absorb it. This is of particular benefit on Frame Relay networks, where the access interface bandwidth and the PVC capacity may be very different. That is why EIGRP is preferable dynamic routing protocol in large hub-and-spoke FR network. To manipulate the interface bandwidth used for EIGRP, use below commands
Router(config-if)# bandwidth 256
Router(config-if)#
ip bandwidth-percent eigrp 1 90      //90% BW for EIGRP

to be continued.