Routing configuration understanding

[chaitu_kranthi]

                   Hi Team,

I am new to routing technology and trying to understanding the WAN technology implemented at all our offices. Could some one explain me how significance of the below config lines and purpose they placed.

Only thing that i understood it EIGRP protocal which is running between our core switches and BT WAN routers. But we have couple more statement in the config line which i didn't understand the purpose.

When we have a EIGRP protocal running between two core switches and BT WAN routers why again we need Default routing and distribute list.

I am just attaching the tolopology diagram and config for your reference. it would be great if some one explains me why they placed and how it work.

!
Primary Core Switch
===========
!

!
!

!
ip prefix-list DEFAULT-ROUTE-ONLY seq 5 permit 0.0.0.0/0
!
ip prefix-list LOCAL-ROUTES-ONLY seq 5 permit 10.0.0.0/8 ge 9
!
router eigrp 90
redistribute static
network 10.0.0.0
passive-interface vlan2
passive-interface vlan3
passive-interface vlan4
passive-interface vlan5
passive-interface vlan6
passive-interface vlan7
passive-interface vlan47
passive-interface Vlan601
passive-interface Loopback0
!
distribute-list prefix DEFAULT-ROUTE-ONLY in  FastEthernet5/27 (Primary BT WAN Router Connected Interface)
!
distribute-list prefix LOCAL-ROUTES-ONLY out FastEthernet5/27  (Primary BT WAN Router Connected Interface)
!

!
ip route 0.0.0.0 0.0.0.0 10.75.252.4 220 (Secondary Core switch interface IP)

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
!
Secondary Core Switch
===========
!
!

!
ip prefix-list DEFAULT-ROUTE-ONLY seq 5 permit 0.0.0.0/0
!
ip prefix-list LOCAL-ROUTES-ONLY seq 5 permit 10.0.0.0/8 ge 9
!
router eigrp 90
redistribute static
network 10.0.0.0
passive-interface vlan2
passive-interface vlan3
passive-interface vlan4
passive-interface vlan5
passive-interface vlan6
passive-interface vlan7
passive-interface vlan47
passive-interface Loopback0
!
distribute-list prefix DEFAULT-ROUTE-ONLY in  FastEthernet5/27 (Secondary BT WAN Router Connected Interface)
!
distribute-list prefix LOCAL-ROUTES-ONLY out FastEthernet5/27  (Secondary BT WAN Router Connected Interface)
!
!

ip route 0.0.0.0 0.0.0.0 10.75.252.3 220 (primary Core switch interface IP)
!

[rais]

Distribute-list in : filters out routes coming in via EIGRP UPDATES through interface F5/27 to whatever access-list DEFAULT-ROUTE-ONLY permits. In this case default route only.

Distribute-list out: Only sends routes depending on Access-list LOCAL-ROUES-ONLY. In this case 10.0.0.0/8 is sent out to the peer on interface F5/27

passive-interface would include that interface IP subnet in EIGRP updates but not create any EIGRP adjacencies/neighbors on that link. EIGRP packets are not sent out of interface defined with passive-interface.

HTH.