Ahorrando al máximo las direcciones IP
Fecha: 8 de mayo del 2020
Escenario
Cuando estudiamos CCNA, aprendemos que con el CIDR y el VLSM no desperdiciamos, o desperdiciamos
menos cantidades de direcciones IP, y utilizando casi siempre subredes /30 en los enlaces punto a punto.
Pero la filosofía del punto a punto nos permite ahorrar aún más, por ejemplo descartando la dirección de
broadcast ya que tenemos solo un host en el otro extremo, para que realizar anuncios a mas ?
Este laboratorio es bastante simple y nos permite implementar una WAN con segmento /31 con solo dos IP
permitidas, lo curioso es que la primer IP del segmento es 10.0.0.0, algo a lo que estamos acostrumbrados a
tener como dirección de red o subred. Este tipo de segmento /31 cumple con la RFC 3021 y permite obtener
128 enlaces punto a punto (128x2=256) contra 64 enlaces /30 (64x4=256).
Inicialmente el enrutamiento lo declaramos con interface de salida, sin IP de próximo salto, luego con IP del
próximo salto (incluyendo la 10.0.0.0) y por último con OSPF, declarando una network 0.0.0.1
Este laboratorio se lo dedico a mi compañero Alejandro Dominguez, con quien tuvimos la experiencia de ver a
alguien pinguear una IP .0 (no por este motivo) y me inspiró para hacerlo.
1.- Configuración inicial:
1.1.- Configuramos en el router A:
1.1.1.- Configuramos interface pseudo-LAN:
Tuve que hacer esto para simular la LAN, simplificar el lab y centrarnos en el segmento WAN.
Router-A#conf t
Router-A(config)#int Loop0
Router-A(config-if)#ip add 192.168.1.1 255.255.255.255
Router-A(config-if)#exit
Router-A(config)#
1.1.2.- Configuramos interface WAN:
Router-A(config)#int fa0/0
Router-A(config-if)#ip add 10.0.0.0 255.255.255.254
% Warning: use /31 mask on non point-to-point
interface cautiously (no es error, es un warning)
Router-A(config-if)#exit
Router-A(config)#
Detalle: este lab se realizó con equipos reales, este es el
resultado en Packet Tracer:
Router(config-if)#ip add 10.0.0.0 255.255.255.254
Bad mask /31 for address 10.0.0.0
Router(config-if)#
Router(config-if)#ip add 10.0.0.1 255.255.255.254
Bad mask /31 for address 10.0.0.1
Router(config-if)#
1.1.3.- Configuramos enrutamiento:
Router-A(config)#ip route 192.168.2.1 255.255.255.255 fa0/0
Router-A(config)#exit
Router-A#
1.2.- Configuramos en el router B:
1.2.1.- Configuramos interface pseudo-LAN:
Router-B#conf t
Router-B(config)#int Loop0
Router-B(config-if)#ip add 192.168.2.1 255.255.255.255
Router-B(config-if)#exit
Router-B(config)#
1.2.2.- Configuramos interface WAN:
Router-B(config)#int fa0/0
Router-B(config-if)#ip add 10.0.0.1 255.255.255.254
% Warning: use /31 mask on non point-to-point
interface cautiously
Router-B(config-if)#exit
Router-B(config)#
1.2.3.- Configuramos enrutamiento:
Router-B(config)#ip route 192.168.1.1 255.255.255.255 fa0/0
Router-B(config)#exit
Router-B#
2.- Verificación:
2.1.-
De conectividad:
Router-A#ping
192.168.2.1 source loop0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout
is 2 seconds:
Packet sent with a source address of
192.168.1.1
!!!!!
Success rate is 100 percent (5/5), round-trip
min/avg/max = 1/2/4 ms
Router-A#
Router-B#ping
192.168.1.1 source loop0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.1,
timeout is 2 seconds:
Packet sent with a source address of
192.168.2.1
.!!!!
Success rate is 80 percent (4/5), round-trip
min/avg/max = 1/1/4 ms
Router-B#
Router-B#ping
10.0.0.0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.0.0,
timeout is 2 seconds:
Packet sent with a source address of 10.0.0.1
!!!!!
Success rate is 100 percent (5/5), round-trip
min/avg/max = 1/2/4 ms
Router-B#
2.2.- Verificamos la tabla ARP:
Router-A#sh
arp
Protocol Address
Age (min) Hardware Addr Type
Interface
Internet 10.0.0.0 - 0017.95c0.aca2 ARPA
FastEthernet0/0
Internet 10.0.0.1
0 000d.658f.84c7 ARPA FastEthernet0/0
Internet 192.168.2.1 0 000d.658f.84c7 ARPA FastEthernet0/0 (es por el proxy ARP y a pesar de no estar en
Router-A# un segmento directamente conectado)
Router-B#sh
arp
Protocol Address
Age (min) Hardware Addr Type
Interface
Internet 10.0.0.0 0 0017.95c0.aca2 ARPA
FastEthernet0/0
Internet 10.0.0.1
- 000d.658f.84c7 ARPA
FastEthernet0/0
Internet 192.168.1.1
0 0017.95c0.aca2 ARPA FastEthernet0/0
Router-B#
2.3.- Verificamos la tabla de enrutamiento:
Router-A#sh
ip route
Codes: C - connected, S - static, R - RIP, M -
mobile, B - BGP
D
- EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1
- OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1
- OSPF external type 1, E2 - OSPF external type 2
i
- IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user
static route
o
- ODR, P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/31 is subnetted, 1 subnets
C
10.0.0.0 is directly connected, FastEthernet0/0
C
192.168.1.0/24 is directly connected, Loopback0
S 192.168.2.0/24
is directly connected, FastEthernet0/0
Router-A#
Router-B#sh
ip route
Codes: C - connected, S - static, R - RIP, M -
mobile, B - BGP
D
- EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1
- OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1
- OSPF external type 1, E2 - OSPF external type 2
i
- IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user
static route
o
- ODR, P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/31 is subnetted, 1 subnets
C
10.0.0.0 is directly connected, FastEthernet0/0
S
192.168.1.0/24 is directly connected, FastEthernet0/0
192.168.2.0/32 is subnetted, 1 subnets
C
192.168.2.1 is directly connected, Loopback0
Router-B#
3.- Verificamos comportamiento de ARP:
Router-A#clear arp
Router-A#
Router-A#debug arp
ARP packet debugging is on
Router-A#
Router-A#ping
192.168.2.1 source loop0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.1,
timeout is 2 seconds:
Packet sent with a source address of
192.168.1.1
!!!!!
Success rate is 0 percent (0/5)
Router-A#
*May 8 12:45:23.703: IP ARP: creating
incomplete entry for IP address: 10.0.0.1 interface FastEthernet0/0
*May 8 12:45:23.703: IP ARP: sent req src
10.0.0.0 0017.95c0.aca2,
dst 10.0.0.1 0000.0000.0000 FastEthernet0/0
*May 8 12:45:25.703: IP ARP: sent req src
10.0.0.0 0017.95c0.aca2,
dst 10.0.0.1 0000.0000.0000 FastEthernet0/0
*May 8 12:45:27.703: IP ARP: sent req src
10.0.0.0 0017.95c0.aca2,
dst 10.0.0.1 0000.0000.0000 FastEthernet0/0
*May 8 12:45:29.703: IP ARP: sent req src
10.0.0.0 0017.95c0.aca2,
dst 10.0.0.1 0000.0000.0000 FastEthernet0/0
*May 8 12:45:31.703: IP ARP: sent req src
10.0.0.0 0017.95c0.aca2,
dst 10.0.0.1 0000.0000.0000 FastEthernet0/0
4.- Modificamos las rutas estáticas:
4.1.- reemplazamos interface de salida por IP del próximo salto:
Router-A(config)#no ip route 192.168.2.1 255.255.255.255
fa0/0
Router-A(config)#ip route 192.168.2.1 255.255.255.255
10.0.0.1
Router-A(config)#exit
Router-A#
Router-B(config)#no ip route 192.168.1.1 255.255.255.255
fa0/0
Router-B(config)#ip route 192.168.1.1 255.255.255.255
10.0.0.0 (rarísimo, sólo con el tío Ernesto)
Router-B(config)#exit
Router-B#
4.2.-
Verificamos:
Router-A#sh
ip route
Codes: C - connected, S - static, R - RIP, M -
mobile, B - BGP
D
- EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1
- OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1
- OSPF external type 1, E2 - OSPF external type 2
i
- IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user
static route
o
- ODR, P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/31 is subnetted, 1 subnets
C
10.0.0.0 is directly connected, FastEthernet0/0
C
192.168.1.0/24 is directly connected, Loopback0
192.168.2.0/32 is subnetted, 1 subnets
S
192.168.2.1 [1/0] via 10.0.0.1
Router-A#
Router-B#sh
ip route
Codes: C - connected, S - static, R - RIP, M -
mobile, B - BGP
D
- EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1
- OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1
- OSPF external type 1, E2 - OSPF external type 2
i
- IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user
static route
o
- ODR, P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/31 is subnetted, 1 subnets
C
10.0.0.0 is directly connected, FastEthernet0/0
192.168.1.0/32 is subnetted, 1 subnets
S 192.168.1.1 [1/0] via 10.0.0.0
192.168.2.0/32 is subnetted, 1 subnets
C
192.168.2.1 is directly connected, Loopback0
Router-B#
5.- Pruebas con OSPF:
5.1.- Eliminamos rutas estáticas:
Router-A(config)#no ip route 192.168.2.0 255.255.255.0 fa0/0
Router-B(config)#no ip route 192.168.1.0 255.255.255.0 fa0/0
5.2.-
Configuramos OSPF:
Router-A(config)#router ospf 1
Router-A(config-router)#network 192.168.1.1 255.255.255.255 area 0
Router-A(config-router)#network 10.0.0.0 0.0.0.1 area 0 (nunca visto, generalmente en CCNA
es 0.0.0.3)
Router-A(config-router)#end
Router-A#
Router-B(config)#router ospf 1
Router-B(config-router)#network 192.168.2.1 0.0.0.0 area 0
Router-B(config-router)#network 10.0.0.0 0.0.0.1 area 0
Router-B(config-router)#end
Router-B#
5.3.-
Verificación:
Router-A#
*May 8 12:23:22.419: %OSPF-5-ADJCHG:
Process 1, Nbr 192.168.2.1 on FastEthernet0/0 from LOADING to FULL, Loading
Done
Router-A#
Router-A#sh
ip route
Codes: C - connected, S - static, R - RIP, M -
mobile, B - BGP
D
- EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1
- OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1
- OSPF external type 1, E2 - OSPF external type 2
i
- IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user
static route
o
- ODR, P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/31 is subnetted, 1 subnets
C
10.0.0.0 is directly connected, FastEthernet0/0
C
192.168.1.0/24 is directly connected, Loopback0
192.168.2.0/32 is subnetted, 1 subnets
O 192.168.2.1 [110/2] via 10.0.0.1,
00:00:39, FastEthernet0/0
Router-A#
Router-B#sh
ip route
Codes: C - connected, S - static, R - RIP, M -
mobile, B - BGP
D
- EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1
- OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1
- OSPF external type 1, E2 - OSPF external type 2
i
- IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user
static route
o
- ODR, P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/31 is subnetted, 1 subnets
C
10.0.0.0 is directly connected, FastEthernet0/0
O 192.168.1.1 [110/2] via 10.0.0.0,
00:00:35, FastEthernet0/0
192.168.2.0/32 is subnetted, 1 subnets
C
192.168.2.1 is directly connected, Loopback0
Router-B#
6.- Configuración de los equipos:
Router-A#sh runn (solo lo mas relevante)
Building configuration...
Current configuration :
669 bytes
!
version 12.4
!
hostname Router-A
!
interface Loopback0
ip address 192.168.1.1 255.255.255.0
!
interface FastEthernet0/0
ip address 10.0.0.0 255.255.255.254
!
ip route 192.168.2.1 255.255.255.255 10.0.0.1
!
end
Router-A#
Router-B#
sh runn (solo lo mas
relevante)
Building configuration...
Current configuration :
828 bytes
!
version 12.3
!
hostname Router-B
!
interface Loopback0
ip address 192.168.2.1 255.255.255.255
!
interface FastEthernet0/0
ip address 10.0.0.1 255.255.255.254
!
ip route 192.168.1.1 255.255.255.255 10.0.0.0
!
end
Router-B#
(2020) Two
is good, three is a crowd
Rosario, Argentina