CCNA in the Age of AI
Hace 3 horas
Una interesante forma de ver la importancia de ARP (RFC 826), es a través del uso de Proxy-ARP (RFC 1027). Conisderemos la red de la figura (extracto de una maqueta que usé para el BSCI), cuya intención es méramente académica, no corrrespondiedo a un diseño adecuado.
Desde un extremo vemos como P1R3 tiene todas las entradas ARP de la LAN directamente conectada (10.1.1.0/24). Ojo que por simplicidad hemos eliminado algunas entradas del debug.
P1R3#terminal monitor
P1R3#debug arp
ARP packet debugging is on
P1R3#clear arp-cache
P1R3#
4w6d: IP ARP: sent req src 10.1.1.2 0009.b72d.d5a8,
dst 10.1.1.1 0006.287a.9680 FastEthernet0
4w6d: IP ARP: sent rep src 10.1.1.2 0009.b72d.d5a8,
dst 10.1.1.2 ffff.ffff.ffff FastEthernet0
4w6d: IP ARP: rcvd rep src 10.1.1.1 0006.287a.9680, dst 10.1.1.2 FastEthernet0
4w6d: IP ARP: creating entry for IP address: 10.1.1.1, hw: 0006.287a.9680
P1R3#sh ip arp | i 10.1.1
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.2 - 0009.b72d.d5a8 ARPA FastEthernet0
Internet 10.1.1.1 0 0006.287a.9680 ARPA FastEthernet0
P1R3#sh ip rou
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
i - IS-IS, 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
10.0.0.0/24 is subnetted, 2 subnets
C 10.1.3.0 is directly connected, Ethernet0
C 10.1.1.0 is directly connected, FastEthernet0
P1R3#sh int | i line|Internet
Ethernet0 is up, line protocol is up
Internet address is 10.1.3.3/24
FastEthernet0 is up, line protocol is up
Internet address is 10.1.1.2/24
P1R1#sh run | b interface
interface FastEthernet0/0
ip address 10.1.1.1 255.255.255.252
no ip directed-broadcast
no ip proxy-arp
speed 100
full-duplex
!
interface FastEthernet0/1
ip address 10.1.1.5 255.255.255.252
no ip redirects
no ip directed-broadcast
speed 100
full-duplex
P1R1#sh ip rou
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
i - IS-IS, 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
172.31.0.0/24 is subnetted, 1 subnets
C 172.31.1.0 is directly connected, Serial0/0
10.0.0.0/30 is subnetted, 2 subnets
C 10.1.1.0 is directly connected, FastEthernet0/0
C 10.1.1.4 is directly connected, FastEthernet0/1
P1R1#sh ip arp | i 10.1.1
Internet 10.1.1.2 0 0009.b72d.d5a8 ARPA FastEthernet0/0
Internet 10.1.1.1 - 0006.287a.9680 ARPA FastEthernet0/0
Internet 10.1.1.6 0 000b.be66.0be1 ARPA FastEthernet0/1
Internet 10.1.1.5 - 0006.287a.9681 ARPA FastEthernet0/1
P1R2#sh ip rou
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
i - IS-IS, 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
172.31.0.0/24 is subnetted, 1 subnets
C 172.31.1.0 is directly connected, Serial0/0
10.0.0.0/24 is subnetted, 2 subnets
C 10.1.2.0 is directly connected, FastEthernet0/0
C 10.1.1.0 is directly connected, FastEthernet0/1
P1R2#sh ip arp | i 10.1.1
Internet 10.1.1.2 0 0006.287a.9681 ARPA FastEthernet0/1
Internet 10.1.1.1 0 0006.287a.9681 ARPA FastEthernet0/1
Internet 10.1.1.6 - 000b.be66.0be1 ARPA FastEthernet0/1
Internet 10.1.1.5 0 0006.287a.9681 ARPA FastEthernet0/1
P1R2#ping 10.1.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.2, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
P1R3#ping 10.1.1.6
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.6, timeout is 2 seconds:
4w6d: IP ARP: creating incomplete entry for IP address: 10.1.1.6 interface FastEthernet0
4w6d: IP ARP: sent req src 10.1.1.2 0009.b72d.d5a8,
dst 10.1.1.6 0000.0000.0000 FastEthernet0.
4w6d: IP ARP throttled out the ARP Request for 10.1.1.6.
4w6d: IP ARP: sent req src 10.1.1.2 0009.b72d.d5a8,
dst 10.1.1.6 0000.0000.0000 FastEthernet0.
4w6d: IP ARP: sent req src 10.1.1.2 0009.b72d.d5a8,
dst 10.1.1.6 0000.0000.0000 FastEthernet0.
4w6d: IP ARP: sent req src 10.1.1.2 0009.b72d.d5a8,
dst 10.1.1.6 0000.0000.0000 FastEthernet0.
Success rate is 0 percent (0/5)
P1R3#sh ip arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.2 - 0009.b72d.d5a8 ARPA FastEthernet0
Internet 10.1.1.1 3 0006.287a.9680 ARPA FastEthernet0
Internet 10.1.1.6 0 Incomplete ARPA
P1R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
P1R1(config)#int f0/0
P1R1(config-if)#ip proxy-arp
P1R3#ping 10.1.1.6
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.6, timeout is 2 seconds:
01:00:56: IP ARP: creating incomplete entry for IP address: 10.1.1.6 interface FastEthernet0
01:00:56: IP ARP: sent req src 10.1.1.2 0009.b72d.d5a8,
dst 10.1.1.6 0000.0000.0000 FastEthernet0
01:00:56: IP ARP: rcvd rep src 10.1.1.6 0006.287a.9680, dst 10.1.1.2 FastEthernet0.!!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 1/2/4 ms
P1R3#sh ip arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.2 - 0009.b72d.d5a8 ARPA FastEthernet0
Internet 10.1.3.3 - 0009.b72d.d5a9 ARPA Ethernet0
Internet 10.1.1.1 1 0006.287a.9680 ARPA FastEthernet0
Internet 10.1.3.4 1 000b.be79.8741 ARPA Ethernet0
Internet 10.1.1.6 0 0006.287a.9680 ARPA FastEthernet0
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