IP unnumbered interface ARPs, simple topology - hard question.
Hello, i've a question regarding ip unnumbered interface.
I have a simple topology:
[USERS]--[SWICTH]-[CISCO GATEWAY]
The gateway is configured with ip unnumberd loopback command:
#sh ip int br
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 unassigned YES unset up up
FastEthernet0/0.2 10.10.10.1 YES TFTP up up
Loopback0 10.10.10.1 YES manual up up
R1#
User configartion: user subinterface with his static route.
R1#show running-config interface fastEthernet 0/0.2
Current configuration : 109 bytes
!
interface FastEthernet0/0.2
encapsulation dot1Q 2
ip unnumbered Loopback0
no snmp trap link-status
end
ip route 172.16.1.44 255.255.255.255 FastEthernet0/0.2
All is working fine.
User is configured with 172.16.1.44 sends arp request and GW replies.
BUT WE ALL KNOW THAT ARP REQUESTS AND ROUTING TABLE ARE NOT RELATIVE, ARP REPLY IS SENT BACK TO INTERFACE WHERE IT RECEIVED.
so the question goes: Why user can't change IP to other than STATIC ip route on gateway states. ? I know that internet to user will not wotk because there will be diferent static route pointing to his interface than his 'fake' ip address. BUT WHY USER DOESN'T GET AN ARP REPLY FROM GATEWAY when user is using invalid IP address ? The question is just about ARP. Since we all know that routes and arp processing is different things, why ARP reply is sent when static route to user is the same as the user ip address.. HOW Cisco router knows that he should'nt sent arp reply to user with invalid IP. The error on gw goes:
*Mar 1 04:42:58.678: IP ARP req filtered src 2.2.2.2 4444.4444.4444, dst 8.8.8.8 c000.3ef0.0000 wrong cable, interface FastEthernet0/0.2
*Mar 1 04:42:58.678: IP ARP rep filtered src 2.2.2.2 4444.4444.4444, dst 2.2.2.2 ffff.ffff.ffff wrong cable, interface FastEthernet0/0.2
Here user sent arp request and gratuitous arp with source 2.2.2.2. How Gateway understand that this request is invalid ? SINCE router doesnt consults his routing table when sending arp reply.
THANKS !!
I have a simple topology:
[USERS]--[SWICTH]-[CISCO GATEWAY]
The gateway is configured with ip unnumberd loopback command:
#sh ip int br
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 unassigned YES unset up up
FastEthernet0/0.2 10.10.10.1 YES TFTP up up
Loopback0 10.10.10.1 YES manual up up
R1#
User configartion: user subinterface with his static route.
R1#show running-config interface fastEthernet 0/0.2
Current configuration : 109 bytes
!
interface FastEthernet0/0.2
encapsulation dot1Q 2
ip unnumbered Loopback0
no snmp trap link-status
end
ip route 172.16.1.44 255.255.255.255 FastEthernet0/0.2
All is working fine.
User is configured with 172.16.1.44 sends arp request and GW replies.
BUT WE ALL KNOW THAT ARP REQUESTS AND ROUTING TABLE ARE NOT RELATIVE, ARP REPLY IS SENT BACK TO INTERFACE WHERE IT RECEIVED.
so the question goes: Why user can't change IP to other than STATIC ip route on gateway states. ? I know that internet to user will not wotk because there will be diferent static route pointing to his interface than his 'fake' ip address. BUT WHY USER DOESN'T GET AN ARP REPLY FROM GATEWAY when user is using invalid IP address ? The question is just about ARP. Since we all know that routes and arp processing is different things, why ARP reply is sent when static route to user is the same as the user ip address.. HOW Cisco router knows that he should'nt sent arp reply to user with invalid IP. The error on gw goes:
*Mar 1 04:42:58.678: IP ARP req filtered src 2.2.2.2 4444.4444.4444, dst 8.8.8.8 c000.3ef0.0000 wrong cable, interface FastEthernet0/0.2
*Mar 1 04:42:58.678: IP ARP rep filtered src 2.2.2.2 4444.4444.4444, dst 2.2.2.2 ffff.ffff.ffff wrong cable, interface FastEthernet0/0.2
Here user sent arp request and gratuitous arp with source 2.2.2.2. How Gateway understand that this request is invalid ? SINCE router doesnt consults his routing table when sending arp reply.
THANKS !!