learning destination MAC if default route uses interface and not next-hop IP

learning destination MAC if default route uses interface and not next-hop IP
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Subject Author Date
learning destination MAC if default route uses interface and not next-hop IP Brad 09-20-2005
Posted by Brad on September 20, 2005, 8:19 am
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I have a router with the following configuration:

service password-encryption
!
hostname suse_class
!
ip subnet-zero
!
no ip domain-lookup
!
interface FastEthernet0/0
ip address 192.168.100.254 255.255.255.0
no shutdown
no ip proxy-arp
ip nat inside
duplex auto
speed auto
!
interface FastEthernet1/0
ip address 10.0.0.1 255.255.255.128
ip access-group inboundfilters in
ip access-group outboundfilters out
no ip proxy-arp
ip nat outside
no shutdown
duplex auto
speed auto
!
!
ip nat pool suseclass 10.0.0.1 10.0.0.1prefix-length 29
ip nat inside source list 1 pool suseclass overload
no ip classless
no ip forward-protocol udp tftp
no ip forward-protocol udp domain
no ip forward-protocol udp time
no ip forward-protocol udp tacacs
ip route 0.0.0.0 0.0.0.0 FastEthernet1/0
no ip http server
!
!
ip access-list extended inboundfilters
evaluate traffic
ip access-list extended outboundfilters
permit tcp any any reflect traffic
permit udp any any reflect traffic
permit icmp any any reflect traffic
access-list 1 permit 192.168.100.0 0.0.0.255
!
line con 0
exec-timeout 0 0
logging synchronous
line aux 0
line vty 0 4
login

The question I have is since I used the interface (fa1/0) in my default
route statement instead of the next-hop IP address how does this router
find out the MAC address for the router on the 10.0.0.0/25 network that
my router is forwarding traffic to out of fa1/0?



NMFall 20%
Posted by Merv on September 20, 2005, 8:28 am
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> how does this router find out the MAC address for the router on the
10.0.0.0/25 > network that > my router is forwarding traffic to out of fa1/0?

It doesn't. It arps for the destination IP address in the outbount
packet and the upstream router responds it it is configured for
proxy-arp.

pointing a static to an interface is a BAD idea; you will want to chane
it to the next hop IP address ( and clear the routers ARP cache (use
shut/no shut if necessary).



Posted by Brad on September 20, 2005, 8:52 am
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Thanks for the reply. That makes sense. If the other router had proxy
arp disabled I guess my router would be unable to communicate to the
outside world?

I did notice that the arp cache was huge, but besides that why is it
such a bad idea?



Posted by Merv on September 20, 2005, 9:00 am
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That is why it is abad idea, that ARP cache has to be scanned
continually by the router. Change to use the next IP hop and it you
control the next hop router disable proxy arp on it also.



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