Hi there 

This is my first time to post a question.

Here is a real scenario which happened a few days ago. Though the problem
has been resolved, i still cannot understand what the cause is.

Customer A has a partner connection to B's network. due to lack of
capability on B's Router/Firewall, one of A's router is plugged directly
onto B's internal LAN(sounds silly, but it is true). 

B's LAN use 10.67.0.0/16 address, of which 10.67.1.x is for servers,
10.67.2.x for routers/switches, 10.67.7.x and 10.67.8.x for DHCP clients.
B's router has 10.67.2.1 addr. 

A's router on B's LAN gets assigned an ip addr 10.67.2.2,but a wrong /24
mask was given by B. since A's users need to talk to B's server, a static
route(ip route 10.67.1.0 255.255.255.0 10.67.2.1) was added. 

A default route is also configured(ip route 0.0.0.0 0.0.0.0 10.67.2.1) on
the A's router.

when this default route was taken off(no obvious reason to point a default
route to B's default router), all B's dhcp clients cannot talk to their own
servers(10.67.1.x) any more even they are on the same subnet.

B's network support was called in, and they found that the A's router is
incorrectly answering ARP requests(by default ip proxy-arp is enabled on the
LAN interface). and somehow the arp respone reaches the client before the
server's, so the client cannot talk to the servers.

the problem later was resolved by rectifying the subnet mask on A's router.
but i still cannot figure out what went wrong when the default route on A's
router was removed.

I'll be much appreciated if anyone can shed some lights on this.

regards

Alec Shi


Senior Support Engineer
Axon Computertime
Auckland
NZ



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