My Stupid Moments: Interface Default Route
Years ago I was faced with an interesting challenge: an Internet customer was connected to our PE router with an Ethernet link and I did not want to include the PE router’s IP address in the default route on the CE router.
The latest IOS release in those days was probably somewhere around 11.x; none of the DHCP goodies were available.
After pondering the problem for a while, I got a “brilliant” idea: if I would use an interface default route, proxy-ARP would solve all my problems. This is the configuration I’ve deployed on the CE-router:
interface Ethernet 0 description Uplink to the ISP ip address 10.0.1.2 255.255.255.0 ! ip route 0.0.0.0 0.0.0.0 ethernet 0
We tested this configuration in the middle of the night and it worked as expected. What do you think happened in the morning?
BTW, I'm curious as to why you didn't want to include the PE router's IP address in the default route?
May be other PE routers started to reply for CE router's ARP queries and hence CE router cannot distinguish between those replies.
Yes you are right. there will be arp entry for each address where cpe trying to send packet to.
2. Your router was running out of memory because the ARP cache was overloaded (not exactly sure how the ARP cache functions... does it expire entries or just eat up resources until... CRASH prolly depends on the router-hardware/version)?
3. Performance slowed to a crawl?
4. All of the above?
"This kind of default route, especially if it is used by a lot of packets to many different destination subnets, can cause high processor utilization and a very large ARP cache (along with attendant memory allocation failures)."
""With this type of configuration, a router performs Address Resolution Protocol (ARP) on the Ethernet for every destination the router finds through the default route because the router considers all of these destinations as directly connected to Ethernet 0""
its not wise to use static route on an interface
This kind of default route, especially if it is used by a lot of packets to many different destination subnets, can cause high processor utilization and a very large ARP cache (along with attendant memory allocation failures)