Category: IP routing
DHCP Response Sets the Default Route
It makes perfect sense in hindsight, but I was nonetheless pleasantly surprised: when the router acting as a DHCP client (configured with the ip address dhcp interface configuration command) receives the DHCP reply packet containing the default gateway option (option #3), it installs a static default route toward that next-hop.
Even better, the default route is installed with the administrative distance 254 (floating static route), making sure that the default route you’ve configured manually or the default route received via a routing protocol are not overwritten.
When is the default-gateway used?
b2#show ip route
Default gateway is 192.168.0.5
Host Gateway Last Use Total Uses Interface
1.2.3.4 192.168.0.10 0:00 13 FastEthernet0/0
Disabling IP routing on a router makes perfect sense if you use it as a (reverse) terminal server or telnet-to-X.25 gateway.
Sinkholes and blackholes
Why is the first ping lost?
When pinging a directly-attached host (end-station) from a router, it's quite common to lose the first reply, as shown in the following example (the same symptom might occur when pinging a remote host that has been inactive).
a2#ping 10.0.0.10
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.0.10, timeout is 2 seconds:
.!!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 1/1/4 ms
Actually, it's not the reply that was lost, the request was never sent out. Whenever a router has to send a packet to the next-hop (or directly attached destination) that has no entry in the ARP table, the ARP request is sent out, but the original packet is unconditionally dropped.
Not All Static Routes Were Created Equal
In his excellent question, Matt reminded me of (almost prehistoric) days when static routes pointing to a connected interface (not IP next-hop) had administrative distance zero. I also remembered that we've had interesting routing problems as those static routes actually behaved like connected routes (and were redistributed into routing protocols with redistribute connected command).
Reliable static routing
IOS release 12.3T introduced an interesting concept: static routes that are used only if an object a router is tracking (for example, next-hop router) is available. Named reliable static routing, this feature allows you to constantly ping next-hop router (or any other IP address) and use the static routes only if the tested destination is reachable.While the reliable static routes are easy to understand and configure, a careless implementation can lead to interesting routing loops or other instability problems. You'll find the needed in-depth design and implementation guidelines in my IP Corner article Small Site Multi-homing (which is one of the prime applications for reliable static routing) in sections Not-so-Very-Static-Routes and End-to-End Connectivity Test.
Unequal load split with static routes
Unequal load-sharing with static routes is almost impossible as there is no configuration command to assign non-default traffic share count to a static route. For example, if you configure two default routes, one pointing to a low-speed interface and another one pointing to a high-speed interface, there is no mechanism to force majority of the traffic onto the high-speed link (IOS ignores interface bandwidth when calculating load sharing ratios).
You can, howerer, use a workaround: if you configure multiple routes for the same prefix pointing to the same interface, that interface will attract proportionally more outbound traffic.