You don't need the "ip classless" command because your default route points to an unnumbered serial interface. If instead it pointed to an IP address that was in the same class as your local Ethernet, then you would have a problem. Here's the classic example:
e0 RouterA s0 ------ s0 RouterB ------>Internet routerA int e0 172.16.10.1 255.255.255.0 int s0 172.16.20.1 255.255.255.0 ip route 0.0.0.0 0.0.0.0 172.16.20.2 routerB int s0 172.16.20.2 255.255.255.0 That confuses Router A. Without "ip classless," it thinks 172.16.0.0/16 is local. But that causes it not to be able to forward traffic to 172.16.20.2, the router on the other end of the serial link that has access to the rest of the world. Try it in a lab, if you get a chance. You'll see that in this situation, you need "ip classless." Priscilla YI Zhou wrote: > > according to many books, ip classless should be used to be able > to use default route. > > but I just found my internet router, actually has no ip > classless. > > which means I am using classful route lookup. > > And this is working fine,who can explain why???? > > note that ip address here is not real one. > > ! > hostname xxx > ! > enable secret xxx > ! > ip subnet-zero > no ip domain-lookup > ip name-server x.x.x.x > > ! > interface FastEthernet0/0 > ip address 210.210.210.62 255.255.255.240 > no ip directed-broadcast > ! > interface Serial0/0 > bandwidth 64 > ip unnumbered FastEthernet0/0 > no ip directed-broadcast > no ip mroute-cache > no fair-queue > ! > no ip classless > ip route 0.0.0.0 0.0.0.0 Serial0/0 > > > Gateway of last resort is 0.0.0.0 to network 0.0.0.0 > > 210.210.210.0/28 is subnetted, 1 subnets > C 210.210.210.48 is directly connected, FastEthernet0/0 > S* 0.0.0.0/0 is directly connected, Serial0/0 > xxx# > > xxx#sh flash > > System flash directory: > File Length Name/status > 1 3612344 c2600-i-mz.120-3.T3 > [3612408 bytes used, 4776200 available, 8388608 total] > 8192K bytes of processor board System flash (Read/Write) Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=53235&t=53231 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
