How a Null route can prevent a routing loop. If you have, for example, 4 networks behind you router.
10.1.0.0\24 10.1.1.0\24 10.1.2.0\24 10.1.3.0\24 And to reduce the size of the routing tables in the upstream routers you summarize those networks to one network. 10.1.0.0\26 and you have a default route point to the upstream router. All is good, until you lose one of your networks. Now you don't have 10.1.2.0\24 in your routing table, *&*^$#! So you send it to the upstream router, your gateway of last resort. The upstream router has your summary route so the packet is routed back to you, %&^$^%^%*! And around it goes... In come the null route to save the day. Add a null route to your summary address and when you have the route in your routing table, more specific prefix, you route it accordingly. However, when a more specific route disappears packets destined to the missing route will match the null route, thus stopping the loop. Hope that explains at least one case. Philip -----Original Message----- From: Chuck [mailto:[EMAIL PROTECTED]] Sent: Wednesday, May 01, 2002 10:18 PM To: [EMAIL PROTECTED] Subject: Re: ip route statement [7:43001] can you provide an example of what circumstance might require the use of a null route to prevent a loop? My lack of imagination is preventing me from deriving my own example. ""Ladrach, Daniel E."" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > The Null interface is typically used for preventing routing loops. > > Daniel Ladrach > CCNA, CCNP > WorldCom > > > > -----Original Message----- > > From: Stanfast Preye [mailto:[EMAIL PROTECTED]] > > Sent: Wednesday, May 01, 2002 2:34 PM > > To: [EMAIL PROTECTED] > > Subject: ip route statement [7:43001] > > > > > > Dear Group, > > > > Why is it necessary to configure all routers in a network > > with "ip route > > xxx.xxx.xxx.xxx null 0" statement before implementing > > migrating to a new IP > > address scheme and DHCP service in the network. > > > > Somebody please help!!!!!!!! > > > > Regards, > > > > Preye. > > > > > > > > > > > > > > > > --------------------------------- > > Do You Yahoo!? > > Yahoo! Tax Center - online filing with TurboTax Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=43123&t=43001 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
