Hi, As I see it:
A router summarizing, let's say a C class, might have in its routing table several entries for subnets of that C class. This router receives a default route from one of its neighbors. Let's assume there isn't the summary pointing to Null0: a packet coming into this router sent to a subnet not present in the routing table, would be sent to the neighbor advertising the default route, which in turn will send it back because it has in its routing table the summary advertised by our router! The summary route pointing to Null0 prevents this behavior, the packet will be discarded if the destination subnet is not found (it is all about that "longest match" issue) HTH Theo ""Hunt Lee"" wrote in message news:[EMAIL PROTECTED].; > Hi all, > I have an EIGRP question. It would be greatly appreciated if someone can > shed some light on this. > > I found the following Routing Table from TCP / IP Vol1 by Jeff Doyle. But I > don't understand why a summary route would be pointing to Null0? > > Jeff explains it as "this route helps to prevent potential black holes when > default and summary routes are used"... which confuses me even more :( > > > Show ip route > > D 192.168.16.0/24 [90/3219456] via 172.20.15.5, 00:41:41, Serial 0 > C 192.168.17.0/24 is directly connected, Ethernet 0 > C 192.168.18.0/24 is variably subnetted, 2 subnets, 2 masks > D EX 172.25.0.0/16 [170/2221056] via 172.20.15.5, 00:41:48, Serial 0 > 172.20.0.0/16 is variably subnetted, 2 subnets, 2 masks > D 172.20.10.0/24 [90/2195456] via 172.20.15.5, 00:41:48, Serial 0 > C 172.20.15.4/30 is directly connected, Serial 0 > D 172.20.15.0/30 [90/2681856] via 172.20.15.5, 00:41:48, Serial 0 > D 172.20.0.0/16 is a summary, 00:00:09, Null0 > > > Please help... > > Best Regards, > Hunt Lee Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=36782&t=36770 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
