Kelly, Thanks so much for getting back to me. However, I still have a few areas that I'm a bit confused about. Please see comments in-line.
Thanks so much again :-) Best Regards, Hunt Lee ""Kelly Cobean"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > Hunt, > The purpose of routing the summary route to Null0 is because it is a > "shortest match" representation of all of the summarized networks, and > therefore is not an actual network that is assigned to an interface. Let's > use the example you gave, where the following subnets: > > 172.20.10.0/24 > 172.20.15.0/30 > 172.20.15.4/30 > > are summarized using the route 172.20.0.0/16. > In the routing table: D 172.20.0.0/16 is a summary, 00:00:09, Null0 Does this summary route to the Null0 interface get automatically created when one uses "ip summary-address eigrp 10 172.20.0.0 25.255.0.0" command? Or do I have to create one manually with: ip route 172.20.0.0 255.255.0.0 null0 > Jeff should really have written his explanation this way: > "this route helps to prevent potential black holes when default AND summary > routes are used." (Note the capital AND, indicating that you are using both > a default route AND summary routes, not either/or.) > > The summary 172.20.0.0/16 includes lots of potential subnets that aren't > being summarized. If the routers routing table didn't include a route for > the summary, then when receiving a packet destined for, let's say > 172.20.55.5 (just a random address that isn't within the range of one of the > 3 routes above, but still within the 172.20.0.0/16 summary), it would look > in the routing table and select the default route (0/0) because there is no > more specific match for the destination address. This is a waste of time > because by advertising the summary network to all of the other routers, you > are telling your EIGRP network that all routes that fall within the bounds > of 172.20.0.0/16 are downstream from this router. Because you are doing > this, there can't possibly be a host 172.20.55.5 upstream (i.e. toward the > default route) from this router. Therefore, any packet addressed to a > destination that isn't specifically addressed by one of the networks being > summarized (i.e. a more specific route), you want to discard it (Null0) > because there is no network or host matching the Destination IP. Let's look > at what would happen to a packet destined for 172.20.55.5 without this route > to Null 0. > > Step 1 - routerA receives a packet destined for 172.20.55.5 > Step 2 - routerA looks in the routing table for the longest match network to > route the packet to. > Step 3 - There is no specific route to the dest. network, so it sends the > packet to the default route with a next hop of routerB Assuming the RouterB interface to RouterA is 172.10.10.1/24 Unlike OSPF & RIP, I know that both IGRP & EIGRP don't understand the "ip route 0.0.0.0 0.0.0.0 172.10.10.1" command, IGRP & EIGRP needs to use "ip default-network" command. However, how do you set a Next-hop IP / or Exit Interface with this command - to point it to the next hop of RouterB? > Step 4 - routerB receives the packet destined for 172.20.55.5 > Step 5 - routerB looks in it's routing table and finds a matching network, > which is the summary advertised by routerA For these following 3 routes, if one summarized them with a more specific summary route, e.g. 172.20.8.0 /21, the IP address 172.20.55.5 will no longer be summarized by the summary route, so wouldn't this fix the problem? 172.20.10.0/24 172.20.15.0/30 172.20.15.4/30 > Step 6 - routerB sends the packet to the next-hop router for the > summary-route 172.20.0.0/16, which is (routerA) > Step 7 - See step 1 > !!! Routing Loop !!! The packet will bounce back and forth until the TTL > is exceeded, then get dropped. > > Let's look at the same packet with the route to Null0 for the summary on the > router that is advertising it (routerA) > > Step 1 - routerA receives a packet destined for 172.20.55.5 > Step 2 - routerA looks in the routing table for the longest match network to > route the packet to. > Step 3 - routerA finds a longest-match in the routing table - 172.20.0.0/16 > Next-hop - Null0 > Step 4 - routerA discards the packet. > > > Does this all make sense? You could get away without this route to null0 on > the advertising router if your routes being summarized covered every address > in the summary range, but as this is not always the case, they include the > route to Null0 to protect valuable resources on the router from being > consumed routing packets that have no destination. > > Hope this helps. > > Kelly Cobean, CCNP, CCSA, ACSA, MCSE, MCP+I > > > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of > Hunt Lee > Sent: Thursday, February 28, 2002 12:20 AM > To: [EMAIL PROTECTED] > Subject: EIGRP Question [7:36770] > > > 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=37518&t=36770 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
