Robert, I believe that your diagram should reflect R1's serial interface to R2 as s0/1 instead of s0/0. This caused me some confusion in trying to figure out the configs. Actually, there is still some confusion given the a duplicate IP (172.16.66.1) assigned to both routers on that serial link -- and that the routing updates are being sourced from that duplicate IP. Perhaps this was your intention???
Anyway, to help answer your question: > I expected that the router would choose netmask belonging to the > subnet of update source (172.16.66.1/25 in this case). This would > make more sense - or maybe I am wrong?? The receiving router, R2 in this case, has no idea what the subnet of the update source is since this information isn't carried in the update itself. Instead, R2 assumes that the advertised RIP network has the same subnet as the network configured on its receiving interface. Since all three of the IP addresses belong to the same major network as the sender, all three are valid candidates for being the intended recipient of the update. The "valid" IP address that R2 seems to be using to interpret the advertised network is obviously 172.16.77.3/29. You make a strong argument that a more logical interpretation would be to use the local IP address 172.16.66.1/25 to interpret the route since it is the only IP address that is on the same subnet as the sending router (since the other IPs configured on the link should, based on normal IP rules, require another router to communicate with the sender). All documentation I've come across and configuration I have done indicates that the receiving router validates the update based on major network only, and then uses the mask of the locally configured address of that network to interpret the incoming networks. So, technically, interpreting the route as 172.16.77.0/29 isn't "wrong" -- it's just one of 3 possible ways of interpreting the advertised network. I'm curious as to whether your configuration works at all given the next-hop address (172.16.66.1) is also a valid IP address on R2. Are you able to ping 172.16.200.1 from R2? It seems to me that R2 should be dropping the traffic instead of forwarding it to R1. If R2 isn't dropping the traffic I'd like to know to why -- Was this intentional and part of your intended lab? Anyway, I hope that this provides some help. - Tom Robert Slaski wrote: > Hi, > > I was almost sure I understood RIP a little. Until today, grrrrrr ;-) > > > > My goal was to inject 172.16.200.0/25 OSPF routes into 172.16.12.0/26 > RIP domain and I tried to use secondary addresses to extend /25 to > RIP domain. > > We have two routers connected via serial link on the edge of a > network: R1 (s0/0) ---- (s0/1) R2 (s0/0) ---- the rest of a network. > > R1 is doing OSPF/RIP redistribution and R2 only in RIP domain: > > router ospf 666 log-adjacency-changes redistribute rip subnets > network 172.16.100.0 0.0.0.7 area 1 ! router rip redistribute ospf > 666 metric 6 passive-interface Serial0/0 network 172.16.0.0 > distribute-list 1 out Serial0/1 ! access-list 101 permit ip > 172.16.200.0 0.0.0.7 any > > > R1: interface Serial0/1 ip address 172.16.66.1 255.255.255.128 > secondary ip address 172.16.12.2 255.255.255.192 no ip split-horizon > > 172.16.0.0/16 is variably subnetted, 4 subnets, 3 masks O IA > 172.16.200.0/25 [110/65] via 172.16.100.2, 02:29:30, Serial0/0 C > 172.16.12.0/26 is directly connected, Serial0/1 C 172.16.100.0/29 is > directly connected, Serial0/0 C 172.16.66.0/25 is directly connected, > Serial0/1 > > > R2: interface Serial0/0 ip address 172.16.77.3 255.255.255.248 > secondary ip address 172.16.66.1 255.255.255.128 secondary ip address > 172.16.12.1 255.255.255.192 clockrate 64000 > > > 172.16.0.0/16 is variably subnetted, 3 subnets, 3 masks C > 172.16.12.0/26 is directly connected, Serial0/0 C 172.16.77.0/29 is > directly connected, Serial0/0 C 172.16.66.0/25 is directly connected, > Serial0/0 > > > Now a RIPv1 update is received: Mar 1 01:12:44.149: RIP: received v1 > update from 172.16.66.1 on Serial0/0 *Mar 1 01:12:44.149: > 172.16.200.0 in 6 hops > > and the routing table looks now: > > C 205.2.3.0/24 is directly connected, Loopback10 172.16.0.0/16 is > variably subnetted, 4 subnets, 3 masks R 172.16.200.0/29 > [120/6] via 172.16.66.1, 00:00:15, Serial0/0 C 172.16.12.0/26 > is directly connected, Serial0/0 C 172.16.77.0/29 is directly > connected, Serial0/0 C 172.16.66.0/25 is directly connected, > Serial0/0 > > We see that an update about 172.16.200.0 was received from > 172.16.66.1 (secondary of serial interface of R1) and installed in > route table. But the netmask was chosen not as I expected: not /25 > subnet locally configured on s0/0 (172.16.66.2/25) has been chosen > but longest-match rule was applied and /29 mask configured on one of > subinterfaces won. > > This behaviour get me confused. Doyle vol.1 doesn't even mention of > choosing masks on receive (this is a great book but lacks of > little-funny-details by the way), and even more detailed and full of > algorithms Zinin's book did not clear this (p.325): "The route mask > is determined as follows. If the network reported is the route > belongs to the same major network as one of the interfaces assigned > subnets - primary or secondary - the route mask is the same as the > subnet mask of the interface's address. Otherwise, the classfull > default address mask is used." > > I expected that the router would choose netmask belonging to the > subnet of update source (172.16.66.1/25 in this case). This would > make more sense - or maybe I am wrong?? > > Could anybody clarify this? > > robert, -- > archives, and subscription info: > http://www.groupstudy.com/list/cisco.html Report misconduct and > Nondisclosure violations to [EMAIL PROTECTED] Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=57168&t=57049 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
