From: CCIE RnS (networker.ccie@gmail.com)
Date: Sun Sep 28 2008 - 08:20:32 ART
You are seeing subnet Mask because of the contiguous subnet mask used by you
on both R1 and R2
RIP v1 is classful and cannot do VLSM because it doesn't advertise the
subnet mask with the routing updates. Every subnet mask on R2 is a
/29, "network
192.168.20.0" under your RIP config to have RIP run on all of the interfaces
and advertise the networks on R2 .
RIP picks up that the networks in same Calss C classful subnet i.e
192.168.20.0 and are using the /29 mask locally even on the outgoing
interface. So it will advertise all of the /29 subnet numbers individually
(without the applicable subnet mask).
Your Debugs shows this below, R1 is receiving subnet numbers but not the
subnet mask.
*Mar 1 00:04:00.775: RIP: received v1 update from 192.168.20.34 on
FastEthernet0/0
*Mar 1 00:04:00.775: 192.168.20.40 in 1 hops
*Mar 1 00:04:00.779: 192.168.20.48 in 1 hops
R1#
On the Other side, when the router R1 receives the advertisements from R2,
default behavior will be to automatically assume a /27 subnet mask on the
subnet number received because that's the locally used subnet mask Calss C
classful subnet i.e 192.168.20.0 on the Router R1. So
Since in your case 192.168.20.40 and 192.168.20.48 are not valid subnet
numbers for /27 mask it assumes it to be a /32 and places it in the routing
table as /32
But if you will check the same on R2 you will see that R2 will be receiving
2 subnet numbers 192.168.20.64 and 192.168.20.128 and both of these subnet
number are valid subnet number for the R2's Local subnet mask /29 hence they
are placed in routing table as 192.168.20.64/29 and 192.168.20.128/29.
Where /29 is the local subnet number of classful class C network address
used on R2. Which causes the subnet mask of prefix changed from /27 to /29
Regards
Sachin
On Sun, Sep 28, 2008 at 1:20 PM, Vibeesh S <vibselva@gmail.com> wrote:
> Hi All,
>
> I have two router connected back to back. I just started configuring rip
> on
> them. I got a lil confused by seeing the route table.
> I want to know how the subnet masks are being shown when i am using ver1 of
> rip. *[192.168.20.48/32 ]*
> I have been practicing a lil while now & my head is aching, am not able to
> reason out. It will be nice if somebody can help me out here.
>
>
> R1 (f0/0) ---------------------------------- (f0/0) R2
>
> R1Config
> --------------
> interface Loopback0
> ip address 192.168.20.129 255.255.255.224
> !
> interface Loopback1
> ip address 192.168.20.65 255.255.255.224
> !
> interface FastEthernet0/0
> ip address 192.168.20.33 255.255.255.224
> duplex auto
> speed auto
> !
> !
> router rip
> network 192.168.20.0
> version 1
>
>
> R2 Config
> --------------
> interface Loopback0
> ip address 192.168.20.49 255.255.255.248
> !
> interface Loopback1
> ip address 192.168.20.41 255.255.255.248
> !
> interface FastEthernet0/0
> ip address 192.168.20.34 255.255.255.248
> duplex auto
> speed auto
> !
> interface FastEthernet0/1
> no ip address
> shutdown
> duplex auto
> speed auto
> !
> router rip
> version 1
> network 192.168.20.0
>
>
> R1 show ip route
> -------------------------
> Gateway of last resort is not set
>
> 192.168.20.0/24 is variably subnetted, 5 subnets, 2 masks
> C 192.168.20.64/27 is directly connected, Loopback1
> R 192.168.20.48/32 [120/1] via 192.168.20.34, 00:00:01,
> FastEthernet0/0
> R 192.168.20.40/32 [120/1] via 192.168.20.34, 00:00:01,
> FastEthernet0/0
> C 192.168.20.32/27 is directly connected, FastEthernet0/0
> C 192.168.20.128/27 is directly connected, Loopback0
>
> R1 Deb ip rip
> --------------------
>
> R1#debug ip rip
> RIP protocol debugging is on
> *Mar 1 00:03:49.935: RIP: sending v1 update to 255.255.255.255 via
> FastEthernet0/0 (192.168.20.33)
> *Mar 1 00:03:49.935: RIP: build update entries
> *Mar 1 00:03:49.935: subnet 192.168.20.64 metric 1
> *Mar 1 00:03:49.939: subnet 192.168.20.128 metric 1
> *Mar 1 00:03:58.463: RIP: sending v1 update to 255.255.255.255 via
> Loopback1 (192.168.20.65)
> *Mar 1 00:03:58.463: RIP: build update entries
> *Mar 1 00:03:58.463: subnet 192.168.20.32 metric 1
> *Mar 1 00:03:58.463: subnet 192.168.20.40 metric 2
> *Mar 1 00:03:58.467: subnet 192.168.20.48 metric 2
> *Mar 1 00:03:58.467: subnet 192.168.20.128 metric 1
> *Mar 1 00:04:00.371: RIP: sending v1 update to 255.255.255.255 via
> Loopback0 (192.168.20.129)
> *Mar 1 00:04:00.371: RIP: build update entries
> *Mar 1 00:04:00.371: subnet 192.168.20.32 metric 1
> *Mar 1 00:04:00.375: subnet 192.168.20.40 metric 2
> *Mar 1 00:04:00.375: subnet 192.168.20.48 metric 2
> *Mar 1 00:04:00.375: subnet 192.168.20.64 metric 1
> *Mar 1 00:04:00.775: RIP: received v1 update from 192.168.20.34 on
> FastEthernet0/0
> *Mar 1 00:04:00.775: 192.168.20.40 in 1 hops
> *Mar 1 00:04:00.779: 192.168.20.48 in 1 hops
> R1#
> R1#u all
>
>
> Thanks,
> Vibeesh
>
>
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This archive was generated by hypermail 2.1.4 : Sat Oct 04 2008 - 09:26:20 ART