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RIP Updates

I have two routers directly connected. Both Routers are running RIP.

Router A

FastEthernet0/0 10.2.1.1 / 24

FastEthernet0/1 10.1.1.1 /24 OR /30

Loopback0 10.10.10.10 /30

Loopback1 10.10.10.20 /32

Loopback3 10.10.10.88 /32

Router B

FastEthernet0/1 10.1.1.2 /24 OR /30

Loopback0 20.20.20.21

Loopback1 20.20.20.30

Fast0/1 are cross connected. When I use subnet mask of /24

on Fast0/1 I see all loopback addresses of Router A on router B.

But, when I use subnet mask of /30 I only see loopback o

on router B routing table.

B#sh ip rou (with mask of /30 on fast 0/1)

20.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

C 20.20.20.20/30 is directly connected, Loopback0

C 20.20.20.30/32 is directly connected, Loopback1

10.0.0.0/30 is subnetted, 2 subnets

R 10.10.10.8 [120/1] via 10.1.1.1, 00:00:11, FastEthernet0/1

C 10.1.1.0 is directly connected, FastEthernet0/1

B#sh ip rou (with mask of /24 on fast 0/1)

20.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

C 20.20.20.20/30 is directly connected, Loopback0

C 20.20.20.30/32 is directly connected, Loopback1

10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks

R 10.2.1.0/24 [120/1] via 10.1.1.1, 00:00:11, FastEthernet0/1

C 10.1.1.0/24 is directly connected, FastEthernet0/1

R 10.10.10.20/32 [120/1] via 10.1.1.1, 00:00:11, FastEthernet0/1

R 10.10.10.88/32 [120/1] via 10.1.1.1, 00:00:11, FastEthernet0/1

From what I understand, even if I use mask of /30 I should see loopback 1/2 addresses of Router A on router B since these are host addresses.

Can anybody explain this behavior. I am running 12.2 IOS on both routers.

thanks...alam

3 REPLIES
New Member

Re: RIP Updates

I am running RIP v1.

Cisco Employee

Re: RIP Updates

The host routes are advertised as long as they fulfill the follwing requirement. In the case the mask on the interface is /30, host routes under the same major net are advertised if the host bits are not all zero. In this case 10.10.10.20 and 10.10.10.88 have their host bits all zero if you apply a 30 bit mask. 10.10.10.22 and 10.10.10.90 for instance would be advertised. The rule is just so there is no confusion between a host and a subnet route (IE: 10.10.10.20/30 and 10.10.10.20/32).

You can apply the same rule for when the interface is configured with a /24 and will see why the two hosts routes are advertised in this case.

Obviously, RIPv2 doesn't have that restriction since it carries subnet mask information in the update message.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
New Member

Re: RIP Updates

Got your point. Thanks.

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