Re: RIPv1 and Update Handling

Hi Keller,

The route won't be accepted because of split horizon rule. Once, the
loopback is removed there is no active interface and hence no rip routes
are present in its database.

Cheers,
Nick

On Mon, Jan 30, 2012 at 5:38 PM, Keller Giacomarro <keller.g_at_gmail.com>wrote:

> Okay, the logic as to why a route is accepted or not, and how RIP is
> processing updates is making my brain melt! I'm hoping you guys can
> give me an assist on this. Here is my current understanding of how
> RIPv1 processes updates. Please correct any of the below statements
> that are incorrect.
>
> * RIPv1 sends updates for all RIP-enabled networks with the full
> network, but no subnet mask information.
>
> * So, for example, 10.20.30.0/24 will be advertised as 10.20.30.0 with no
> mask.
>
> * If the router has a connected interface in the same classful network
> as the update, it will use that interface's mask for the update.
>
> * For example, if the interface the above update was received on is
> 10.20.40.0/28, the route will be installed as 10.20.30.0/28.
>
> * This also applies to connected interfaces that are active in RIP but
> were not actually involved in the receiving the route.
>
> * So, if the update is received on link 20.30.40.0/24, but there is a
> connected loopback that is 10.99.99.0/25, the route will be installed
> as 10.20.30.0/25.
>
> I'm especially confused by the loopback part, as I have a router that
> has a loopback assigned 150.1.10.0/24, and it will not accept a RIPv1
> update of 150.1.0.0 (as seen via 'debug ip rip') into its routing
> table. If I shutdown the loopback, the route is installed. To
> compound my confusion, I don't really understand why SW2 is
> advertising 150.1.0.0 instead of the component subnets (as seen in the
> show output below). It advertises other networks that are not on
> their classful boundaries (advertises 155.1.37.0/24 as 155.1.37.0,
> even though it's a Class B), why not the loopback network?
>
> Here are the relevant configs:
>
> ***SW2***
> router rip
> network 150.1.0.0
> network 155.1.0.0
> no auto-summary
> !
> interface Port-channel1
> no switchport
> ip address 155.1.108.8 255.255.255.0
> ip rip send version 1
> ip rip receive version 1
>
> SW2#show ip route | inc 150.1.
> 150.1.0.0/16 is variably subnetted, 9 subnets, 2 masks
> R 150.1.7.0/24 [120/3] via 155.1.58.5, 00:00:07, Vlan58
> R 150.1.6.0/24 [120/3] via 155.1.58.5, 00:00:07, Vlan58
> R 150.1.5.0/24 [120/1] via 155.1.58.5, 00:00:07, Vlan58
> R 150.1.3.0/24 [120/2] via 155.1.58.5, 00:00:07, Vlan58
> R 150.1.2.0/24 [120/2] via 155.1.58.5, 00:00:07, Vlan58
> R 150.1.1.0/24 [120/2] via 155.1.58.5, 00:00:07, Vlan58
> R 150.1.0.0/16 [120/2] via 155.1.58.5, 00:00:07, Vlan58
> R 150.1.9.0/24 [120/4] via 155.1.58.5, 00:00:07, Vlan58
> C 150.1.8.0/24 is directly connected, Loopback0
>
>
> ***SW4***
> router rip
> network 150.1.0.0
> network 155.1.0.0
> no auto-summary
> !
> interface Port-channel1
> no switchport
> ip address 155.1.108.10 255.255.255.0
> !
>
> SW4#show ip route | inc 150.
> 150.1.0.0/24 is subnetted, 1 subnets
> C 150.1.10.0 is directly connected, Loopback0
> SW4#show ip rip database | inc 150.
> 150.1.0.0/16 auto-summary
> 150.1.10.0/24 directly connected, Loopback0
>
> Any input, thoughts, or other corrections of my understanding are
> greatly appreciated!
>
> Keller Giacomarro
> keller.g_at_gmail.com
>
>
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Received on Tue Jan 31 2012 - 20:59:20 ART