CCNP Route Redistribution

[justvistin]

Hi All,

Attempting the CCNP Academy course and I've hit a section I'm just not grasping. I can understand what the course is saying, but not WHY. If I don't understand the reasoning behind it, I feel I wont be able to apply it.

Section 5.2.3 Redistributing Routes from Classless to Classful Protocols advises:

'A common problem with redistributing routes between RIP and OSPF is that RIP does not advertise routes out an interface if those routes are on the same major network, but have a different mask than that particular interface.'

See

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093fd9.shtml#ospf_longer

for a scenario.

I feel I'm missing something basic here (and possibly obvious!). Anyone able to offer an explanation ?

Thanks in advance.

 

[lerdalt]

I'm a little rusty on redistribution, so hopefully I won't add to your confusion. What I'm seeing in the example, is specifically RIPv1 which will not be subnetted beyond the class A/B/C masks.

I'm betting (haven't labbed it myself) that if you were to setup a router similar to the one in between the OSPF and RIP networks, and setup the routing, a "show ip route" will show a route for 128.103.88.0/24 via rip. And it will only want to see a 128.103.35.0/24 redistributed from OSPF into RIP.

RIPv2 would change this as well, I believe.

 

[justvistin]

Hi Lerdalt,

Thanks for responding.

I understand that as RIP v1 is classful it would only 'want to see' a 128.103.35.0/24 route, but I'm not grasping why the longer subnet mask on OSPF size of the network (/28) stops the GW-2 router from redistributing the route into RIP alltogether ?

The implication in the text is that if the subnet on the OSPF side was the same as the RIP side (ie /24 on both) the problem woudln't arise.

 

[unclerico]

hopefully a full understanding of the process that RIPv1 goes through when sending/receiving route updates will clear things up bit.

the process that RIPv1 goes through when deciding to send an update is:

•Is the subnet information part of the same major net as the interface that sources the update?

?No: Router 1 summarizes at the major net boundary and advertises the network.

?Yes: Does the network have the same subnet mask as the interface that sources the update?

?Yes: Router 1 advertises the subnet.

?No: Does the network have a /32 mask ?

?Yes: If it is RIP, then the network is advertised. If it is IGRP, then Router 1 drops the network.

?No: Router 1 drops the network.

When receiving an update:

•Is the subnet received in the update on the same major net as the interface that received the update?

?Yes: Router 2 applies the mask of the interface that received the update. If the advertised network has a host bit set in the host portion of the update, Router 2 applies the host mask (/32). In the case of RIP, it continues to advertise the /32 route to the subsequent router, but IGRP does not.

?No: Do any subnets of this major net already exist in the routing table, known from interfaces other than the one that received the update? The network in this update should be a major net unless the link between the two routers is an unnumbered link, in which case it is possible for the update to contain subnet information.

?Yes: Router 2 ignores the update.

?No: Router 2 applies a classful mask. If the update came across an unnumbered link and contains subnet information (bits in subnet portion of network are set), then Router 2 applies a host mask

Here's the full text directly from Cisco:

http://www.cisco.com/en/US/customer/tech/tk364/technologies_tech_note09186a0080093fd8.shtml

I hate all Uppercase... I don't want my groups to seem angry at me all the time! =)
- ColdFlame (vbscript forum)