From: Baety Wayne SrA 18 CS/SCBX (Wayne.Baety@kadena.af.mil)
Date: Tue Oct 01 2002 - 23:55:17 GMT-3
So far 4 solutions that work...
Yes, what you've stated is called tunnelling the subnet with secondaries
(1st Solution). That would work in this instance because there is only 1
router in his diagram involved on the RIP side. It's viable as long as he
remembers to place a secondary network throughout his RIP domain on each
router link (sorta like creating a path) to maintain connectivity and
reachability information about that particular subnet size. (RIP silently
drops information about subnets across subnets with a differing subnet size
but part of the same classful network). It may be better to use a different
classful network altogether (2nd Solution) and rely on auto-summarization
and advertisement of the classful network. He could also subdivide the
smaller size prefix with secondary networks into multiple networks that
match the larger subnet size of the RIP domain (3rd Solution). To get this
into the routing table static addressing would be the the preferred method
but using secondaries on one of the router links in the OSPF domain and
relying on proxy-arp to determine destination next-hop would also work.
Solution 2 and 3 require modification of relatively a few routers and
probably can be more easily documented than Solution 1. A 4th Solution
(that was mentioned) is to create a tunneled interface matching the subnet
size of the RIP domain with the endpoint of the tunnel running to each RIP
router requiring reachability information about that subnet size. This is
really just Solution 1 with performance drawbacks for the sake of
convenience and possibly smaller subnet utilization (if every router doesn't
require reachability information). Both Solution 1 and 4 require
provisioning a new subnet for each router link just for the sake of carrying
information about subnets that have a different subnet size in the routing
table: something that can be quite wasteful if deployed in a large scale
fashion.
WAYNE A. BAETY, SRA, MCSE, USAF
18th Communications Squadron
DSN: 634-9927
Commercial: 011 81 611 734 9927
-----Original Message-----
From: folivore [mailto:folivore@hotmail.com]
Sent: Wednesday, October 02, 2002 1:47 AM
To: Baety Wayne SrA 18 CS/SCBX; 'Young K. Bae'; ccielab@groupstudy.com
Subject: Re: What if OSPF routes have shorter masks than RIP?
For the original diagram, the easiest way is to configure a 172.16.x.0/25
network as secondary IP on whatever interface between R1 and R2, and use
neighbor command in RIP on this secondary IP.
----- Original Message -----
From: "Baety Wayne SrA 18 CS/SCBX" <Wayne.Baety@kadena.af.mil>
To: "'Young K. Bae'" <ybae@cisco.com>; <ccielab@groupstudy.com>
Sent: Monday, September 30, 2002 11:24 PM
Subject: RE: What if OSPF routes have shorter masks than RIP?
> Non 'ip route' method (relies on proxy-arp or auto summarization)
>
> Choose multiple secondary networks that complete the /25.
>
> Put the secondary addresses on an OSPF enabled interface. Make sure
> it's
a
> real interface involved with packet forwarding (i.e. not a loopback
> interface) otherwise you will kitchen sink packets destined for the
> more specific routes.
>
> Turn Proxy-ARP on for all OSPF routers that could route the more
> specific /26's and that are directly connected to the advertising OSPF
> router (Any router advertising the /25 LSA) (This step can be ignored
> if you're
dealing
> with a classful routing protocol that auto summarizes and you use two
> address ranges that cross a classful boundary).
>
> Do not insert the /26's into your OSPF domain.
>
> Inject the /26's into your RIP domain.
>
> OSPF (/25)-----Any-------RIP (/26)
> /26 OSPF <-- /26
> /26 Router <-- /26
>
>
> There are two problems with this. The first, is that you will have to
> choose an ip address that is not being used anywhere else in your
> network
to
> use for the secondaries (the normal ip address conflict detection code
will
> not work). Whatever you choose would need to be heavily documented.
>
> I tested this by using three routers.
>
> Lo1 (/25:1)
> |
> |
> A----------B---------C
> /25:2 /26:3
> /26:1a
> /26:1b
>
> Router A answered ARP packets for addresses within the range of /26:1a
> and /26:1b because it had /25:1 in its routing table. Also, Router B
summarized
> the /26's into a /24 before advertising it to Router C since you will
> be crossing a classful boundary. However, this technique will also
> work for classless routing protocols even without relying on the auto
summarization.
>
> Because /26:1a and /26:1b was in the routing table Router B was able
> to advertise them to Router C via RIP.
>
> Router C was able to ping the Loopback on Router A (and get replies)
because
> /26:3 was in the OSPF domain and /26:1a and /26:1b was in the RIP
> domain.
>
> The second problem with this method is that Router B will be ARPing
> for
all
> addresses in the /26:1a and /26:1b ranges and therefore would need to
> increase it's arp cache size if the proxy addresses become significant
> in number.
>
> But since you are dealing with a classful protocol that automatically
> summarizes subnets when crossing classful boundaries, you don't even
> need the secondaries (just choose an addressing scheme that would fit
> the results). This would probably be the answer that a testing
> proctor would want you to come up with.
>
>
> WAYNE A. BAETY, SRA, MCSE, USAF
> 18th Communications Squadron
> DSN: 634-9927
> Commercial: 011 81 611 734 9927
>
>
>
> -----Original Message-----
> From: Young K. Bae [mailto:ybae@cisco.com]
> Sent: Saturday, September 28, 2002 9:51 AM
> To: ccielab@groupstudy.com
> Subject: What if OSPF routes have shorter masks than RIP?
>
>
> Guys,
>
> I'm sure this question has been asked many times before, but can
> someone kindly explain to me again? There are some /25 routes in OSPF
> Area0 that
I
> need to redistribute into RIP v1. The serial link that connects R1
> with
R2
> has a /26 mask. How can I inject /25 OSPF routes into a classful
> routing domain in a case such as this?
>
> R1 --- 172.16.12.0/26 (RIPv1) --- R2 ---- 172.16.100.0/29 (Area1) ---
> R5 172.16.200.0/25 (Area0)
>
> Thanks in advance,
>
> Young
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