9 replies to this topic

[redscorpion69]

Ok so here is the topology I'm running, it's pretty simple:




o---------o----------o
R1 R2 R3

R1 is running OSPF. R2 is running OSPF on f0/0 (interface to R1), and EIGRP on f0/1 (interface to R3). R3 is running EIGRP.
I am emulating connected interface on R2 with loopback0.
Now, when i redistribute connected into R2's router eigrp process, R3 will install a new route:

D EX 192.168.10.0/24 [170/156160] via 10.6.2.1, 00:00:05, FastEthernet0/0,

and R1 which is running OSPF will not install a route. As i imagine, this is happening because of split-horizon which
prevents R3 to send update on f0/0 on which it learned about the route to begin with.

Ok, the problem i have is, if i instead redistribute connected under R2's router ospf process, R1 will now have a route:

O E2 192.168.10.0/24 [110/50] via 10.5.2.2, 00:30:22, FastEthernet0/0,

BUT R3 will never get the update. If split horizon is not preventing R1's OSPF to advertise a route back to R2 which is using mutual redistribution, and therefore to R3
what is preventing R3 from learning about the route?

Can someone explain this to me, I'm missing a concept here ?

Edited by redscorpion69, 25 January 2009 - 10:10 PM.

[ciberkot]

ok, if you redistribute the connected loopbackon R2 into the EIGRP domain, your OSPF domain will not have any route to loopback bacause you don't redistribute EIGRP into OSPF somehow (don't you?), the same is if you redistribute your Loopback into OSPF only.
in this particular case you should redistribute your loopback into both routing domains.

[ManishBehal]

Are you specifying a seed metric when you redistribute into EIGRP?

[tunerX]

You are redistributing connected. When you redistribute the connected into EIGRP or OSPF, the connected route remains in the routing table.

When you do mutual redistribution from a routing protocol to another your are redistrbuting what is currently in the routing table for that protocol. The connected route will show up as connected and will appear as connected in the routing table. Even by turning of split horizon and receiving the same route back the the router running both protocols will still keep the connected route in its table. The only way to get it across R2 is to have the route show up in either EIGRP or OSPF depending on the direction. I don't think it is possible to have a connected route show up as a route from a routing protocol, it will always remain connected.

Edited by tunerX, 25 January 2009 - 11:32 PM.

[zgx]

TunerX already said this but I want to try to clarify it even more.

Whenever you redistribute between two protocols you will redistribute two things:

1. The routes for the protocol install in your routing table. You can verify by doing a "show ip route <protocol>" to see what those are.

2. You will also redistribute routes that are announced via that protocol. You can often verify that with "show ip route X.X.X.X" on single routes. It is also easy to check by looking at your network statements or doing "show ip protocols" to see what interfaces you are announcing into the routing protocol.

This is pretty much how redistribution works so this is very important to understand.


So you are redistributing connected into OSPF and you have mutual redistribution for OSPF and EIGRP. And you wonder why the connected route does not show up in EIGRP right?

Let’s check out the redistribution criteria and see how we do.

1. "show ip route ospf" will that show us a route to the connected interface? Answer: No, since the connected interface has a better administrative distance then the OSPF route.

2. Is your connected route included in your OSPF network statement? Answer: No.

Conclusion: Your connected route will not be redistributed into EIGRP.

[redscorpion69]

You are redistributing connected. When you redistribute the connected into EIGRP or OSPF, the connected route remains in the routing table.

When you do mutual redistribution from a routing protocol to another your are redistrbuting what is currently in the routing table for that protocol. The connected route will show up as connected and will appear as connected in the routing table. Even by turning of split horizon and receiving the same route back the the router running both protocols will still keep the connected route in its table. The only way to get it across R2 is to have the route show up in either EIGRP or OSPF depending on the direction. I don't think it is possible to have a connected route show up as a route from a routing protocol, it will always remain connected.

Hmm i think i understand what you mean. Ok follow me on this:

in first case, when i redistribute connected into eigrp, the split-horizon prevents R3 to advertise connected back to R2 and then to R1.

However, split-horizon is irrelevant here in a sense that, when R2 receives connected route which is advertized back to him (no matter from R1 or R3) it will not put it into
its routing table because connected always has AD of 0, therefore it will drop the update. SO the only way to actually have connected in all 3 routing tables is to redistribute
connected in both eigrp and ospf.
Right so far?

[tunerX]

Yeah thats right. If you wanted to verify you can turn off split horizon on R3 and capture the packets going back to R2. You will see the loopback network coming back into R2.

[zgx]

You have a few options Red Scorpion,

1. Redistribute connected into both OSPF and EIGRP

2. Redistribute connected into one protocol and including the connected interface in a network statement for the other one.

3. Include the connected interface in a network statement on both routing protocols.

4. Using a static route and redistribute that might also work.

And there are probably additional ways to do it.

[redscorpion69]

I was working on similar problems and i just forgot about couple of things (you know how concentrating on one issue, you total forget about other even so bluntly obvious).


Btw this is a problem from Doyle's TCP/IP Vol I, where he presents the same topology only uses different distance vector protocols. He explained that routes will never be advertised back because of split-horizon. But he never mentions it wont redistribute back even without split-horizon because of superior AD of connected in R2's routing table. That's why i was confused since OSPF doesn't look at split-horizon.

Thx for the discussion, love you guys.

Edited by redscorpion69, 26 January 2009 - 12:46 AM.

[zgx]

Darby credit Brian McGahan. He explained it so well for me in this free V-seminar http://classroom.int....com/p42549585/. I can highly recommend it to anyone who wants to learn more about redistribution.