Troubleshooting Metric Redistribution

This topic is to discuss the following lesson:

Rene,
Hi. Great explanations as always. One quick clarification - in the offset list example towards the end of the network lesson you show applying the offset list to the F0/1 interface. of R2. I assume that you must also apply it to the F0/0 as well (like you did in the example following using distribute-lists)?

Many thanks,
Thomas

Hi Thomas,

Glad to hear you like it! In my example, R4 was advertising the route with the lower better but it could also been R3 (depends on which router converges first) so yes, you should also apply the offset-list to the interface facing R2.

R3 and R4 can never advertise anything useful about network 1.1.1.0 /24 to R2 so we should ignore anything they send.

Rene

Hi

I was creating this topology in GNS 3 (version 3.10) . The routers I am using are C-3725 .

When I configure the fa interfaces between R1 and R2 . I get the following message “IP addresses may not be configured on L2 links”.

Could some one guide me where I am going wrong.

 

Thank You

romesh

Hi Romesh,

You probably “inserted” the switch module in the router instead of the routed interfaces.

The interfaces on the switch module are always L2, routed interfaces are L3. Try one of the other interfaces in the slots and it should work.

Rene

Dear Rene,

Referring to your “R2 has learned about network 1.1.1.0 /24 from R1 with a hop count of 6. I used an offset-list on R1 to increase the hop count for this topology, I’ll show you why in a bit.,”

Can you please help showing how to set hop count up to 6?

I have tried the following:

R2(config)#router rip

R2(config-router)#offset-list LOOPBACK_R1 in 6 fa0/0

but the hop count is still one from R2 result.

 

Thanks.

Makara NGY

Hi Makara,

I have an example here:

Rene

Hi Rene, there is actually something even deeper to this that you may not have realised. Let’s say the network is stuck in the loop currently, and R2’s best path is via R4 to reach 1.1.1.0/24. Okay? So since R4 told R2 this is the best path via RIP, this ALSO means that since RIP is distance vector, it means split-horizon would then suppress advertisements of 1.1.1.0/24 from R2 towards R4. So EVEN IF you change the AD on R4, once the loop is active, lowering the AD of RIP or increasing the AD of OSPF external routes should not affect anything. Since R4’s RIP database won’t even have a route to 1.1.1.0/24 inside of it, the only path to 1.1.1.0/24 is via R3. So how would you handle this in the lab exam, if you are stuck in this scenario & can’t make the AD adjustments actually work? Just ran into this a moment ago in a lab :slight_smile: And I even turned split-horizon off to try make it work, but it actually didn’t work lol.

Hi Stephen,

The “core” of this problem is that R2 is learning the 1.1.1.0/24 route through R4, something that should never happen since it’s a route that went from RIP > OSPF > RIP. We call it a metric redistribution issue since RIP doesn’t have any way to differentiate between an internal or external route…the lower metric from R4 wins. This really is a routing loop which is bad :slight_smile:

Once R2 installs the route through R4, it will no longer advertise 1.1.1.0/24 to R4 so the only way for R4 to get to 1.1.1.0/24 is R3…keep in mind that this is not much of an issue…it’s sub-optimal routing but not a routing loop :smile:

The only way to fix this sub-optimal routing problem so that R4 uses R2 as the next hop is to make sure that:

* R2 is able to advertise 1.1.1.0/24 to R4.
* R4 doesn’t install an OSPF route for 1.1.1.0/24.

You can achieve this with redistribution filtering or offset-lists.

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Hi Renee and Andrew,
Kinda confused, in the beginning you started off by saying “In this lesson we’ll take a look at metric based redistribution problems and how to fix them.”, but the illustration is all about using Administrative Distance (AD) to fix redistribution problems. I thought metric, for example, is defined as cost for OSPF and K values for EIGRP while AD is the reliability factor for example, 90 for EIGRP and 110 for OSPF. They are not the same, are they? Can these two terms be used interchangeably?

Respectfully,
Syed

Hi Rene,

Please disregard my above post / question. After reviewing it carefully, i noticed it’s all about metric, not AD. It’s my bad!
Sry for taking your time.

Respectfully,
Syed

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u r the best :+1: well explained i swear to GOD

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Great document! Thanks Rene!