EIGRP External to Internal

Answered Question
Feb 21st, 2008
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Hello,

I have short question about EIGRP. Is there a way to change routes known as EIGRP type External to EIGRP Internal? Using redistribution, route-maps, etc.


Best regards,

Correct Answer by ruwhite about 9 years 2 months ago

I think you might be able to set the eigrp route type using a route map now, but I've never played with it, so I'm not certain. OTOH, I would be really really careful with this sort of thing--I wouldn't even recommend setting the admin distance for externals and internals the same.


Suppose you have a simple situation:


EIGRP internal-->other protocol-->EIGRP external


When you redistribute, you lose your metrics, so it's very possible that an external redistributed from eigrp into another protocol, and then back to eigrp, can easily be preferred over the original eigrp route, causing a permanent routing loop. I would guess the odds are really high this would happen in normal operation.


So, if you're going to do this, make certain/double certain/triple certain that any eigrp routes redistributed into another protocol are _never_ reflected back into the same EIGRP process, under any conditions. This is a huge recipe for disaster.


I once filed a feature request asking this to be changed, so you simply couldn't change the preference between externals and internals. I think it's still open and active, but it's not any sort of road map at this point.


HTH.


:-)


Russ

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Jon Marshall Thu, 02/21/2008 - 05:43
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Hi


If you are trying to get your router to consider both Internal and External EIGRP routes to the same destination based on their metrics only ie. not AD, then you can modify the distance under your EIGRP config ie.


router eigrp 1

distance eigrp 90 90


Internal EIGRP normally has an AD value of 90 and external EIGRP has an AD value of 170. The above config makes the AD the same for both ie. 90 so then the router will simply choose the path based on the best metric.


Note that you need to apply this config to all your routers.


HTH


Jon

Correct Answer
ruwhite Thu, 02/21/2008 - 08:30
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I think you might be able to set the eigrp route type using a route map now, but I've never played with it, so I'm not certain. OTOH, I would be really really careful with this sort of thing--I wouldn't even recommend setting the admin distance for externals and internals the same.


Suppose you have a simple situation:


EIGRP internal-->other protocol-->EIGRP external


When you redistribute, you lose your metrics, so it's very possible that an external redistributed from eigrp into another protocol, and then back to eigrp, can easily be preferred over the original eigrp route, causing a permanent routing loop. I would guess the odds are really high this would happen in normal operation.


So, if you're going to do this, make certain/double certain/triple certain that any eigrp routes redistributed into another protocol are _never_ reflected back into the same EIGRP process, under any conditions. This is a huge recipe for disaster.


I once filed a feature request asking this to be changed, so you simply couldn't change the preference between externals and internals. I think it's still open and active, but it's not any sort of road map at this point.


HTH.


:-)


Russ

k.lapczuk Fri, 02/22/2008 - 03:20
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Thanks Russ, that is clarification I needed. I'm making certain that those routes are not reflected back, but here, in my deployment it'd be good to have such feature.

I simply need that some routes redistributed from other EIGRP proces are better than the ones from original EIGRP process, but in case of failure of the external one, the routes from original should be working. I don't want to switch distances external <-> internal, because there are some external routes redistributed in EIGRP, that should stay external with distance 170.

In route-map now you can configure 'set metric external/internal' but it is not valid (working) for EIGRP.


Best regards,

Krzysztof

ruwhite Fri, 02/22/2008 - 07:55
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As long as you're certain the routes won't ever reflect into the original process, you can use the admin distance trick for now.... I need to tie making it so the admin distance can't prefer internals over externals, and the ability to change a route to internal, together in the system, so they get fixed at the same time (if I can arrange that).


:-)


Russ

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