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Community Member

EIGRP question about how default route is processed

Hello, I have a basic setup:

R1---R2---R3---R4

R1, R2 and R3 have EIGRP running. R2 has a default route that says to take R3. R3 has a default route to take R4.

I want to change the default route in R2 to a new internet connection but my EIGRP also redistributes statics so I was concerned with R2 sharing this new static route with R3. I do not want R3 to know about this route and just continue using the route it already has configured pointing to R4.

I set this up in a lab and swapped out default routes in R2 and everything seemed to work exactly as I want it to. I did a show routes in R3 and it looks like it knows nothing about the new default route in R2. If all the router's EIGRP process says to redistribute static why is R3 not noticing the new default route? Since R3 already has a default route statically configured does it just ignore other default routes shared to it via EIGRP?

Thanks for the help!  -Mark  

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Hall of Fame Super Blue

Re: EIGRP question about how default route is processed

Mark

If R3 has a statically configured default route then yes it will use it's own rather than the one from R2.

If on R3 you configured the static default route with an AD > 170 then it would use R2s EIGRP advertisement. This is because when you redistribute into EIGRP those routes become EIGRP external routes with an AD of 170. A static route has an AD of 1 so it is always going to be preferred unless you make it's AD > 170.

Jon

7 REPLIES
Hall of Fame Super Blue

Re: EIGRP question about how default route is processed

Mark

If R3 has a statically configured default route then yes it will use it's own rather than the one from R2.

If on R3 you configured the static default route with an AD > 170 then it would use R2s EIGRP advertisement. This is because when you redistribute into EIGRP those routes become EIGRP external routes with an AD of 170. A static route has an AD of 1 so it is always going to be preferred unless you make it's AD > 170.

Jon

Community Member

EIGRP question about how default route is processed

Thank you Jon!

Hall of Fame Super Blue

Re: EIGRP question about how default route is processed

Mark

Apologies, it's late here and my brain is not working properly. The last post still stands but to expand on it a bit -

1) if you wanted to make sure R3 did not see the default route you could use an outbound distribute list on R2 on the interface connecting to R3 and deny the default route or you could an inbound distribute list on R3's interface to R2 to deny it.

Either would work but as i say it won't be used anyway.

In terms of whether R3 actually sees it, yes it will. You won't see it in the routing table but if you look at the EIGRP toplogy table you should see it. This is where my brain is not working properly ie

"sh ip eigrp topology"  i believe should not show it but i could be wrong

"sh ip eigrp topology all-links" should show it because this shows all EIGRP received routes

Jon

Community Member

EIGRP question about how default route is processed

Thanks Jon for clarifying. I forgot to mention that I knew I could've used route maps to block the distribution but thanks for the advice regardless. I wanted to see what the less invasive way resulted in, (which seems to work as planned). I'm going to do some more testing on Monday and I'll definiately use that EIGRP command you provided. I'll be learning much more about routing in the coming weeks as I study for CCNP ROUTE...

Hall of Fame Super Blue

EIGRP question about how default route is processed

Mark

No problem, glad to help.

If you have the kit handy could you actually do the "sh ip eigrp topology" and "sh ip eigrp topology all-links" commands and see which one or both actually show the default route.

It's bugging me i can't work it out but my brain is literally shutting down as we speak

Jon

Community Member

EIGRP question about how default route is processed

That all links command displayed it, here is what I see:

R3#sh ip eigrp topology

EIGRP-IPv4 Topology Table for AS(10)/ID(172.130.1.2)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,

       r - reply Status, s - sia Status

P 10.1.0.0/16, 1 successors, FD is 28160

        via Connected, FastEthernet0/0

P 172.30.1.0/30, 1 successors, FD is 28160

        via Connected, FastEthernet0/1

P 192.168.130.0/24, 1 successors, FD is 30720

        via 172.30.1.1 (30720/28160), FastEthernet0/1

P 0.0.0.0/0, 1 successors, FD is 28160

        via Rstatic (28160/0)

R3#sh ip eigrp topology all-links

EIGRP-IPv4 Topology Table for AS(10)/ID(172.130.1.2)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,

       r - reply Status, s - sia Status

P 10.1.0.0/16, 1 successors, FD is 28160, serno 106

        via Connected, FastEthernet0/0

P 172.30.1.0/30, 1 successors, FD is 28160, serno 1

        via Connected, FastEthernet0/1

P 192.168.130.0/24, 1 successors, FD is 30720, serno 2

        via 172.30.1.1 (30720/28160), FastEthernet0/1

P 0.0.0.0/0, 1 successors, FD is 28160, serno 110

        via Rstatic (28160/0)

        via 172.30.1.1 (156160/128256), FastEthernet0/1

Hall of Fame Super Blue

EIGRP question about how default route is processed

Mark

Thanks for that.

Thinking a bit more clearly now so that is what i would have expected to see

Jon

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