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Community Member

EIGRP on PTP Links

I am studying for my bsci test and have a lab setup with a simple PTP link between 2 routers. The link is over a serial connection on a pair of 2500 routers. I configured EIGRP on both routers and added the respective LAN networks into eigrp on each. After adding the PTP /30 networks into eigrp on both sides, everything works fine.

My question is that on my companies production network, I have a PTP link between 2 locations (DS3) and do not have the /30 network added to eigrp and routing works fine. Why is this?

Each side is setup with the respective LAN network added to EIGRP, but that's it (other than a re-distributed default route).

What am I missing...?

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Accepted Solutions
Hall of Fame Super Gold

Re: EIGRP on PTP Links

Richard

Assuming that this is the explanation then I would say that the good news is that it is still working - even when logically it should not. The bad news is that if either router reloads (or perhaps if one of the interfaces stops and restarts) then it will not work. I would certainly suggest that the configuration should be changed to have network statements that cover these serial links on both routers.

HTH

Rick

11 REPLIES
Hall of Fame Super Bronze

Re: EIGRP on PTP Links

The WAN interfaces must have EIGRP enabled in order to route between devices. You can verify with show ip eigrp interfaces command.

It sounds like they decided to include a major network under EIGRP routing process that covers your LAN and WAN subnet. For instance, if your WAN subnet is 10.0.0.1/30 and your LAN subnet is 10.1.0.0/24 and your EIGRP routing process has

router eigrp 1

network 10.0.0.0

no auto-summary

Both WAN and LAN subnets will be running EIGRP.

Community Member

Re: EIGRP on PTP Links

The summary network is added to eigrp on the LAN switch.

The connection is L3 switchA-->routerA-->DS3 PTP<--routerB<--L3 switchB.

SwitchA is running eigrp and has the summary address for the entire site (Site A). RouterA's lan interface is in this range. Router A's eigrp only has the network that the lan interface is in.

SwitchB is running eigrp and has the summary address for it's entire site (Site B). Router B's lan interface falls in this summary, as well as the ptp /30 network on the RouterB serial and RouterA serial. Router B only has the lan network added into eigrp.

Since the /30 net falls in the summary address on SwitchB, this adds the /30 networks to eigrp on both router a and router b? Is this because all devices are in the same AS and have at least one of their networks added into eigrp?

Thanks

Hall of Fame Super Bronze

Re: EIGRP on PTP Links

Let's see show ip interface brief and show ip eigrp interface from those 4 devices.

Hall of Fame Super Gold

Re: EIGRP on PTP Links

Richard

I agree with Edison that seeing the output of show ip interface brief and of show ip eigrp interface will be very helpful in understanding what is going on and hope that you will post it.

I am wondering about this statement in your post:

Each side is setup with the respective LAN network added to EIGRP, but that's it (other than a re-distributed default route).

and wondering if there is only a single network statement in router eigrp and would ask that in addition to what Edison has requested that you also post the EIGRP configuration.

I have seen an implementation that would produce most of the symptoms that you describe. The cusomer router has a point to point serial configured with ip unnumbered which points to their fast ethernet interface. And a single network statement puts both interfaces into EIGRP. Could your situation be similar?

HTH

Rick

Community Member

Re: EIGRP on PTP Links

switchB...

Community Member

Re: EIGRP on PTP Links

Attached are the outputs. The PTP /30 is carved out of the Site B IP scheme. The entire scheme for this site is added to eigrp on switch B.

I gather that this has to be where the PTP links get added into eigrp, as this is the only place where the networks are added.

Switch B will be in the next post.

Thanks

Hall of Fame Super Gold

Re: EIGRP on PTP Links

Richard

Thanks for posting the additional information. It is quite puzzling to me. It does clearly show that the serial interfaces are EIGRP interfaces. But it shows that the interfaces do not match a network statement on either router.

I believe that I have seen a similar situation once. It was a situation in which there had been network statements for the interface, the interface had initialized and become an EIGRP interface, and then the network statements were removed so that the interface no longer matched a network statement. I was surprised to discover that the interfaces remained active EIGRP interfaces - at least until the interface was shut/no shut or until the router reloaded. I wonder if the same thing could have happened on your router?

HTH

Rick

Community Member

Re: EIGRP on PTP Links

Yes, that's why I couldn't figure out how routing was still working.

That is a possibility. I actually was just at this site re-addressing the entire location. I changed so many addresses, I don't recall if I first added these to eigrp. I may have, and then removed them.

Hall of Fame Super Gold

Re: EIGRP on PTP Links

Richard

Assuming that this is the explanation then I would say that the good news is that it is still working - even when logically it should not. The bad news is that if either router reloads (or perhaps if one of the interfaces stops and restarts) then it will not work. I would certainly suggest that the configuration should be changed to have network statements that cover these serial links on both routers.

HTH

Rick

Community Member

Re: EIGRP on PTP Links

Thanks for the help

Hall of Fame Super Gold

Re: EIGRP on PTP Links

Richard

It has been an interesting discussion about an unusual problem. This is the kind of thing that makes the forum so interesting and helpful. Thank you for using the rating system to indicate that your question was resolved (and thanks for the rating). It makes the forum more useful when people can read about a question and can know that they will read a response that resolved the question.

I encourage you to continue your participation in the forum

HTH

Rick

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