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Help with frame relay lab

rileymartin
Level 1
Level 1

I setup a simple frame relay config between two routers in a lab and I can't get OSPF routes to populate the routing tables on each router. Everything worked fine when I wasn't using subinterfaces. The DCE side says up and up for both the physical interface and the subinerface however on the DTE side only the physical interface says up and up. The subinterface on that side says down and down. I tried adding frame-relay map commands on both sides but that didn't help. Each router and their connected switch shows up on one another's 'show cdp neighbor' so layer two seems to be working fine so I don't understand why the serial0.1 subinterface says down and down. I included the interface configs below. Any help would be appreciated. Thanks.

2600 Router

-----------

interface Serial1/0

no ip address

encapsulation frame-relay

clock rate 64000

cdp enable

frame-relay intf-type dce

!

interface Serial1/0.1 point-to-point

ip address 192.168.126.162 255.255.255.252

frame-relay interface-dlci 162

2514 Router

-----------

interface Serial0

no ip address

encapsulation frame-relay

cdp enable

!

interface Serial0.1 point-to-point

ip address 192.168.126.161 255.255.255.252

ip ospf network broadcast

frame-relay interface-dlci 161

1 Accepted Solution

Accepted Solutions

Riley

I would suggest at least 2 changes in the config in this post:

- remove the no keepalive under interface serial0. You needed that for your test environment. You do not want it in production.

- I would remove the ip ospf network broadcast from the subinterface. The subinterface defaults to OSPF network point-to-point which will work well. The network broadcast would work but it introduces more overhead into what OSPF must do (check for DR/BDR, elect DR/BDR, the point to point knows there will be only a single neighbor while the broadcast maintains the possibility of other neighbors).

While some people do ip unnumbered in a hub and spoke with many spokes, my experience is that it is more common to configure IP addresses and subnets on each link.

HTH

Rick

HTH

Rick

View solution in original post

12 Replies 12

Richard Burts
Hall of Fame
Hall of Fame

Riley

I notice this line in the config:

frame-relay intf-type dce

This command is configured to indicate that you are doing frame relay switching on the router. Since you did not include much of the router config I can not tell whether you are really doing frame relay switching on the router, but I doubt it. I suspect that you configured it believing that it would indicate that the interface is connected to a DCE cable. But that is not what the command does. I suggest that you remove this command and let us know if the behavior changes.

HTH

Rick

HTH

Rick

I made a little progress by adding no keepalive statements to the physical interfaces on each router. After I added the no keepalive statements the subinterface on the DTE side went up and up. But I still don't see ospf routes on either side. I removed the frame-relay intf-type dce statement but still no luck.

I was able to identify another problem with my config where each router can't even ping their own subinterface. I'm obviously missing something and don't understand the difference between using the physical interface and a subinterface. I got everything to work fine when my config was just on the physical interface but now on the point-to-point subinterface I can't get it to work.

DELL ACORD
Level 1
Level 1

Do you have a frame-relay switch in between the two routers? If so can you post that frame-relay switch configurations?

If not. In the configuration that you have posted. I don't see the command frame-relay switching on either router.

Riley

Am I correct in understanding that the two routers are just connected back to back with a serial cable? If so there is no frame relay switch in between. In that case I would recommend making the DLCI the same value on both ends - either both 161 or both 162.

HTH

Rick

HTH

Rick

This is just two routers connected back to back with a DCE/DTE serial cable.

I removed the frame-relay map statements and configured both sides of the connection with the same DLCI and as soon as I did that, EIGRP and OSPF immediately started talking. I tried removing the no keepalive statements but as soon as I did that the link went down and down so I had to put them back. I'm guessing since the routers aren't actually connecting through a frame relay switch I have to turn off the LMI status inquiry messages?

I don't understand why on this link between point-to-point subinterfaces both sides need the same DLCI and when I was just using the physical interfaces it worked with different DLCI's, one for each router?

Riley

I am glad that you have got it working now.

Your understanding of why you need to turn off keepalive is correct. Both routers are sending LMI querries and expecting to receive LMI responses. But there is not anything there to generate the response. Configuring no keepalive tells the router to not bother with LMI querries and responses. (It might be interesting to back and configure again the frame-relay intf-type dce and see if that affects the keepalive issue.)

The issue with the DLCI value on the physical interface being different but being a problem on the subinterface has a fairly simple explanation. When configuring frame relay on the physical interface it is treated as a multipoint interface and it can have multiple DLCIs on it. When you configure the subinterface it is a point to point interface and it can have only a single DLCI. If the router is configured with 161 and it receives a frame with DLCI 162 it assumes that this is an error and does not process it.

HTH

Rick

HTH

Rick

Thanks for your help. The ICND Intro and ICND books don't explain it enough.

So what's the difference in the configuration that I'm doing in my lab with two routers back to back and the configuration that I would do with an actual frame relay circuit?

Riley

The big difference between what you are doing with 2 routers back to back and what you would be doing with an actual frame relay circuit is that the actual frame relay circuit would have a frame relay switch (or multiple frame relay switches) in between the routers so that a router talks to a frame relay switch (which may talk to other frame relay switches) to get to the other router.

HTH

Rick

HTH

Rick

So if I connected two sites together with frame relay I would use the actual configuration listed below? Or would I need to change or add any statements? For a hub and spoke topology when you have a central site with many remote sites do people more typically use ip unnumbered or is that unnecessary with private ip addressing?

interface Serial0

no ip address

encapsulation frame-relay

no keepalive

cdp enable

!

interface Serial0.1 point-to-point

ip address 192.168.126.161 255.255.255.252

ip ospf network broadcast

frame-relay interface-dlci 100

Riley

I would suggest at least 2 changes in the config in this post:

- remove the no keepalive under interface serial0. You needed that for your test environment. You do not want it in production.

- I would remove the ip ospf network broadcast from the subinterface. The subinterface defaults to OSPF network point-to-point which will work well. The network broadcast would work but it introduces more overhead into what OSPF must do (check for DR/BDR, elect DR/BDR, the point to point knows there will be only a single neighbor while the broadcast maintains the possibility of other neighbors).

While some people do ip unnumbered in a hub and spoke with many spokes, my experience is that it is more common to configure IP addresses and subnets on each link.

HTH

Rick

HTH

Rick

Thanks, you've been a big help.

Riley

I am glad that my suggestions have been helpful. Thank you for using the rating system to indicate that your issues has been resolved (and thanks for the rating). It makes the forum more useful when people can read about an issue and can know that they will read the solution that resolved the issue. I encourage you to continue your participation in the forum.

HTH

Rick

HTH

Rick
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