WAN Clarification

Unanswered Question
Jan 9th, 2008

I need a clarification on the following:

- We have about 25 sites that are linked to a core router. The 25 sites have different types of WAN links, some are on Frame Relay, P-To-P, or ATM. After reveiwing the network diagram,I inherited from a former Ntwk Engr, and and the routers' config files, I became unclear on why the remote sites routers having Frame-Relay and P-to-P linked to the Frame-Relay cloud; however, on the core router configuration file, each one of the remote sites has an ATM interface configured for it. Can someone explain to me how a Frame-Relay or P-to-P link on one end is linked to an ATM on the other end?

Thanks in advance,

sK

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Richard Burts Wed, 01/09/2008 - 09:15

SK

This is a service available from some providers. At the HQ you have a single ATM interface with a virtual circuit for each remote site. The provider takes each virtual circuit and delivers some of them as ATM, delivers some of them as Frame Relay, and delivers some of them as p-t-p. The provider is doing the protocol translation in the middle translating the ATM to Frame Relay or p-t-p as appropriate.

As the various remote sites were provisioned someone in your organization negotiated with the provider to provide the most appropriate (and probably cost effective) technology at the remote site while maintaining a single protocol at the HQ.

HTH

Rick

skhirbash Wed, 01/09/2008 - 10:40

Thanks for the prompt response. I have completed reveiwing and analyzing the config file on the core router at HQ and discovered that all of the ATM and Frame-Relay at some of the remote locations have an point-to-point ATM subinterface on the core router; however, none of the P-T-P remote sites has any ATM subinterface configured for it on the core router!! When I say p-t-p, I mean the serial interface on the remote router is assigned an ip address and no encapsulation specified! Where do the p-t-p remote routers(sites) link to?

Thanks again,

sK

mheusing Wed, 01/09/2008 - 09:29

Hi,

There are devices, which do ATM-Frame Relay interworking. Basically the L3 payload is copied from an AAL5 frame into a frame relay frame and vice versa. This has been standardized by the ATM and FR forum.

You can order the service from some SPs doing that for you. One of the Cisco devices capable of doing it is the IGX, but routers also are capable of doing ATM-FR interworking.

Have a look at

http://www.cisco.com/en/US/tech/tk39/tk53/tsd_technology_support_protocol_home.html

for further details.

For an IGX, have a look at

http://www.cisco.com/en/US/products/hw/switches/ps988/products_configuration_example09186a0080094564.shtml

for routers further details can be found f.e. at

http://www.cisco.com/en/US/products/ps6441/products_configuration_guide_chapter09186a0080080f5f_4container_ccmigration_09186a0080775258.html

Hope this helps! Please rate all posts.

Regards, Martin

skhirbash Wed, 01/09/2008 - 10:40

Thanks for the prompt response. I have completed reveiwing and analyzing the config file on the core router at HQ and discovered that all of the ATM and Frame-Relay at some of the remote locations have an point-to-point ATM subinterface on the core router; however, none of the P-T-P remote sites has any ATM subinterface configured for it on the core router!! When I say p-t-p, I mean the serial interface on the remote router is assigned an ip address and no encapsulation specified! Where do the p-t-p remote routers(sites) link to?

Thanks again,

sK

mheusing Wed, 01/09/2008 - 10:53

Hi,

If your network plan does not tell you, I would suggest to traceroute the interface IPs and check which devices in your network connect you to the remote sites.

Alternatively you could telnet to the other IP in the /30 subnet usually found on a P-to-P link and check which device answers.

Regards, Martin

Richard Burts Wed, 01/09/2008 - 11:03

SK

I wondered about that when I read your post. I am familiar with the ATM/Frame Relay interworking but had not seen ATM to p-t-p interworking.

In addition to the suggestions that Martin makes about traceroute or telnet, I might suggest that if CDP is running on the remote router that show cdp neighbor (or show cdp neighbor detail) would be a good way to find where the p-t-p is terminated. Or if you are running a dynamic routing protocol you could look at who the neighbors are from the remote router.

HTH

Rick

skhirbash Wed, 01/09/2008 - 12:39

Silly question! I learned the PPP protocol on my CCNA studies but not the p-t-p. What is the difference?

Thanks in advance,

sK

Richard Burts Wed, 01/09/2008 - 12:45

sK

p-t-p is an abbreviation for point to point and is frequently a generic way to describe a point to point link. A point to point link may run any of several encapsulations including PPP, HDLC, or Frame Relay.

HTH

Rick

skhirbash Wed, 01/09/2008 - 12:54

And if the encapsulation is not specified, I would assume that the default encap is HDLC, correct?

sK

Richard Burts Wed, 01/09/2008 - 13:12

sK

Yes the default is HDLC if the encapsulation is not specified on a serial interface.

HTH

Rick

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