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New Member

An MPLS Design Question

I am trying to get a handle on MPLS network design. Somewhere in the CCO technical documentation pages (http://www.cisco.com/warp/public/105/mpls_vpn_basic.html)

it is said that PE routers consist of:

Software - Cisco IOS® Software Release 12.1(3)T(Release 12.0(5)T includes the MPLS VPN.)

Hardware - Any Cisco router from the 3600 series or higher, such as the Cisco 3660 or 7206.

What I would like to know, if anybody out there can help me, is this :- What is required for an MPLS backbone router? The same web-page implies that the backbone routers need to be ATM-enabled...is this necessary? Could I use a similar hardware platform as a backbone router (P-router) that I use for a PE-router?

10 REPLIES
New Member

Re: An MPLS Design Question

LSRs at the core of the MPLS backbone are usually either Cisco 12000 GSR's (PoS, GE, FE interfaces)Cisco 7200 (PA-A3, FE on NPE) and Cisco 7500 series (PoS, PA-A3/A1, FE, GE interfaces)routers OR LightStream 1010 ATM Switch or Catalyst 8540 MSR1.

Cisco 4x00, Cisco 36x0 and Cisco 2600 have only edge functionality over the ATM interfaces.

Important for a backbone router is that the available interfaces support MPLS in IP (shim) or ATM (vc label). Historically MPLS ATM was supported first on LS1010, 7200 and 7500's. 7200VXR would also be used as Tag Switch Controllers for BPX ATM Switches. I think MPLS over Ethernet is new and only supported by GSR FE and GE, 7200 I/O Controller with Fast Ethernet port, PA-FE, PA-2FEISL, GEIP (7500 only). When you run MPLS over Ethernet in a switched environment due to the 4 byte label overhead and especially with 802.1Q, watch out for the support MTU on your Catalysts. Only 5000, 6000, 2948 and 2924XL support jumbo frames.

New Member

Re: An MPLS Design Question

I was planning to do something a little different with MPLS. I want to implement it over a pure IP backbone. Imagine Cisco XXXX routers (whatever they might be) with DS-3 or OC-1 interfaces switching packets over a network....for budgetary reasons, I would like to know how low I can go with the backbone router platform. Given the fact that DS-3 interfaces are available for the 36XX range, would this be technically feasible?

New Member

Re: An MPLS Design Question

DS-3 interfaces don't connect to a packet backbone unless you use a platform like 7600OSR or 12000GSR's.

DS-3 is PLCP and ATM cells carrying IP packets (RFC1483R or similar) so not pure packet (like PoS).

Cisco 4x00, Cisco 36x0 and Cisco 2600 have only edge functionality over the ATM interfaces, be that DS-3, E3. So, that is the only interface you can use to connect to a MPLS network (with Edge functionality).

At the core, label switching is only supported by 7500, 7200 or 12000GSR Cisco Router Platforms.

The lowest you can go, is 7200 and run it over the FE ports on the NPE.

New Member

Re: An MPLS Design Question

whoa...wait a minute. Have I completely misunderstood the entire theory and application of MPLS?

Mr GAUTSCHI, and anybody else out there who can help me : Is it, or is it not possible to run MPLS over any medium OTHER than ATM? To my imagination, it's rather like running a Java VM over another operating system - it seems basically like an unnecessary extra layer... Is there no way of creating an MPLS-based backbone (ie. a router-physical medium-router link) without having to get into ATM ?

New Member

Re: An MPLS Design Question

Cisco Implementation of MPLS!

You have not misunderstood the "theory" of MPLS. I don't know from where you take this statement about running MPLS only over ATM. In Cisco kit MPLS is implemented as outlined in the my previous emails. It very clearly says that there are 100Base-T and 1000Base-T interfaces in high-end router platforms (OSR, VXR and GSR) which are MPLS capable.

It also says that on low end platforms you have to use ATM interfaces. According to the 12.1 Code release MPLS is also supported in PoS interfaces on 4x00 and 36x0 in the edge routers. I am not aware of any other interfaces that support CEF and MPLS. We're talking complex chip architecture here that allows buffering and CEF lookups, maybe even per port memory chip like 12000. My guess is, that the platform architecture of those "small" routers does not allow it. MPLS over Frame Relay is only supported on Cisco's 7000 Series. I think Ascend has got MPLS enabled Frame Relay switches. I think Cisco is not involved in rfc3034 (MPLS over FR) work...?

Hello Cisco! Do you ever read these conversations? We need some help here...

New Member

Re: An MPLS Design Question

Today Cisco MPLS Product Manager replied to me saying that MPLS runs on 4700 and 3x00 on the PoS and DS-3 interfaces with PE and P functionality. Well that is good news!

And MPLS over Frame Relay is enabled on all MPLS capable platforms.

New Member

Re: An MPLS Design Question

According to the MPLS resource center FAQ:

"" a. What are the differences between MPLS and ATM?

MPLS brings the traffic engineering capabilities of ATM to packet-based network. It works by tagging IP packets with "labels" that specify a route and priority. It combines the scalability and flexibility of routing with performance and traffic management of layer 2 switching. It can run over nearly any transport medium (ATM, FR, POS, Ethernet...) instead of being tied to a specific layer-2 encapsulation. As it uses IP for its addressing, it uses common routing/signaling protocols (OSPF, IS-IS, RSVP...) "

This implies that it should be possible to run IP-only MPLS without having to get anywhere involved in ATM?

New Member

Re: An MPLS Design Question

You are completely right. But, RFC's provide guidelines for vendor implementation. It does not say that every vendor has implemented every flavor of these technologies. Due to hardware constraints re-engineering of existing control planes is not always feasible. Cutting an IOS code "MPLS over everything" for a piece of hardware that is not used in Frame Relay, ATM or optical-IP core networks, I have my doubts about it.

New Member

Re: An MPLS Design Question

It's true that production runs etc. can't be cancelled, and that there is a lot of legacy ATM and Frame Relay hardware that can be "re-programmed" to run MPLS, but does this mean that the physical layer absolutely HAS to be IP over ATM ? I'm not sure if I'm making sense, or making myself clear, but I hope you get the distinction I'm trying to make. This may sound silly, but can I not run MPLS over PPP ?

New Member

Re: An MPLS Design Question

There are 3 different MPLS implementation in Cisco kit:

1. ATM cell header

gcf -(vpi - vci)- pti - clp - hec - data

-----(l a b e l)------------------------

2. PPP header (for PoS)

ppp header - label header - layer 3 header

3. LAN MAC header

MAC header - label header - layer 3 header

You can run PPP over MPLS (AToM).

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