OSI - Below layer helping above layer (Routing Protocol over MPLS)

[John Peterson]

Hi, from my CCNA I understood that in the OSI layer protocols in the layer below help protocols in the layer above perform there tasks. A simple example is where layer 2 aid layer 3 in packet transmission.

I have a network where routers at different sites connect in via MPLS but transverse through different MPLS networks. I don't have any control of the MPLS as the technology is provided by our ISP. At both ends we have Cisco devices which are capable of running a routing protocol e.g. RIP or EIGRP.

I may be wrong but, MPLS is a layer 2 feature where routing protocol are Layer 3, then I am able to routing a routing protocol between these devices where the packet would travel different MPLS network to get to our cisco device?

Any thoughts would be appreciated.

[Kevin Dorrell]

As far as you are concerned, the MPLS cloud is a point-to-point tunnel, so you can treat it as layer-2. It may be using all sorts of protocols to join the ends of the tunnel, but your routing protocols are safely confined to the tunnel. The MPLS cloud transport mechanism never even sees them.

Kevin Dorrell

Luxembourg

[John Peterson]

Thanks, when you say tunnel is that from our Cisco devices to the other Cisco device. Or devices between?

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[Kevin Dorrell]

The tunnel is from the edge router on one of your sites to the edge router on your other site. Once a packet goes into one end of the tunnel, it doesn't come out (or rather, it doesn't get de-encapsulated) until it reaches the far end. And that includes your routing protocol PDUs.

[John Peterson]

Thanks, but I may be wrong but doesn't the packet get de and encapsulated when it does through different networks from one site to the other. As I'm sure the ISP has different networks in between our sites?

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[amigomnemonik]

MPLS attaches the label to your packet and traverse this packet across the edge routers on your ISP network without modifying your packet (content) let's say even of the ethical reasons.

So let's say they arrive on one side as IP packet, ISP does not modify the packet except adding extra label on the top of it so they know how to route it to the end edge device on the ISP network, then it leaves ISP and goes out toward your network. This make the taversing transparrent.

That's why MPLS allows you to run routing protocols between two end points. i.e. two Data Centres in a similar fashion as GRE tunnel interfaces.

[John Peterson]

Thank you.

Is it always the case where the lower layer helps transmission to the above layer, as it seems it in this case?

[John Peterson]

Hi,

I've setup a home LAB, where I have theree routers:

Router A:

Internal IP: 192.168.10.1/24

Ext IP: 90.0.0.1/24

Router C:

Fa0/0: 90.0.0.2/24

Fa0/1: 40.0.0.2/24

Router C:

Internal IP: 192.168.20.1/24

Ext IP: 40.0.0.1/24

There is a static router in router a and c which allows ping between both routers (a and c) Router C is only there to separate the subnets.

I have tried to run a routing protocol between routers a and c but there are not able to form neighbours. The only way a

neighbours is formed is when I include router B in the routeing protocol.

From my understanding as long as the devices can ping each router then they should be able to form neighbours?

[Richard Burts]

For most routing protocols forming neighbor relationship requires that they both have interfaces in the same subnet. That is not the case in your lab and is the reason why a dynamic routing protocol is not working.

HTH

Rick

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[John Peterson]

I am guessing in an MPLS cloud the devices are on the same subnet.

You said most routing protocols, which dymanic protocol could I use when interfaces have different subnets?

[Richard Burts]

John

The dynamic routing protocol that does not require being on the same subnet is BGP.

As a side note - your original question was about routing and MPLS. Are you implementing MPLS in this lab? In my experience when you want to use a routing protocol over MPLS you run a dynamic routing protocol where your neighbor router is the Provider Edge router which would be on the same subnet as your router. The provider forwards the routing information through the MPLS cloud to the PE router on the other side of the cloud. And that PE router is neighbor to your remote router, and is on the same subnet.

HTH

Rick