Question about MPLS and routing protocols

GeekyChick

So I'm trying to figure out where MPLS fits in. Please someone tell me if I'm understanding this right. SONET, DWDM and CWDM are just the physical way the light is sent down the fiber. OSPF, IS-IS (Link State), RIP(Distance Vector) are interior gateway routing protocols used within Autonomous Systems. BGP is an exterior gateway routing protocol which connects Autonomous Systems. So, where does MPLS fit in? Is it also a routing protocol? Do I have this all right? TIA

pevangel

MPLS is typically used to connect multiple sites to each other through a service provider's network. A company with multiple remote sites usually want all sites to have some type of connectivity to each other in order to share resources. The service provider using MPLS services can appear as a router, a switch, or a cable to these sites. Company sites that are geographically diverse can be configured like a single LAN thanks to MPLS applications.

GeekyChick

Thanks! That was a pretty thorough explanation. Since it's across the internet there's no privacy I'm guessing??

Would you say that MPLS is an Interior Gateway Protocol, like RIP or OSPF?

Edit - forget my privacy question. I just saw that a VPN could be used with it.

pevangel

MPLS is used for tunneling packets/frames so the applications enabled by MPLS are called VPNs. The only time you actually send routing information to the service provider is when you get an L3VPN service from them where the provider appears as a giant router in the cloud. Picture the router in the cloud having logical routers within it for each customer. Each logical router with their own separate routing table.

I wouldn't call it an IGP. MPLS is a technology that allows you to create tunnels called LSPs. There's a protocol used for label distribution that's similar to an IGP which is LDP or Label Distribution Protocol. Once an IGP and LDP is configured on all the routers, you'll have a full mesh of dynamic tunnels. When you need to send a packet from A to Z, you go through a tunnel (LSP) instead of each router in the middle needing to process the packet.

GeekyChick

I gotta wrap my brain around that. So, it's used for a large company to have its own personal WAN. The packets are tunneled by MPLS through the ISP. Routing doesn't happen, all connections within the company's WAN gets the data.

Btw, I never thought of routers having a mesh. I think it's so fascinating.

Russell77

It does not have to be a large company application. It also does not have to have a gateway to the internet. Pre MPLS companies would use point to point data connections between remote office sites. If you have more than one site you could run everything in a hub and spoke pattern to a main site or have point to point data connections between all sites (too expensive). Better to think of MPLS as a lan connection over a long distance connection.

GeekyChick

So, MPLS is just a way to tunnel traffic across long distances. It doesn't use IP for destination it uses tags (not sure of name). Apparently it's used a lot, I'm assuming. What I'm not getting is how does it work on the internet with the routing protocols? Does it replace routing protocols when the destination is always the same, for example within a company ? It doesn't work with OSPF or RIP? OSPF, and the like, does it's own thing?

Dieg0M

Where does MPLS fit? I've often asked myself the same question. By itself MPLS is just a way to forward packets based on labels and it does not provide any significant improvements over the traditional IP based forwarding model. However, if you start looking at the applications and services that it enables (L2/L3 VPN's, TE and FRR), that's where it really starts to shine. Service Providers and Large Enterprises will often deploy MPLS as it allows them to push the intelligence of the network to the edge and keep the core simple thus allowing to scale. Scaling is very important for any large company as it is almost impossible to operate and maintain a large complex network. Another important use case of MPLS is it's ease of integrating different transport technologies(including legacy ones) like ATM, Ethernet, Frame Relay, HDLC, PPP over an agnostic core. A lot of these points might not be clear to you, they were not for me in my first few years in networking.

Often people think they are "using MPLS" because they are connecting to an MPLS carrier through a L2 or L3 VPN. Don't be confused, running MPLS and connecting to an MPLS carrier is not the same. Many small enterprise connect to an MPLS carrier but mostly large enterprise run MPLS and most of the time it is for scalability and/or traffic segmentation purposes.

Edit: some typos

Dieg0M

GeekyChick wrote: »

Apparently it's used a lot, I'm assuming. What I'm not getting is how does it work on the internet with the routing protocols? Does it replace routing protocols when the destination is always the same, for example within a company ? It doesn't work with OSPF or RIP? OSPF, and the like, does it's own thing?

BGP routes the internet, MPLS is mostly used to run a BGP free core(amongs other things) for the ISP. See BGP free core designs for more info. A routing protocol is still needed to establish the LSP (Label-switched Path) between the LER's (Label edge routers) loopbacks but it is run internally only and not facing other carriers or external entities. Different use cases for Service Provider networks and enterprises. For SP it is mostly to provide VPN's with good SLA's and QoS, for Enterprises it is mostly for security through traffic segmentation on the WAN and sometimes extending to the DC.

networker050184

MPLS is just a header used for making forwarding decisions not much different than an IP address. So in an MPLS network forwarding decisions are no longer based on destination IP address. This is what allows you to implement the L3 and L2 VPNs people are describing.

Now you need something to distribute these labels. That's where things like LDP, RSVP, even BGP and other routing protocols come into play.

GeekyChick

Ok, I think I got it. It just encapsulates a packet with a label. The label is popped off when it hits a router and then the IP address is used to route to destination(or whatever the protocol is). It's used as a way to transfer packets where they don't care what it contains, because it doesn't look at the inside only the label. That's probably good enough for now. When you throw in L3 VPNs I start thinking, wait a minute I thought IP wasn't used. (I'm guessing that's just what's inside the MPLS packet, not MPLS itself.) Thank you guys.