MPLS and routing protocols

[Horacio Gutierrez]

Hello,

Can you run ospf and eigrp on a mpls cloud or is mpls only meant for bgp, also If its possible to run ospf and eigrp on mpls what are the advantages and disadvantages of running bgp on mpls cloud versus ospf and eigrp on a mpls cloud. Another question I have what is the difference between ibgp and ebgp where is that run can I get example with a company in 3 cities that connect thru an mpls cloud that includes ibgp and ebgp and one site is the main site to go out thru the Internet. My last question what are the disadvantages and disadvantages between ATM and frame relay versus mpls?

Any help would be really appreciated.

Thanks,

[Jose Solano]

Hi,

You can run OSPF and EIGRP over MPLS, now about the other questions If you understand how both work, then you realize the determinant is whether to BGP peers are using the same AS or a different AS.  For routine eBGP, you're peers are very likely to use different ***, so you wouldn't have a choice.  For internal BGP usage, I suppose you could use different (private?) ***, down to individual BGP peers allowing them to operate with eBGP rules.

The pros and cons of using eBGP internally vs. iBGP would be due to the differences in peering rules and routing rules With eBGP rules you don't need to create a full mesh as you do with iBGP (or reflectors or confederations).  With eBGP rules, you're less likely to require another IGP if the peers share a common subnet, or if they don't, need special multi-hop configuration along with extra routing information external to BGP itself.  With eBGP rules, AS hops might indicate actual best path.

Although in theory I guess you could use eBGP rules with private *** within the same logical AS, it could make it difficult to peer with more then one external public AS.  In other words, this isn't normally a choice, so curious how you envisioned using pros and cons to select between eBGP vs. iBGP.

About Frame relay vs MPLS

In terms of where they lie in the OSI model, frame-relay is a Layer 2 technology instead of using MAC addresses to switch data such as in Ethernet we switch data using DLCI's.

MPLS is slightly harder to pin to an OSI Layer as it has functions that belong to both Layer 2 & 3 and 4.

MPLS labels packets as they enter an MPLS enabled network this is done by Label Switch Router (LSR's), depending on what the packet is and where it's going it can potentially have multiple labels.

Actual MPLS PDU's operate at Layer 2, however LDP which is the control-plane part of MPLS operates at Layer 3 & 4 (UDP port 646).

So that's the main difference's from an OSI point of view but I suppose in terms of how they work there kind of similar although MPLS has extra features that frame-relay doesn't such as Traffic Engineering.

Hope this helps.

[Horacio Gutierrez]

Hello,

Let me give a scenario with a college network in 4 cities thru a mpls cloud. Where all four colleges in different cities connect to each other to form a private network. Also one college out of the 4 cities is the site where traffic goes thru to go to the Internet.  So what cities router would run ibgp or ebgp with the network design I gave and could they use the same AS or different AS?

[daniel.dib]

If one college has the Internet connection then it sounds like a hub and spoke design. Should all colleges be able to communicate directly or should they always go through the hub?

Are the IP addresses at the hub owned by the college? Do they have their own registered AS? How are they connecting to the ISP? Are they running BGP to the SP?

As you see, there are many things to consider in a design. Unless the other colleges has a need to advertise public networks it should be fine to use a private AS. On the hub site remove-private-as could so it would look like that all networks are advertised via the hub, hiding the internal topology.

The spokes would run eBGP with the service provider. The same AS or different AS could be used on all sites.

Daniel Dib
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[Horacio Gutierrez]

All Colleges are able to communicate with each other thru MPLS cloud no need to go thru the hub however when all colleges want to go to the Internet not the private network they go out that one college router keep in mind that router has firewall in between the router and other routers. So Im assuming but correct me if Im wrong you would run IBGP on all the the routers instead of EBGP and the router with Internet Connection as well as private network connection would run both IBGP as well as EBGP. Pretty much its a mesh topoly not a hub and spoke topology with exception of one router that give others routers access to internet.  Also, can you explain the difference between private AS and register AS with the SP and if I go into a router which commands will let me know if its running EBGP or IBGP?

Thanks,

[daniel.dib]

It doesn't matter much if you run iBGP or eBGP in this case but it's most common to run iBGP so that SP can save on AS in use.

The reason to get own AS is mainly for multihoming.

To see if running iBGP or eBGP look at which AS the neighbors are using. Sh bgp ipv4 uni all sum and sh bgp ipv4 uni nei x.x.x.x

Daniel Dib
CCIE #37149

Please rate helpful posts.