Why using eBGP instead of MPLS to connect multiple branch office network ?

[eigrpy]

Hi We have multiple branch office networks in different locations. We connect them to each other through MPLS. but I notice some diagram where eBGP connect them to each other. Why do they use eBGP instead of MPLS to connect them to each other ? Anyone can explain it ? Thank you

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

MPLS requires hardware to support it, not all does.

BGP is a routing protocol, that many routers (and L3) switches can support.

So, it might depend on what your platforms support.

There could be other reasons, not all mutually exclusive.  Too little information to suggest why your topology is configured as described.

[Jon Marshall]

They are two different things.

MPLS is the technology used to forward the packets across a WAN.

It is used by SPs within their WAN to forward packets.

So your sites may all be connected to an MPLS WAN although your routers typically will not be running MPLS.

But you must advertise and receives routes between your sites so they know how to get to each other.

To do that you need a routing protocol (usually) and that is what BGP is doing.

So your site routers (CE devices) will be running BGP and exchanging routes with the SPs PE routers and these PE routers are where the MPLS network starts.

You don't need to worry about the MPLS implementation details unless of course you are building your own.

Jon

[Masoud Pourshabanian]

BGP and MPLS are two very different protocols. They can be used with each other, but you can not compare them. BGP is a routing protocol exchanging routes between routers while pure MPLS is designed to forward packets faster and MPLS VPN is designed to separate IP traffic of different (lets say) customers in layer 3 networks to provide more security.

Let me give you a simple example. You have a router connected to several customers. You want to have a different routing table( virtual routing table) for each customer and you do not want to mix their IP traffic. The solution here is MPLS VPN.

Now, you have several virtual routing tables(VRFs) in a router. How to exchange this routing tables across your network? There must be an option that helps you to differentiate each route and routing table. Here BGP helps. BGP is not used only for MPLS VPN. But when it is used along with MPLS VPN, it is able to carry some options helping destination routers determine which route belongs to which virtual routing table.

So, you implement MPLS VPN and create some virtual routing tables(VRF) for each customer. BGP exchanges routes inside VRF. When networks is convergent, MPLS VPN keeps traffic of different customers separated. 

MPLS VPN can implemented without BGP if network is small. Lets say only 2 or 3 routers.

Hope it helps,

Masoud

[eigrpy]

Thanks for your reply. I know usually BGP is one of MPLS components. Here is my question: BGP I am talking about and asking is BGP without MPLS. I do not know if the eBGP can also connect different branch office network together located in different sites without MPLS involvement. This way, BGP might replace MPLS. Can I say like that ? 

[Jon Marshall]

BGP is a routing protocol not a transport medium.

So if you are connecting your sites up using something other than MPLS then yes you can use BGP or any routing protocol for that matter, to exchange routes.

All BGP is doing is exchanging routes.

You still need a physical WAN to forward the packets.

Jon

[eigrpy]

We are using MPLS to connect multiple sites and we know MPLS well. The function of MPLS is to connect remote sites network together, and then the remote networks can function like LAN. 

But I just see some one using eBGP connect multiple sites. It could be star mode or line mode (router1(ebgp)---router2(ebgp)----router3(ebgp)--- .... ). It looks like eBGP does not have that kind of function of MPLS in this topology. Why they are using eBGP. Maybe, the only reason why using eBGP is to send routes like ospf or eigrp. but eBGP have more powerful routing function.

We can say only ISP use eBGP, right ? none-ISP use eBGP ? if so, for what ? just for routing ? Thank you

[Jon Marshall]

The function of L3 MPLS is simply to provide a WAN to connect remote sites.

When you do this you need to exchange routes with your SP and you can use BGP or if they support them other routing protocols.

So you would use BGP at each site.

Are you sure you are not talking about VPLS which is a L2 network over a L3 MPLS WAN.

VPLS would make all your sites seem like the were on a LAN not a WAN.

Many companies use BGP for either MPLS connectivity or internet connectivity where they connect to multiple ISPs so it's not just ISPs although BGP is the routing protocol of the internet.

Yes BGP is used for routing but it is quite a complex protocol and gives you a lot of options for influencing the path traffic takes.

Jon

[eigrpy]

I saw some engineer using eBGP to connect office network(or client) at different sites. The reason that I post my question here is to try to know why they use eBPG. for what purpose they use eBGP. The below is my thought. Do you agree ? 

I suppose the topology is like this:  

office network1 --- ISP1-router1 --- router 2--- ...... --- router9---router10-ISP10-office network10

we can connect office network1 and office network10 through several ways:

L2L vpn, mpls, these two are mostly used for the connection.

Now I think eBGP also can have them(router1 and router10) connected. we just configure eBGP at ISP1-router1 and ISP-router10 and make the two router as eBGP neighbor. I do not know if this is practical. I guess the weakness of eBGP in the topology is no security and no faster than mpls. Thank you

[Jon Marshall]

You are still not quite understanding the difference.

When you say BGP is no faster than MPLS you are not comparing like with like.

MPLS is your WAN and this connects sites.

If the sites in your topology above are using BGP then there must be some sort of WAN to transport the packets.

All BGP does is exchange routes that is it.

It cannot forward packets without something to forward them over.

This may not be an MPLS WAN but it is a WAN of some sort.

Jon

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

As Jon as noted, eBGP and MPLS is not "like for like".  Your question is somewhat like asking at some sites they use OSPF with stubs but at others Ethernet, why is that?

eBGP is a certain usage of BGP, so it would compare to other routing protocols for why you would use it. eBGP vs. iBGP is somewhat like comparing OSPF with, or without, stub areas; i.e. usage variants.

MPLS is sometimes referred to as level 2.5, as it provides a L2 and some L3 features.  The latter, is more like comparing IP to a routing protocol, or L3 routing to a routing protocol.

But to answer your question why, some sites are using MPLS and others eBGP is like asking why some sites have Ethernet and others OSPF.  We don't have sufficient information to say.  Both can be used together or separately because they serve different purposes.

[Masoud Pourshabanian]

"We are using MPLS to connect multiple sites and we know MPLS well. The function of MPLS is to connect remote sites network together, and then the remote networks can function like LAN"

Your definition is close. I just need to change MPLS to MPLS VPN and LAN to a private network; however, it can be a logical LAN also.

BGP has several capability. BGP can be used to work with MPLS VPN and also can be used to propagate internet routes.

Service providers use MPLS VPN to separate network of different customers. You as a customer see your network as a private network and do not need to be concern about MPLS. It is service provider useing BGP to advertise routes of different customers because BGP has some features which work well with MPLS VPN.

Generally,

BGP is a routing protocol designed to exchange Internet routes across Internet ISPs. BGP has a lot of features and attributes can be used to manipulates routes.

Actually, it is possible to use BGP inside an autonomous system, but it in not common because configuration of BGP is much more complicate than IGP (OSPF, EIGRP) and  it also needs more resources.

The common use of BGP is at the edge of companies network. Each company or better to say autonomous system has internal routing protocol to propagate internal addresses and BGP at the edge network to propagate internet routes.

For example, your company which has its own IGP make a request to an ISP to receive Internet. BGP can be established between the Edge your company and your ISP to receive internet route. Your service provider may use MPLS.

MPLS and BGP are very broad concepts. They have different capability.

I suggest you read different versions of BGP and different applications of MPLS.

BGP > IBGP, EBGP, BGPV4

MPLS >MPLS VPN, MPLS TE, Ethernet over MPLS, MPLS QOS

[Jon Marshall]

Just to add to Masoud's post.

I agree with everything he says but there is a slight terminology issue here.

A LAN is a LAN and if you want to connect them to each other you use a MAN or a WAN.

If you use a L3 MPLS solution then that is a WAN and your LANs at each site connect to this and route between sites.

If you use a L2 MPLS solution such as VPLS then the WAN in effect becomes an extension of your LAN so yes you could then say all your sites are in effect on a LAN.

Not trying to be pedantic or win an argument but it's still not really clear what type of MPLS solution the OP has.

Jon

[Masoud Pourshabanian]

Thanks Jon for pointing out. I should have said logical LAN or as you said extension of LAN or expansion of broadcast domain over IP/MPLS. I did not understand your last sentence. What you meant by OP?

Best,

Masoud

[Jon Marshall]

Hi Masoud

OP = Original Poster ie. the author of this thread.

Jon