BGP

[deepanprasanth]

How are data center prefixes advertised to branch offices and vise versa with BGP?

[Julio E. Moisa]

Hi

Please correct me if I understanding wrong the question. If you want to advertise a prefix to a other bgp peer you need to use the network command line, example:

router bgp 100
network 172.25.1.0 mask 255.255.255.0

Take in consideration 2 things before to advertise:

1- The prefix to be advertised must have an entry on the routing table where the BGP is running

2- You must advertise the prefix with the same subnet mask displayed on the routing table.

Hope it is useful

:-)

[Dmitri Popkov]

Another way to advertise is

1) To use redistribution from others sources, link connected, static or IGP

2) To make an aggregate route.

But the main goal of BGP advertisement it the existence of route in RIB (it can be even static route to Null)

[Joseph W. Doherty]

The other posters have addressed how to get routes into BGP, so BGP can share network prefixes.

However, I also wanted to mention, in an example of DC and branches sharing BGP routes, you might be using iBGP and an IGP might be in use too.  If it is, the complexity might take a jump up, as among other things, summarized addresses might be what's being shared within BGP.

[deepanprasanth]

Thanks Joseph...

I am student studying about BGP routing with massive confusion.. Please see below.

I don't know how BGP works between head office and multiple branch office separated geographically which are connected by different service providers running BGP.

If i am able to access company secured network drive from branch office, how this connection is happening ? 

Correct me if i am wrong, That network drive IP must be advertised to eBGP peer at head office and only those subnets will be advertised to branch office by ISP on agreement customer.

Moreover, Why do we need iBGP ? Can't we redistribute the needed routes from any IGP(eigrp or ospf) to BGP on edge router ?

[Joseph W. Doherty]

Hard to say what's going on in the network you describe, not really enough information.

Moreover, Why do we need iBGP ? Can't we redistribute the needed routes from any IGP(eigrp or ospf) to BGP on edge router ?

In general, iBGP is used when eBGP is also being used.  BGP itself is used, outside the Internet, again generally, when complex routing polices are being used.  However, SPs often use it, with customers, because SPs often are also ISPs, and they are used to using BGP.  It's also handy when customers want Internet routes.  (BTW, often SPs/ISPs are also using an IGP too, but hidden from customers.)

Yes, you can redistribute IGP routes into BGP (and/or the converse), but often IGP routes are provided to BGP by null static route statements.  This is done to avoid route churn in BGP.

[deepanprasanth]

Head office --------------isp-----------------Branch office

Above setup running BGP protocol....

Routes from branch office was advertised to ISP eBGP peer, how ISP ensures prefixes belongs to particular customer and route them to other offices ?

Do ISP knows about customers advertised subnets ?

[Julio E. Moisa]

Hi

It depends if you are using public BGP AS you will have connectivity through Internet and you are explicitly saying who is your neighbor through the command line:

neighbor x.x.x.x remote-as < public AS>

Now, as I know the ISP must provide the public BGP AS to use.

https://www.apnic.net/get-ip/faqs/asn/#public-asn

Other option is if the ISP creates a MPLS VPN L3 scheme on its network infrastructure, it will be transparent for the client.  The ISP will represent the traffic of the client into a VRF, this VRF will be created on both ends and it will be forwarded through the ISP network.

http://www.cisco.com/c/en/us/products/ios-nx-os-software/multiprotocol-label-switching-mpls/index.html

I think there are other options like dark channel between 2 ends.

http://www.interoute.com/what-is-dark-fibre

Please rate the comment if it is useful

:-)

[Joseph W. Doherty]

Do ISP knows about customers advertised subnets ?

Yes and no.  The "know" about them as the carry they through their network, but other than passing them along, they don't normally care about them, i.e. they often don't use those subnets themselves.

Routes from branch office was advertised to ISP eBGP peer, how ISP ensures prefixes belongs to particular customer and route them to other offices ?

There are different ways to accomplish that.  If we're dealing with private address space, as Julio described, provider is likely using MPLS and VRF.  The latter allows each customer to have an isolated routing topology.  (Basically, it's like there's only the provider and one customer's network.)

If we're dealing with public address space, a customer's prefixes are usually made known to the Internet.