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Beginner

BGP Preferred Path - Dual DC

Hi everyone,

Looking for some advice.

We're looking at building 2 connections and tunnels with an end client, one of the connections will reside in DCA and the other in DCB.

The end client will be advertising the same prefix, lets say 192.168.24.0/24.  All traffic will be initiated outbound to the client site. We're looking at doing an Active/Standby setup where all traffic will traverse DCA (no load sharing), unless DCAs connection goes down, then DCB will take over and route traffic out of the secondary link.

My thoughts are to first ensure the prefixes are accepted/flooded via BGP in DCA and DCB.  Ensure IBGP is setup in between DCA and DCB to ensure both sites are sharing the routes.  My final idea was to set up a route-map and prefix list in DCA on the bgp peering router with a higher local preference, for eg, 200.  If the routes are flooded throughout both cores, the routes should traverse DCA (preferred path), until that route is no longer active, and then DCBs route will be installed in the routing table.

Is this setup and solution correct, or does anyone have advice on how they would design it? As another solution thought of, can the client advertise the prefix to us via DCA with higher priority so no route manipulation is required on my side? A third solution though of would be to accept difference prefixes from both sites, but I'm trying to figure out a solution with the one prefix advertised to both sites to save address space and complexity with the network/security controls.

Thank you!

8 REPLIES 8
VIP Advisor

Re: BGP Preferred Path - Dual DC

Hi

Are you controlling client A advertisement or ask the client to advertise this prefix With a specific as-path?
Your solution is correct. When is possible I prefer manipulating the prefix at the source which means you advertise same prefix with different as-path and when received at the other end the shortest the as-path the more preferred is the path.
If you have multiple prefixes, you can have another solution that'll be to ask the client to advertise their prefixes with a specific community on neighbor A and a different on neighbor B then you can play with it using local-pref inbound but more complex solution.

Using any of these make sure you'll reach the destination using DCA over DCB. But i believe you also want to make sure the traffic from client A back to you is also taking the right path and not have a traffic coming from DCB but going back through DCA. Using as-path prepend on your side facing your neighbor is also the solution to avoid asking him to configure any local-pref.

Thanks
Francesco
PS: Please don't forget to rate and select as validated answer if this answered your question
Beginner

Re: BGP Preferred Path - Dual DC

Hi Francesco,

The client will be advertising the prefix to us.  We will be sending outbound traffic from our site to the clients pointing towards the advertised subnet.  Would the simplest solution to simply have the client AS Path Prepend the prefix advertised out of DCB?

Thanks

VIP Advisor

Re: BGP Preferred Path - Dual DC

Hello


@kramer977 wrote:

Hi Francesco,

The client will be advertising the prefix to us.  We will be sending outbound traffic from our site to the clients pointing towards the advertised subnet.  Would the simplest solution to simply have the client AS Path Prepend the prefix advertised out of DCB?

Thanks


Not really - AS-path preference is further down the route-path selection process, local preferance would be better option on received routes

 



kind regards
Paul

Please rate and mark posts accordingly if you have found any of the information provided useful.
It will hopefully assist others with similar issues in the future
Beginner

Re: BGP Preferred Path - Dual DC

I figured it would help to create a quick diagram to show the bigger picture (attached)

Most end users who are initiating traffic is primarily flowing through DCA.

With both prefixes advertising, I will be accepting them via BGP from both DCs via AS2, AS3, AS4.

Maybe this clears things up a bit better, would love to hear what options are available from both the client and our end? If we have the client AS Path Prepend on the secondary circuit, do I also need to add preference for the outbound traffic?

VIP Advisor

Re: BGP Preferred Path - Dual DC

Hello

as stated previously I would suggest you utilise Local Preference PA for path selection within you DCs for the received routes from you clients additionally you could also incorporate bfg community values this way dc rtrs wiil assign LP on the routes assigned to a particular bgp community - meaning you could have many clients with routes coupled  in say two community values and have the dcs path select on these two values as to which path they take for each community 



kind regards
Paul

Please rate and mark posts accordingly if you have found any of the information provided useful.
It will hopefully assist others with similar issues in the future
VIP Advisor

Re: BGP Preferred Path - Dual DC

Local-pref is always preferred over as-path prepend.

Actually the diagram helps understanding the big picture because on my head it was the other way around (inverted client and you).
Anyways, on your side you can use local-pref as you mentioned at the beginning for outbound communication (you --> to client).
On client side you designed it at 2 separate sites but behind it there are some other devices interconnecting both DC right? Are you sure DCA is always the default path for them? In this case, i would advertise a longest as-path to DCB to make sure the return traffic will always take the right return path.

FYI: As-path advertisement could make sense of your client can do it also but you don't have to use local pref on your side otherwise it won't make sense.

Thanks
Francesco
PS: Please don't forget to rate and select as validated answer if this answered your question
VIP Advisor

Re: BGP Preferred Path - Dual DC

Hello @Francesco Molino 

Looking at it again you could possibly be right -

"The end client will be advertising the same prefix, lets say 192.168.24.0/24.

All traffic will be initiated outbound to the client site

The diagram shows a heading of "Client" with DCA/DCB underneath it -  then a Heading of "OUR SITE" with end users.
So I am now confused!



kind regards
Paul

Please rate and mark posts accordingly if you have found any of the information provided useful.
It will hopefully assist others with similar issues in the future
Beginner

Re: BGP Preferred Path - Dual DC

For inbound direction you can use AS path prepend. It's the easiest option. For outbound direction, use LP.

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