BGP Design Considerations when Dual-Homed

[marioderosa2008]

Hi all,

I am designing BGP route redistribution between two AS's which are dual-homed...

should I follow the same rules as if the AS was a transit AS between different AS's...?

For example, should I consider iBGP on all routers in the path as a fully mesh? Or should I force BGP Synchronisation?

Any advise appreciated!

thanks

Mario

[marioderosa2008]

To add more information...

we use OSPF as our IGP and we are redistributing between OSPF & BGP. We recieve thier full BGP routing table and then only redistribute summaries in to OSPF rather than the full BGP routing table.

We currently have one link up and working at the moment. But now all the fun is about to start as we need to add in a second resilient link making us dual-homed.

thanks

Mario

[JohnTylerPearce]

Is there on router connected to both ISPs, or two routers, with each connected to one, and a link between them?

[marioderosa2008]

Hi,

there are two ASR routers each with its own BGP peer and they are both connected together in Area0 of OSPF.

Hope that makes sense

Mario

[JohnTylerPearce]

Marioderosa,

Thanks for the updated information. You don't need to worry about BGP synchronization at all, I would just turn that default off, which it very may be by default, depending on IOS verison.

Will the access rate be the same on both ISPs? For instance, will ISP-A have 100MBps and ISP-B have 100Mbps as well?

Also do you have a provider independent network or is your public range provided to you by your Internet povider?

You can do several things such as:

- Configuring iBGP between your two routers, with each router having an eBGP peer with the directly connectd ISP.

- Configuring dual static routes for load-sharing etc...

Are you wanting to have one as primary and secondary, or both used at the same time?

Sorry for all the questions, I'm just trying to come up with a good design for you, that meats the goals that you want.

[marioderosa2008]

Thanks for the quick response...

The bandwidth is the same on both links, 100mbps...

we would like to have a dedicated primary and backup scenario....

our public range is provided by our ISP's...

Our setup will be the first option which is running iBGP between the two ASR's and each ASR peers with an eBGP neighbor...

Thanks

Mario

[JohnTylerPearce]

Marioderosa,

Thanks for the information.

When you said "our public range is provided by our ISPs", does this mean that each ISP assigns you a block?

Such as ISP-A assigns you 100.100.100.0/27 and then ISP-B will provide you 200.200.200.0/27?

The first option should work for outbound, from that information I can see so far. You can setup IP SLA with tracking.

You can configure two default routes and configure the one going to ISP-A with the default Administrative Distance

and the other default route to ISP-B with an Adminstrative Distance of 5 for example.

This will mean all outbound traffic goes out of ISP-A and then you can configure IP SLA with tracking to track that route and

if the IP SLA can't ping 8.8.8.8 for example, then take out the default route goign to ISP-A and then use the default route going to ISP-B.

Of course, since you have two ISPs, and if they are both giving you a block of "their public address space", then the return traffic may not go back to ISP-B regardless of how your outbound traffic is.

Also, with this method, I would just ask them to send you a default route, since your not doing load-sharing/load-balancing they're really isn't a point to having the full route, as traffic ill primarily go out of ISP-A unless ISP-A is down.

[marioderosa2008]

Hi John,

sorry, I havent explained this fully... This is not connecting to an ISP as such.

We are two companies which are merging networks...

My Company, has installed CE routers in our two datacentre's so that we can have two resilient links to our sister company's MPLS network.

So public ranges are not really an issue at this stage...

We are dul homing to the same autonomous system rather than multi-homing to two different autonomous systems.

Hope thats clearer...

Thanks

Mario

[JohnTylerPearce · Accepted solution]

Maridoerosa,

Thanks for the update, I understand now. So from what I understand, you have installed two CE routers that are

connected to your new sister company's MPLS network, for obvious redundancy reasons.

How are the routers at the other end configure? Because, form what I understand, the two CE routers are running OSPF in your data center, and then have a connection to your sister company's MPLS network, is this through iBGP/eBGP or straight L2?

If it's running iBGP, I would think multiple mutual redistribution should work with BGP and OSPF and whatever the other side, and jst configure filter-lists etc, to prevent route feedback and administrative distances if you have too.

Then you could create IP SLA like I said previously but going to the other side of your company's sister network.

[marioderosa2008]

Hi John,

yes our CE routers have eBGP neighbours in our Sister company. Our CE routers will have an iBGP relationship between them. Both our CE routers will redistribute summary addresses in to OSPF...

Currently we recieve the full BGP routing table from our Suster company. We then summarise the routes and redistribute them in to OSPF.

Going back the other way, we redistribute the full OSPF routing table in to BGP, but then configure summary routes to advertise to eBGP neighbors.

yes we intend to use route-baps with filters such as route tagging...

Thanks

Mario