Search this Blog

Friday, November 5, 2010

EIGRP, eBGP routing, HSRP - Static Route/Gateway ?

We have some questions about general data flow going from downstream access routers up to the internet edge and lastly though multihomed eBGP peers.

We attached a basic document to help show what I'm talking about. The red lines on the drawing is the data flow we'd like to achieve. We'll be receiving full routes from both ISPs, and only directly connected routes from our directly peered clients. We want to primarily use the dark fiber, as the latency will be much better than our leased line (and we can easily expand via CWDM). D to B will be used for fail-over only, least initially (Might use load sharing and variance on it).

1.) What static routes do we advertise from routers A and B downstream to C/D and E/F? As we want data to flow from E/F using equal cost load balancing to C and D, and then flow through C to A. Once the data reaches A, then the packets should take the best route to the internet via A or B (and on B through ISP #1, or Peer). We want to advertise the default route(s) necessary so that a routing decision happens based on the full routing tables from both ISPs (and peers) and doesn't automatically go right through the router following a default route to ISP#2 and therefore waste having full internet routes and iBGP, etc. Basically how do we make sure (via default route(s) and configuration) packets that arrive at either A or B actually utilizes the full routing tables to decide which path to take going outbound to the internet?

2.) Should we get a static route sent from our ISPs via eBGP?

3.) Do we need to use HSRP at A/B for any reason, or will EIGRP handle the fail-over and advertising the default routes in the event A or B drops? We don't want to have any links inactive and on standby if we can help it. (we will be using EIGRP at the access layer).


On A and B, you inject a default route into EIGRP. Then you must play with EIGRP metric so that the link over the dark fiber is preferred over the link over the leased line.

The best way to influence EIGRP metric is by setting the 'delay' value on the interfaces. Put a higher (worse) delay on the interfaces over the leased line.

Once this is done, the traffic should go via E->C->A and F->C->A (I don't see the need of messing up with metrics to use F->D->C->A).

Now between A and B, you must configure an iBGP neighborship, so that they can exchange the routes they learn from their respective EBGP peers, and chose the best route based on BGP best path selection . Please click here (cisco login required) for more information.

You can also play a lot with BGP attributes (local pref, as-path etc...). More config examples about BGP multihoming here:

Last thing: no need for HSRP. HSRP is to provide a redundant default gateway to hosts, not between routers. EIGRP and BGP will take care of the redundancy if a link or a router goes down.

Citation - This blog post does not reflect original content from the author. Rather it summarizes content that are relevant to the topic from different sources in the web. The sources might include any online discussion boards, forums, websites and others.

1 comment :

  1. Great precise info, I've been searching on this topic for a while. Bookmarked and recommended
    windows 7


/* Google Analytics begin ----------------------------------------------- */ /* Google Analytics end ----------------------------------------------- */