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

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

I'm attaching a basic document to help show what I'm talking about.  The red lines on the drawing is the data flow I'd like to achieve.  I'll be receiving full routes from both ISPs, and only directly connected routes from our directly peered clients.  I want to primarly 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 failover only, least inititally (Might use load sharing and variance on it).

1.)  What static routes do I advertise from routers A and B downstream to C/D and E/F?  As I 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).  I 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 defualt route to ISP#2 and therefore waste having full internet routes and iBGP, etc.   

Basically how do I 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 I get a static route sent from our ISPs via eBGP?

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

Are there any gotchas that I should consider or think about?

Thanks,

Nathan

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

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

Hello!


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 (see http://www.cisco.com/en/US/partner/tech/tk365/technologies_tech_note09186a0080094431.shtml).

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

http://www.cisco.com/en/US/partner/tech/tk365/technologies_configuration_example09186a00800945bf.shtml

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.

Regards,

Herve

3 REPLIES
Cisco Employee

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

Hello!


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 (see http://www.cisco.com/en/US/partner/tech/tk365/technologies_tech_note09186a0080094431.shtml).

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

http://www.cisco.com/en/US/partner/tech/tk365/technologies_configuration_example09186a00800945bf.shtml

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.

Regards,

Herve

New Member

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

Herve,

Thanks for the reply, that definitely clarifies some of the questions I have.

I still would like to know what static route do I inject at both A and B?  Is it the next hop interface to ISP#1, and #2?  If so, should I let the ISPs sent that in the BGP tables?

About the iBGP, I understand A and B will compare and each others route maps and make up their own best route tables based on it, but if the default routes point straight to the ISPs, would the packets from C be checked against the entire route map and if no matches found, THEN it will take the default route to the ISP??  (sorry for the run-on!)

Cisco Employee

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

Hello,

Since you'll receive the full Internet table form the two ISPs, where your static default route is pointing does not really matter: typically you'll configure on A and B a static default to null0 and redistribute it into EIGRP. In that way the traffic from your local network will reach the A and B routers following the default route, and will be routed based on the more specific routes that you learn from BGP. This is the only goal of this static default: advertise a default gateway into your network.

Note that the ISPs might as well advertise to you a smaller set of the Internet table + a default route. If they do this, instead of redistributing a static default route into EIGRP, you would redistribute the BGP default route into EIGRP (but only the default route, use filters... otherwise there will be too many routes into EIGRP).

Herve

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