BGP and Default Routes

[conmurph]

Hi All,

I'm working on a practice lab at the moment and trying to understand BGP and default routes in a network. I've attached diagram with a bit of information. From my reading I understand that you shouldn't redistribute BGP into and IGP such as EIGRP. Instead you should use default routes and inject them into your IGP to point towards the default gateway.

In my example I have 3 routers, R7, R5 and R12, running BGP between 2 autonomous systems. I have EIGRP running on both autonomous systems.

Looking at AS711, instead of having a static route, ip route 0.0.0.0 0.0.0.0 fa0/1 or ip route 0.0.0.0 0.0.0.0 fa0/2, which point to either R5 or R12, would it be better to use ip route 0.0.0.0 0.0.0.0 null0 on R7 and then inject this into EIGRP? That way instead just using either fa0/1 or fa0/2 to get to AS 911, the null0 would allow the EIGRP routers to get to R7 and then BGP on R7 would choose the best path to AS911.

Is this the best practice or is there a better way to achieve this?

Cheers,

Conor

[Reza Sharifi]

Hi Conor,

With EIGRP, there are a couple of ways that Cisco recommends for redistributing default route into EIGRP.

one is useing ip route 0.0.0.0 0.0.0.0 with next hop command and the other one is IP summary command

Have a look at this link:

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml#defaultrouting

HTH

Reza

[Lei Tian]

Hi Conor,

My thought is if the AS 911 doesn't own the default, or it doesn't advertise a default to AS 711, you don't want configure a static default route point to R5 or R12. It is fine to either creating a default to null 0 and redistribute into EIGRP or using summary-address inject default into EIGRP.

If the AS 911 advertises BGP default to AS 711, and you don't want redistribute it into EIGRP. On that case, you need to use summary-address inject default into EIGRP, and you need to change the default AD (5) to a higher value, so it won't overite the default route learned from AS 911 via BGP.

Another note, for static default route, it is better to use next hop ip; a default points to interface will cause the router ARP for all packets.

HTH,

Lei Tian

[james-worley]

Conor

Why do you want a default route in your IGP? IGPs 'should' only carry infrastructure routes.

In a typical service provider senario with AS911 as the upstream AS and AS711 as the the customer, AS911 would send a BGP default to AS711. Assuming AS711 has a full mesh of iBGP neighbours then the AS will automatically forward the eBGP learnt route via R5 and R12.

James

[conmurph]

Thanks everyone for your help. I'm not sure how this would be setup in a real environment. If AS 711 was a large company and 911 was an ISP, would 711 only run BGP on the routers connecting to the ISP rather than have a full mesh of iBGP peers within the organization?

The reason I thought to use a default route in the IGP was because the routers connecting to the ISP would use BGP and the remaining routers in the company would use EIGRP. So I used the default route in EIGRP to send all traffic destined for the internet towards R7. Is this not the best method? Sorry if it's a bit unclear, I'm trying to understand the best practices for real life scenarios.

[james-worley · Accepted solution]

Conor

In the senario you desribe using a EIGRP defuault route to R8, 9 and 10 is valid and is 'normal' for a lot of Enterprise customer where R7 is the CPE of some managed service.

James