BGP Multi-Home connections and default route.

[alalli]

Good Day,

I have a question about the use of static default-routes and BGP.

We have a multi-homed BGP network with a primary and secondary Internet provider.

We have 2 Border routers running BGP with an iBGP neighbor relationship between their loopback interfaces.

Currently, the Primary Internet Service Provider is sending us a full BGP routing table and no default-route.

The secondary ISP sends us quite a large number of routes and a default-route.

The Border routers also have a static default-route pointing to Null0 with an admin distance of 250.

I tried to test the failover capabilities of the routers and found that when I put the Primary ISP neighbor into shutdown, that my network was down in spite of the fact that I had all of these routes and the default route from the backup ISP.

I have also noted that when I run show ip route, that the route pointing to null0 is picked as the gateway of last resort. I was thinking that this should be the default-route from the backup ISP.

I think that, perhaps, I should not have these static default-routes pointing to Null0.

I am a BGP newbie, so please forgive me if I am not understanding the protocol; but I think that the static default route gets put in the routing table because it has the highest weight (32K ).

I think that i should be deleting these statics to null0.

I have also requested that my primary ISP send me a default-route as well.

Can anyone shed any light on this?

Also, what are the repricussions from removing the static-default route without one supplied yet from the primary ISP. Perhaps none, since i have one from the Secondary ISP.

Thank you very much.

Any help at all is appreciated.

Regards,

Amanda

________ ____________

| | | |

|--| Bdr1 | --eBGP-- | Backup ISP |

| |________| |____________|

|

|iBGP

| ________ ____________

| | | | |

|--| Bdr2 | --eBGP-- |Primary ISP |

|________| |____________|

[prafuljaded · Accepted solution]

Hi Amanda,

I don't understand why you have a default static pointing to Null0.Can anybody explain it better ?

I have a similar multihoming setup as yours but I am accepting Service providers' routes around 6000 routes totally from both ISPs.And I have a static default route to my primary ISP (ISP1)and another default to my secondary ISP (ISP2) with higher admin distance, so that my outgoing traffic prefers ISP1 for unknown routes.

Regards,

Praful

[alalli]

Good Day Mate,

I don't know why the router has been configured this way. It does not make any sence to me at all.

I like how you all have done it with the two static routes pointing the the different service providers. I think you have just given me the answer. I wonder if anyone else can confirm it.

Thank you anyway for your help mate :)

[mjoaquin23 · Accepted solution]

default static is sometimes used for loop prevention (prevent traffic from bouncing between defaults facing each other) or to just trick the ebgp if your desire is to advertise the default route to your provider to influence incoming traffic. usually this would be followed by a network 0.0.0.0 command, but in this case, this has no relationship with the default that is being received from your ISP, which only has influence on your outgoing traffic. if you're not using any other dynamic routing protocol, then i think you can delete the static default. and since only your secondary ISP is sending the default, you can immediately confirm its functionality even without bringing down the primary link.

you can implement the 2 static default route scheme only for outgoing traffic and also have the luxury of blocking incoming advertisements from your provider to minimize memory utilization.