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eBGP & iBGP for Dual ISP


I need help on our current design made by our previous net admin. The topology I have given below is just to show you the connections.

And somehow we want to lessen down time going to the Internet whenever any one ISP link goes down physically.

Test Scenario A:

- Shut the port facing ISP A on Router1 leaving ISP B the only link to the Internet.

- No Internet downtime from LAN 10.x.x.x/24

Test Scenario B:

- Shut the port facing ISP B on Router0 leaving ISP A the only link to the Internet.

- Internet downtime of approx 3 minutes from LAN 10.x.x.x/24

- After 3 minutes, Internet is reachable automatically via ISP A

I am not sure about this but I have a theory on Test Scenario B why it has a downtime of 3 minutes and here it goes:

The iBGP link

hold time is 180, keepalive interval is 60 seconds

The eBGP link

hold time is 180, keepalive interval is 60 seconds

so whenever the ISP B goes down it waits for the hold time of 180 seconds (~3 minutes) and then after the convergence, it will use the ISP A link automatically.

Our goal is to minimize down time going or even no down time to the internet with this design.

Can someone help me in achieving such goal?

Or any design out there that gets our goal?

Any configurations e.g. BGP timer change that's needed in iBGP and eBGP links?

Your inputs will be much appreciated.


IT Monkey

Hi Jerhone,

Where are you doing NATting? On Router 0 and Router 1? Then my suggestions are as follows:

1. Remove the key word "always" in the command "default-information originate always", so that OSPF will advertise a default only when it has a default route; Adjust the metric so that one of the routers is the primary router.

2. On both routers, use "ip route  x.x.x.x track x" command to install a default route only when tracking is successfull;

3. Adjust the tracking timers agressively to achieve fast convergence;

4. Adjust OSPF timers to the possible lowest values.

If no NATting is involved, you will mostly depend on ISP routers' BGP timers for convergence because your subnets have to be advertised out to ISP for returning traffic.

Hope this helps.



I appreciate your quick response!

NAT is done in Router2.

I will definitely look into your suggestions and simulate it when am in the office later.

Sent from Cisco Technical Support iPhone App

IT Monkey

Hello Jerhone,

You should definitely remove keyword always from default-information originate always command. OSPF should advertise default route only if eBGP route is received from ISP.

As Chuan suggested you should prefer one link over another (primary/backup), to do this modify LOCAL_PREF inbound and AS_PATH (prepending outband).

Do you advertise same routes to both ISP? There could be problem with return traffic, so as I modify BGP attributes.

Set BGP timers to lower values, for example #neighbor XYZ timers 30 60

In case you shut down interface toward ISP, BGP neighborship will flap and routes via that link will be immediately flushed from routing table, so no waiting for hold timer expiration in your test should happened.

Best Regards

Please rate all helpful posts and close solved questions

Best Regards Please rate all helpful posts and close solved questions

Hi blau grana,

I appreciate your response on this. We will be doing some testing on both of your recommendations and update you guys what happend and what is configured.

Again, many thanks!

IT Monkey