BGP failing with 3850

[Damir Reic]

Hi all,

we were assured by a consultant that instead of getting Cisco ASR router we can do BGP routing with 3850 (we only have 2 ISPs connected to our network) but now we have a problem where we can't reach some public IPs from our network when both ISPs are connected, if only one is connected everything is OK. We currenty hold some routing tables on the 3850 (i think around 4000). Since we are stuck now with 3850, can we for example just have 2 routes (to each ISP) without any routing tables so those ISPs handle traffic routing? The crucial part is that we have auto failover to other link if one of the ISP links go down. I am not BGP experts so if question is silly my apologies.

Thanks!

[Richard Burts]

I do not remember having done it but I would think that running BGP on 3850 should be quite possible. I would have some concern about the size of the BGP tables with two ISP peers. I would suggest that you arrange with both ISP to provide only the default route - or you can configure an inbound filter that accepts only the default route. With BGP to two ISP and accepting only the inbound default route there should be no issue with table size and you would have automatic failover if one ISP develops a problem.

There are some things to consider such as whether you treat both ISP as equally attractive or treat one as primary and the other as backup. There are also potential issues such as how you are doing address translation when you have two active ISP (which could possibly be the issue you are experiencing).

HTH

Rick

[Damir Reic]

Hi Richard,

having one ISP to act as primary and other one as secondary is acceptable, we only care about the availability and that's our only concern. If ISPs only provide us default routes, how fast failover can we expect in case 1 ISP goes down? If there are fw ms difference in paths that something we don't care about as long as everything is available.  

In this scenario can we expect problems in case packets are coming in from a secondary ISP if the source gets closer route to our AS over that secondary ISP (can loss of packets happen)?

Thanks

[Richard Burts · Accepted solution]

I agree with Akash that it would be good to try to find out what is the problem when you have both ISP connected. I am assuming that the problem is that you have problem based on the number of routes being advertised to you. But perhaps the problem is something else. We can solve the problem better when we are sure that we know what the problem is.

There are alternatives to BGP. You could configure a static default route to one ISP and a floating static default route to the other ISP. To get failover you would also need to configure IPSLA and to track connectivity to the first ISP. And that does not address the question of how you would advertise your address space to one or both ISP. So I think that as long as you are using two ISP that BGP is the optimum solution.

You ask about the possibility of packet loss if some data is sent to you via the second ISP. I would not think that this would cause a problem directly and I have worked with customers who routinely receive data from two different ISP with no problem at all. But depending on how you are doing address translation (assuming that you are doing address translation) there could be problems if you receive data from one ISP and send the response using the other ISP.

HTH

Rick

[Damir Reic]

Hello Richard,

thank you for detailed answer. I'll see what can be done.

Regards!

[Akash Agrawal]

Hi,

When BGP session is up with both ISPs and you are not able to reach few public ips, sourcebased traceroute to one such ip would help to check where packet is getting dropped. I hope we have atlest one default route.

Regarding how many routes, should we accept -- Default route + BGP prefixes of ISP1, ISP2 and its directly connected customers can be accepted. If you receive only default router and prefer it via ISP1 and want to reach customer of ISP2, then unnecessary latency would be added.

Regards,

Akash