dual isp cisco 3850??

[Muhammad.nabeelch]

Hi experts, 

we are using cisco 3850 (layer 3 switch) to connect to isp and then connecting it to aruba controller for access points. Now customer wants to add 1 more secondary ISP as a backup so if main ISP is down the secondary ISP should start working immediately. Kindly tell if it is possible to implement this is layer 3 switch. Thanks.

[Jaderson Pessoa]

@Muhammad.nabeelch hello,

you can use a IP SLA to do it.

Exemple:

track 1 ip sla 1 reachability
delay down 30 up 30 

ip sla 1
icmp-echo 8.8.8.8 source-interface GigabitEthernet0/0/1
verify-data
threshold 100
timeout 100
frequency 30
ip sla schedule 1 life forever start-time now

ip route 0.0.0.0 0.0.0.0 201.10.10.5 name ISP1 track 1
ip route 0.0.0.0 0.0.0.0 201.10.10.10 20 name ISP2

if you need more information, check it: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipsla/configuration/xe-3se/3850/sla-xe-3se-3850-book/sla_ftp.html

[Richard Burts]

There are several things that we do not know about this environment and that makes it difficult to give good advice.

1) we do not know if this implementation uses static routes to forward to the ISP or uses a dynamic routing protocol. And we do not know the basis of the forwarding logic used by the ISP to forward traffic from Internet to this network (static routing or dynamic protocol). Once we know this it should be possible to identify what needs to be done to make the second ISP work.

2) we do not know what kind of address space this network is using - is it address space assigned by first ISP or is it provider independent address space? If it is provider independent address space then adding a second ISP is fairly easy. If it is provider assigned address space then a second ISP may be problematic. You would have to have some negotiation to get permission to send traffic to second ISP with addressing from first ISP. And you would have to work out how to get the Internet to forward your traffic to the second ISP while there are problems with first ISP. I have accomplished this when running BGP with both ISP. Without BGP I don't know how you would get it to work.

3) we do not know if this network is performing address translation on it Internet bound traffic. Since the 3850 does not support address translation this is a bit of a challenge. So we do not know how much of a problem this might be in bringing up a second ISP.

If we can get these questions clarified then we can give better advice about how to make this work.

HTH

Rick