Configure 3G wireless as a backup line with layer 3 cisco switch??

[Fida jlassi]

Hi all,

We have an existing GPRS modems to tranfer data between 2 different sites, This connection is a little bit slow at no more than about 114kbps, the idea is to add a 3G modem, so the solution will be based on a dual communication lines which are the 3G network and GPRS network.

The GPRS line will be the primary and the 3G will be secondary, This redundancy will offer a high level of communication availability between both sites.

is it possible to configure this redundancy with a cisco switch layer 3? if it's the case did you have a tutorial or a link that describe how to do this work with a layer 3 ciso switch?

any information will be helpful for me, thanks

[Fida jlassi]

could you help me please

[anisaini]

Hi Fida,

Yes you can achieve that logic.

This can be done by using a floating static route with the primar having higher preference

ip route 0.0.0.0 0.0.0.0

ip route 0.0.0.0 0.0.0.0 200

but only above config might have some loopholes like when the primary is down  it will not switch to back-up as the route still exists in routing table.

so we need to have some mechanism which will withdraw the first route when there is no traffic passing through primary link, this can be achieved by track feature with ip sla.

use "track feature" on 1st static route which will keep track of the primary route and if reachability is lost it will remove the first static route and the next available route will be in routing table.

after track config

-----------------------------------

ip route 0.0.0.0 0.0.0.0 track 1

track 1 ip sla 1

ip sla 1

icmp-echo 4.2.2.2 source-interface

ip sla schedule 1 life forever start-time now

ip route 4.2.2.2 255.255.255.255 permanent

ip route 0.0.0.0 0.0.0.0 200

i found another good link which explains the same

https://supportforums.cisco.com/docs/DOC-6078

Hope this helps.

[Fida jlassi]

Ok, thank you anisaini, I will test it and go back to you.

Only one thing to be sure before starting the purchase procedure, a layer 3 switch can do this work or I need a router?

[Fida jlassi]

Could you help please, if it's possible to use the cisco layer 3 switch instead  of cisco router ??

[manish arora]

Hi Fida,

The Cisco 3750 Layer3 switch does support the IP SLA configuration.

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_55_se/configuration/guide/swipsla.html

Manish

[Fida jlassi]

Hi,

I start the purchase procedure and I get the SF300 to do this work.

I start the basical configuration with only the ip route like you described in the first email:

ip route 0.0.0.0 0.0.0.0

ip route 0.0.0.0 0.0.0.0 200

The command is accepted by the switch but it is impossible to ping from the internal network to the external one after the ip route command.

other question please did I need to cerate vlans in this case?

[Fida jlassi]

Any Answer?

[cadet alain]

Hi,

You cannot NAT on a switch(except 6500 and 5500) so if you want to communicate with outside networks from private IP you need to NAT and in this case you need a router or firewall to do so.

Regards

Alain

Don't forget to rate helpful posts.

[Fida jlassi]

Hi,

Please I feel confused, because in the previous  post, manish arora  said that the switch the Cisco 3750 Layer3 does support the IP SLA configuration and according to anisaini with the SLA configuration we can assure the failover between the 3G and the GPRS line.

That's why I start the purchased procedure and start the configuration.

Please advise did the switch layer 3 ensure the fonctionalities that I described in the first post?

Really I need your help Experts

Regards,

Jlassi Fida

[cadet alain]

Hi,

if you have'nt got any Layer3 device capable of doing NAT like modem/router or firewall then you won't communicate on the Internet because  as I've mentioned before a switch can't do NAT.There is also another problem which is that some modem/routers won't NAT other subnets but their directly connected one.

Regards

Alain

Don't forget to rate helpful posts.

[Fida jlassi]

Hi Cadet,

Thanks for your answer.

Could you please advice on how to do the router configuration for the failover between two lines?

more descripbtions are found above in my first post.

Thanks again.

Regards,

Jlassi Fida

[cadet alain · Accepted solution]

Hi,

The config is the one provided by anisaini but you'll have to modify your NAT like this:

ip nat inside source route-map PRIMARY interface x/x    Primary interface

ip nat inside source route-map SECONDARY interface y/y  Secondary interface

int z/z     Inside interface

ip nat inside

int x/x

ip nat outside

int y/y

ip nat outside

access-list 99

permit x.x.x.x  y.y.y.y   where x.x.x.x is your inside addresses subnet and y.y.y.y is the corresponding wildcard mask

route-map PRIMARY

match ip add 99

match interface x/x

route-map SECONDARY

match ip add 99

match interface y/y

Regards

Alain

Don't forget to rate helpful posts.

[Fida jlassi]

Hi,

Only to be sure, following is the architecture, please confirm that in this case I need a router instead of a layer 3 switch

Regards,

Jlassi Fida