Configuring Redundancy w/ 3 WAN Links (2 Routers and 4G as Tertiary Route)

[michael.lorincz]

Hello, I'm trying to determine the best approach for a somewhat complicated redundancy scenario. Initially we have setup an HSRP failover mechanism between 2 routers which has worked great. Things are getting interesting now that the customer wants to add 4G to the mix. Here's a little background first.

Currently:

Remote site has 2 routers, a 1921 as primary with VPLS connection to HQ core switch. They also have an 881 onsite with a DSL connection which hosts a couple of DMVPN tunnels that home back to a couple of routers at HQ. The two remote site routers are setup in an HSRP arrangement with the 1921 setup w/ IP SLA tracking next hop reach ability, decrementing priority in the event of a failure. The 1921's default route is learned from HQ over the VPLS connection.

The 881 has a static route pointing out the DSL interface. In the event the 1921 fails reaching its next hop the 881 takes over and send everything out the WAN port. So far so go.

Going to:

Time to spice it up, the customer wants to add a 4G card to 1921 and offer 4G as a last resort. If the VPLS goes down default gateway for data changes to 881, if  the 881's DSL goes down then I'll need a mechanism to forward toward the 1921 for egress from 4G. I figure the easiest way to do this will be to configure IP SLA to track reach ability. I'd then tie that tracker to a static route pointing out DSL w/ a floating static route pointing back to 1921. So if the DSL does die then routing table will point all traffic to the 1921. I'd like to avoid messing with HSRP further if possible (feel free to interject if you think this isn’t a good approach).

So the packets are now arriving back on the 1921, VPLS is down and 4G is the only way out... here's where I get fuzzy. Customer only wants to utilize 4G at these times, otherwise it wouldn’t be active. I know I have to setup a dialer which kicks in when interesting traffic is detected but I'm not exactly sure how to pull that off in this case. 

Any help would  be appreciated. I hope everyone's day is going well!

{C}

[Nagendra Kumar Nainar]

Hi Michael,

If I understood it clearly you plan to have a floating static route on backup router pointing worads 1921. So if DSL is down, 881 will still be hsrp active and point towards to 1921.

I think you can play around with hsrp priority to have the traffic flow direclty to 1921 if DSL is down. 

1. Configure HSRP with higher priority say 100 on 1921 and track primary VPLS link. If it is down, reduce the priority to 80

2. Configure 881 with priority set to 90 and track DSL interface (or any address towards dsl interface). If it is down, reduce the priority to 70

Regardig 4G config, I will leave it others. Its been a while and I myself needs some refreshment on the technology/config.

HTH,

Nagendra

[michael.lorincz]

Thanks Nagendra, your understanding is correct. I figured I could use HSRP or alternatively the IP SLA w/ floating static route on the 881 but I'm more interested in how I could trigger the 4G routing from the 1921 in this case. Thanks again!

- Mike