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Have two sites with Cisco 2520 grid switches connected over fibre (OPGW, to be precise).

  • Site #1 is the existing site, with a 2520 connected to Site #2 (via fibre). There is an existing 3G backup connection elsewhere in that network (via a 1900 + 3G WIC).
  • Site #2 is the new site, solely 2520 switches, and needs a backup 3G connection.

The current plan is to connect a 3G router (i.e. Sierra Raven XE) to a second port on the Site #2 edge switch that terminates the OPGW fibre. Ideally, if there's a comms failure on the Site #2 switch FE0/1 port connected to the OPGW (i.e. damage, contractor bumps a FOBOT, etc) the switch should pick up on this and fail over to the 3G router sitting on FE0/2.

My understanding is that setting up an IP SLA on the Site #2 edge switch that hits (via icmp-echo) an IP within Site #1 is the 'simplest' approach here.

e.g.

Site #2 edge switch:

ip sla 1 icmp-echo 192.168.x.x` frequency 5 timeout 500 ip sla schedule 1 life forever start-time now track 1 ip sla reachability

ip route 0.0.0.0 0.0.0.0 192.168.x.x track 1 ip route 0.0.0.0 0.0.0.0 FastEthernet0/2 20 ! metric of 20 to ensure it only kicks in when track 1 fails

Site #1 edge switch stays 'as is'.

Have I missed anything here? Can we do this purely with IP switches?

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Other than the inherent issues with routing to an ethernet interface -- which is an arp table muderous case of proxy-arp -- I see no problem. (I'm assuming your ping target is the other switch, not some machine that could be randomly "unreachable")

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  • Correct, ping target will be the other switch. What potential issues will proxy-arp introduce in this case? – elithrar Nov 5 '14 at 3:02
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    a) something out on f0/2 will have to answer the ARP request for things that aren't it, b) every IP address to which you wish to communicate will have an ARP entry. Unless that switch is a gateway for other machines, (b) won't be such a huge issue. (others have had SERIOUS issues because this config faux pas.) – Ricky Nov 5 '14 at 3:57

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