Re: wan link failover?

Mangesh Kamerkar · ‎11-19-2010

hi all,

currently i have 2 leased line between our HO and Branch office, one is ethernet link and other is serial,

our ethernet link is 4 mb and i want to use it as my primary link. if it fails my serial link should take up. also i should get a notification about ethernet link failure

please suggest me the best way to configure my routers.

Peter Paluch · ‎11-19-2010

Mangesh,

I assume you are not using any routing protocol between your HQ and Branch.

Using static routing, your goal should be easy to accomplish by configuring the routing in such a way that routes over the Ethernet connection are preferred, and only when the Ethernet interface fails, the Serial interface will take over.

Let's assume that we are at the Branch, and the network located at the HQ is 10.0.0.0/24. Furthermore, assume we are using Fa0/0 and S0/0/0 as interfaces towards the HQ site. The next hop for routes via Fa0/0 is 1.1.1.1 and the next hop for routes via S0/0/0 is 2.2.2.2.

The routing on the Branch would be set up as follows:

ip route 10.0.0.0 255.255.255.0 Fa0/0 1.1.1.1

ip route 10.0.0.0 255.255.255.0 S0/0/0 2.2.2.2 2

This would have to be done for all networks located on the HQ site. The HQ site would have to configure this analogously for networks located at the Branch.

Note that the route over the S0/0/0 interface has its administrative distance set to 2 (the very last number in the command) and therefore it is less preferred than the route through Fa0/0. This way, if Fa0/0 is working, the route to 10.0.0.0/24 will go via Fa0/0. If the interface gets disconnected, your router will use the S0/0/0 to reach the 10.0.0.0/24, and if Fa0/0 is later made operational again, router will start using it.

This configuration reacts to interface disconnection/operation, but it is not able to verify if the entire route from Branch to HQ via Fa0/0 is workable. If the ISP has a problem but the Fa0/0 is still up/up, routes will not be replaced. To solve this, you would either need to run a routing protocol over the interfaces (if that is possible), or use active tests (IP SLA) to determine the validity of a route.

As for notifications, each up/down interface transition is logged on the console, and also can be sent to a Syslog or SNMP server for further processing. The Syslog is probably easier to set up, there are lots of Syslog servers out there for Windows and Linux (for example, Kiwi Syslog Server for Windows or syslog-ng for Linux). On your router, you just configure sending messages to the appropriate machine where the Syslog server is running:

logging host 192.0.2.1

logging trap informational

I assume here that the syslog server is at 192.0.2.1.

Best regards,

Peter

Talha Ansari · ‎11-19-2010

Hi,

I would like to add one more way which would help to overcome this.

You can form a GRE tunnel between your HO and branch office through the fastethernet ports and set keepalives on the tunnel interfaces. When the service provider link goes down the ethernet port stays up. By configuring keepalives on the tunnel interfaces will make the tunnel interface go down once keepalives is missed and this make the route to converge to serial link. Make sure you have a higher AD route configured through your serial port.

# int tunn 0

# keepalive 10 3

You will only require an additional subnet of /30 to be applied on the physical fastethernet interfaces for source and destination purpose of tunnel formation. But you can use "any" subnet that is not been used in your network, as you do not need to route this subnet.

Also when the tunnel interface will go down a syslog will be generated to inform you.