Solved: Failover from serial to Ethernet?

Eduardo Ramirez · ‎08-14-2010

I have two sites connected via a Point to Point T1 link. In each side of the point to point link I have two different subnets (i.e. 192.168.2.0 and 192.168.3.0). I have a a full copy of each other on each side of the link (just in case it goes down). At both ends of the PTP link I have a cisco router. I understand that If I have both the serial and the ethernet interfaces plugged in at the same time, the router will always use the Ethernet interface, because it is a directly connected network. Can I still set it up with two paths to the same place (ethernet and serial) and leave the ethernet connection disconnected until it needs to be up? In other words, my serial link goes down, so I plug in my ethernet interface to the standby network and bring it up.

Nagaraja Thanthry · ‎08-14-2010

Hello,

If it is directly connected network, then you cannot force the router to

send the traffic over T1 link. What I was suggesting is possible if the

router is routing over Ethernet link as well (Ethernet link should have

address other than 192.168.x.x address).

Regards,

NT

View solution in original post

Nagaraja Thanthry · ‎08-14-2010

Hello,

If I understand you right, your network topology is something like below:

192.168.2.0 -- RTR ======T1/Ethernet====== RTR -- 192.168.3.0

If the network is as above, you do not need to disconnect the Ethernet

cable. You can use static routes and force the traffic to go over T1 and use

Ethernet only when T1 goes down:

ip route 192.168.x.0 255.255.255.0 254

Now, since the ethernet metric (cost) is higher than the T1, it will be used

only when T1 is down. Alternatively, you can also use a routing protocol and

increase the cost of the Ethernet link.

Hope this helps.

Regards,

NT

Eduardo Ramirez · ‎08-14-2010

OK. So you can manipulate the router and tell it that the serial link has a lower metric (even-though it is not directly connected like the ethernet interface) than the ethernet interface? If so, what method would you use? Any specific routing protocol that would be better or can it be done using static routes?

Nagaraja Thanthry · ‎08-14-2010

Hello,

Can you please elaborate on what you mean by directly connected? Would it be

like:

192.168.2.0 -- RTR -

Ethernet (100.1.1.x)-- RTR -- 192.168.3.0

If that is how it is, then yes, you can configure the routing protocol like

RIP/EIGRP/OSPF and force the traffic to go through the other link.

Regards,

NT

Eduardo Ramirez · ‎08-14-2010

I mean that when you have a network on a router's ethernet interface, it is going to put it on the routing t

able as a C route (directly connected), which by default has lowest metric (preferred route). You are saying that I can manipulate

that and give preference to the serial link to the network over the directly connected ethernet interface? If so, the routing will swap over to the other link

when the link goes down? And what about when the link comes back up? Should it recalculate and start routing over the preferred serial link?

Nagaraja Thanthry · ‎08-14-2010

Hello,

If it is directly connected network, then you cannot force the router to

send the traffic over T1 link. What I was suggesting is possible if the

router is routing over Ethernet link as well (Ethernet link should have

address other than 192.168.x.x address).

Regards,

NT

Eduardo Ramirez · ‎08-14-2010

Thank You for your Replies NT.

paolo bevilacqua · ‎08-14-2010

You must not have the same subnet in different places. That prevents any basic routing and IP sane design to work.

Once you have remvoed the duplication, then you can do whatever you want.