Redundant links between subnets

Ghazan Haider · 2006-06-20T15:02:52+00:00

We have two subnets connected by two wireless connections. The wirelessconnections are unreliable.The wireless connections have IP nodes on each end.. thus we have the10.0.0.250 and 10.0.0.254 links on our 10.0.0.0/24 subnet and the10.0.1.250 and 10.0.1.254 nodes on the 10.0.1.0/24 subnet.I'd like to use the links (a) redundantly and (b) in a load-balancingfashion. We have two cisco 1841 routers in each subnet acting as afirewall and defaultroute. I know etherchannel/trunk will make twolayer 2 links redundant. But I dont know what to use.I've been reading on VRRP HSRP and GLBP, but they make routersredundant. This would work if the wireless nodes were cisco, alasthey're not. I read about DLSw+ but that works on layer2 as well.Then I've been digging around OSPF and EIGRP, which might solve theproblem. But OSPF links go down having not received hello packets 3times or so. That may take 10 seconds of total downtime before IP isrouted through the other link. Our application is tolerant of upto 2-3seconds of delay (TCP/ODBC), but will break if packets are dropped withdestination host unreachable. I dont know if in this case OSPF willresend the packet through the other link so nothing is lost. If itdoes, and hello packets are frequent enough, my problem is solved.But I'm wondering about other cisco shrink-wrapped technology that justworks for redundant layer 3 routes without a full-blown routingprotocol. Can any expert here give me a better clue before I giveOSPF/EIGRP a shot?Yes I've scoured the cisco site, news groups and for thepast 5 days.

Jun 20, 2006 22 Replies

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Ghazan Haider 20 years ago

We have two 1841s. So the connection is between two cisco routers over two non-cisco routers. I havent tried it but I was under the impression static routes could be entered for the other cisco router on each router for the sake of hello packets, and the neighbor command can be used to specify the neighbor for hello packets. If this doesnt work, you're right I'll have to resort to GRE tunnels.

I wonder if I can do GRE tunnels just for the hello packets. Another thing I just thought of was packets in each side will go over to the other side's cisco router before entering the other side's subnet, since the destination subnet should be behind the cisco router running the IGP. I wonder if the hello packet triggers can be used to bring up/down different routes.

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Merv 20 years ago

Its not quite that simple, because each routing protocol can have additional conditions that must be met.

EIGRP certainly does and that condition being that neighbours MUST be addressed from the same subnet which you will not be true in your particular circumstance.

OSPF will require testing. OSPF may form an adjacency and allow LSA exchange. The real test is whether it will insert the learned LSA's into the main routing table given that the neighbours are on two different subnets. If it does you are homefree. You can then tune the hello timers as required.

As your stated objective is to make your connectivity more robust, keep the setup as simple.

And as Vince indicate what out for the MTU hit with GRE tunnels.

Also for GRE tunnels remebber to configure a static route for the tunnel destination address to prevent recursive lookups.

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Vincent C Jones 20 years ago

You can do GRE tunnels just for the hello packets BUT... 1- It is ugly and error prone. 2- Even if you get it right, last time I checked Cisco only updates static routes once per minute unless a link has failed hard, so your average recovery time will be over 30 seconds and be nowhere close to meeting your availability requirements.

Been there, done that, been burnt...

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