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Traffic diversion in EIGRP / OSPF

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Feb 10th, 2014
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Dear All,


I am new to routing protocols.

I need to connect 2 branches with redundant links.

My requirement is to use only one link as a active and second link in standby. Both links having same Bandwidth.

What protocol I should use, ospf or eigrp?

how much time it take for failover ?

if primary link is having some fluctions, what commands i need to use to do failover the traffic if i use ospf or eigrp.


If links having same bandwidth, then load balancing is by default enabled in case of eigrp or ospf?


Please help me out with doing this small setup.

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Jon Marshall Mon, 02/10/2014 - 12:54
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Okay which do you want ie you say you want one as backup and then you talk about load sharing equal cost paths.


If they are both Cisco routers you can use EIGRP and add a delay to the interfaces of the backup link on both routers so the other link is preferred. If it is just these two routers then even with the delay the backup link should be seen as a feasible successor so failover will be very quick.


Perhaps you could explain a bit more about your topology. Is it literally one network at each site connected by a single router with two links on each router ?


Jon

viral_cisco Mon, 02/10/2014 - 13:06
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Hi Jon, Here is my topology... could u pls clear my concept by replying on my questions.

These are cisco routers... link bandwidth is same for both links. by default does it load balance? i need to configure active standby scenario.

image.jpg

Jon Marshall Mon, 02/10/2014 - 13:37
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Yes, by default it load balances across equal cost paths.


But you are using HSRP on the router so it wont see two equal cost paths anyway ie. all traffic goes to the HSRP active router so it will smply use it's own link. So you could simply match up the HSRP active on both sides so that the same link was used for traffic both ways.


If those are serial links you could also track the state of the serial link at both ends and failover if the serial link went down.


The only failure scenario that would cause a problem would be if the LAN interface of the HSRP active router failed. Then it will failover but the other end does not because it's serial link connection is stll up. So it sends traffic to the router with the failed interfaces.


To get round that you could always use IP SLA tracking or maybe EEM to shut the serial interface if the LAN interface fails and then the other end would failover as well.


Can you clarify whether those links are serial point to point links ?


I am just not sure what a routing protocol gives you in this instance.


Jon

viral_cisco Mon, 02/10/2014 - 14:18
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Thanks jon for your reply....

my links are with point to point ethernet drops...


i aggred with ur comments on HSRP. but if one link goes down, does failover happpen automatically?

Jon Marshall Mon, 02/10/2014 - 14:40
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It depends where the failure is. If they are point to point links then if one end fails the other end should go down as well. However if these are being provisoned by a provider they may well appear point to point but they may go through provider switches in which case if one end fails the other might still think it is up.


The only way to test if to shut one end down and see if the other end goes down as well.


Lets assume they do behave like that and you are using HSRP. You are also tracking within HSRP the status of the point to point link -


1) if the link itself fails  then both routers should see the WAN interface go down and because you are tracking with HSRP both router failover to the othe HSRP router so it all works


2) if the WAN interface on either router fails the other router's WAN interface should go down and again because of HSRP tracking both routers failover.


3) if either active router at each end of the link fails then again the other router's WAN interface should go down and both routers failover.


but -


4) if the LAN interface on one of the active routers fails then it fails over to the other router. But the active router at the other end does not failover because it's WAN link is still up because it was the LAN interface that failed on the router not the WAN interface.


All of the above as i say depends on whether those links act as true point to point links. If they don't then you definitel can't rely on HSRP with tracking for any failover.


So it depends. And if you wanted to be sure of failing over in all scenarios then you may need additional configuration.


Jon

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