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Friday, October 29, 2010

Suggestion needed on setting up a DR (sort of ) in place?

At one site we have an MPLS single link with subnet connected to L3 switches.At the other site we have another MPLS single link with subnet connected to L3 switches.These should be configured Active/Passive , like all MPLS traffic should be flowing through site1 as long as that is up and thru site 2 if site 1 goes down.

These sites are connected with 2 P2P links . The problem we have , is that the requirement is to have a sort of DR in place for these buildings in such fashion , that if site1 MPLS goes down all traffic should flow through the site2 MPLS link.

The L3 switches will have a default route to the internet and static routes to point to the MPLS router as next-hop.There is no protocol running anywhere now .Building 1 is the existing building of this customer and building two will be coming up soon .What they want (save some cost as well) is : - Currently they have 2 MPLS at the existing site(Building1) which they want to split and move 1 link to the new site. From there , the story we have just narrated follows.They can not get the ISP to do any custom config so this is the issue they are faced with .Once they split these links , how will these two sites be aware of each other and more over do failover.


If you are not exchanging routes with the ISP then you can do this. Lets assume site 1 is active and site 2 is passive in normal operations,.This also assumes the the 2 MPLS routers can route to each other via EIGRP.

1) Configure a static default-route on site 1 router that connects to MPLS pointing to the next-hop for the site 1 MPLS connection.

2) redistribute this static into EIGRP. Note this will mean the static has an AD of 170 on all other routers in both sites.

3) Configure IP SLA on the site 1 router to track the availability of the MPLS link. You need this because you are not exchanging routes with your provider so you won't automatically know if the link has failed.

4) On site 2 router that connects to MPLS configure a floating static default-route pointing to the next-hop for the site 2 MPLS link. Note the AD of this route must be > 170.

5) On the same site 2 router redistribute static into EIGRP.

This works as follows -

1) The site 1 router connecting to MPLS redistributes the static into EIGRP. All routers in both sites see this route and they will all see it with an AD of 170.

2) The site 2 router also sees this route and because it has a lower AD than it's own static route then it uses this one. It will not insert it's own static route into the routing table and hence into EIGRP because it's own static route has a higher AD.

3) Under normal conditions all traffic follows the default-route out via site 1.

4) If the link or the remote provider router fails then site 1 router removes the default-route from it's routing table. It no longer gets redistributed into EIGRP and so site 2 router now no longer receives it.

5) site 2 router now installs it's default-route and redistributes into EIGRP and all routers receive this route. Note all routers except site 2 will see this route with an AD of 170.

6) If the link comes back up at site 1 IP SLA will reinstall the removed static route and then this will get propogated to site 2 router. Site 2 router will then see it's own default route with an AD > 170 and the new one with an AD of 170 and will use the new one. So it removes it's own default route from the routing table and stops redistibuting into EIGRP.

7) All intermediate routers will now use site 1 again.

Note - when site 1 comes back up and installs it's own route and redistributes every intermediate router will have 2 default-routes in their routing table temporarily until the site 2 MPLS router receives the site 1 redistributed route. Once it does and it removes it's own default-route that will no longer be redistributed into EIGRP and so all the intermediate routers will only have one default-route left which will be via site 1.

Citation - This blog post does not reflect original content from the author. Rather it summarizes content that are relevant to the topic from different sources in the web. The sources might include any online discussion boards, forums, websites and others.

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