CCIE Topic: 1.4e OSPF Loop-Free Alternate Fast Reroute (LFA FRR)

hey everyone charles judd here back with another ccie topic this time we're finishing out the ospf blueprint section wrapping up with the loop free alternate feature let's talk about why and how this is used and then we'll jump into a command line and take a look at some configuration and verification in ospf there's no concept of a feasible successor as we have in eigrp the loop-free alternate feature in ospf is a way that we can have rerouting around a failed link and that creates a loop-free path to the same destination you may also see this referred to as loop-free alternate fast reroute or lfa fr in various cisco documentation sources just as with the fast reroute mechanism we looked at in eigrp the lfa feature allows ospf to quickly switch over to a backup route during a failure and that convergence can happen within 50 milliseconds to understand how this works think about using ospf without any rerouting mechanism if we have a failure in the primary network path to a particular destination then ospf would have to rerun the shortest path first algorithm to find a new path to the same destination lfa allows ospf to determine a backup next hop for every primary next hop and install this information into the cef table this works because since ospf is a link state protocol all routers within the same area will share an identical link state database if we take a look at this topology you can see that there are three paths to get from the source router of r1 out to the 5.5.5.5 loopback address that's configured on r5 we can go through r2 r3 or r4 we have different cost metrics for each router as well that you can see noted here so obviously the preferred path or the shortest path is going to be through r2 which you can see has a cost of 21 and in fact if we say show ip route 5.5.5.5 you'll see that this does in fact go through the 12.12.12.2 link which is r2 and the route metric we can see here is listed as 21. if we say show ipospf route we can see that our path to the 5.5.5.5 network goes through again 12.12.12.2 which is router 2 over the local gigabit ethernet 1 and again we see the cost is 21. if we again take a look at our show iprout 5.5.5.5 command again you see that currently our routing table has 12.12.12.2 as the next hop as we would expect and we don't have a backup route available of course with ospf a failure over this link would require shortest path first calculations before a backup route would be installed so we can also say show ipsef 5.5.5.5 and that also shows us our next hop information again with no backup route listed we see the next hop is router 2 over our local gigabit ethernet 1. so let's look at now enabling loop free alternate fast reroute we can do that under our ospf process id so under global configuration mode say router ospf 1 and the command we want is fast hyphen reroute i'll also point out that i am using a csr 1000v series router here so depending on your router model you might not have the fast reroute commands available this is commonly used in service provider environments so you're much more likely to see this feature on the more robust router models if we look at contextual help you can see that we have the options for keep all paths and per prefix so for now let's choose per prefix and we follow that with the enable keyword if we look at contextual help you can see we're able to configure the area that we want to have fast reroute on and we can set a priority so let's start with saying area and of course everything in this particular network is in area 0. if we look at contextual help again we're left with the prefix priority so let's put that in and if we look at our options underneath that we have both high and low the difference is that with low priority all loop free alternate routes are calculated at the same time if we choose high priority this means that any loopback interfaces or any prefixes with a slash 32 mask those will be calculated a bit earlier than the other prefixes so in my case i'm just going to choose the low option and hit enter to accept that and that's it it's actually really really simple to configure we see a message from bfd bi-directional forwarding detection where we have a new session created and that's used to detect a forwarding path failure so now if we break out and we say again show iprout 5.5.5.5 notice we still have our first hop address of 12.12.12.2 listed here but additionally now we have a repair path listed of 13.13.13.2 which is of course router 3 connected over the local gig 2 interface same thing if we say show ipsef 5.5.5.5 we're going to see our next hop of router 2 and we also have a repair attached next hop of router 3. now if we jump over to r2 let's go here let's go under interface gig zero slash zero which is the interface that's connecting back to r1 and let's just say shut that's going to trigger our lfa fr mechanism if we go back to r1 and let's say show iprout 5.5.5.5 and now you can see that our first hop is router 3 at 13.13.13.2 and we have an additional repair path listed of 14.14.14. which is of course router 4. above that you'll see that our bfd session was destroyed once we shut down that interface and we created a new bfd session and you can see that is with the neighbor router 3 so we had a new session established to track this in case we have a failure on router 3 then we can install another backup route and as we were looking at those you might have noticed that we did have an ospf adjacency message coming to our console letting us know our dead timer had expired so hopefully you can see the advantage here rather than waiting for that dead timer to expire we had almost immediate failover with the loop free alternate fast reroute mechanism within ospf if we go back to r2 and let's say no shut just to bring that interface back up once that comes back up we're going to see an ospf adjacency re-establish with router 1. if we go back to router 1 we see some new console messages letting us know that one of our bfd sessions was destroyed with router 3 and we have a new session created with the neighbor router 2 so if we say show iprout 5.5.5.5 we're back to using our original route over router 2 and we have our original repair path as well going over router 3. let's go back on router one under router ospf one and let's take a look at the additional command that we mentioned which is fast hyphen reroute and the one that we skipped over was keep all hyphen paths you might be able to guess what this does this command will actually keep track of all of our possible backup paths rather than just the primary and a single backup path so let's hit enter let's break out let's say show iprout 5.5.5.5 you'll notice that looks exactly the same as before same thing if we say show ipsef 5.5.5.5 we have our next hop and we have a repair route but if we say show ip ospf rib 5.5.5.5 we want to see a different story here notice that we see our initial path of 12.12.12.2 that has a flag of rib meaning that this is the route that we're primarily using at the moment we see underneath that we have a repair path of 13.13.13.2 having the flags rib and repair letting us know that this is an active repair route but here we also see a route going over 14.14.14.2 which is of course router 4. that has the flags of ignore and repair and that means this was a possible repair route but it's been ignored for the time being because we have a better option in the network now we would not see this additionally ignored route to router 4 without the keep all paths option that we just used if you're using something such as a cisco asr 1000 running cisco ios xe you also have the option to configure certain repair path tiebreaker attributes which aren't available on my specific model of router and those work similar to the way bgp uses attributes so that's a look at the ospf loop free alternate fast reroute feature also called lfa fr i hope you found this content useful and i want to thank you sincerely for watching

Channel: Charles Judd

Views: 2,526

Keywords: cisco, ccie, cisco enarsi, ccie enterprise infrastructure, cisco enarsi 300-410, cisco encor 350-401, ccie lab, my ccie journey, ccie training, ccie blueprint, section 1.4, 1.4e ospf optimization convergence and scalability, shortest path first, 1.4e ospf loop-free alternate fast reroute, LFA FRR

Id: PUapMB0kiNY

Length: 9min 53sec (593 seconds)

Published: Mon Nov 30 2020