350-501 SPCOR Cisco CCNP Service Provider – Weight & Local Preference part 5

8. Clear IP BGP Sessions

In this video I’ll explain you how to clear the BGP sessions whenever you make any changes to the BGP routing policy. Now, one thing, if you remember, we have seen in our previous sessions, like whenever you make any changes, let’s take an example. You got a router which is connecting to multiple routers, multiple exit paths, and whenever we apply any local preference attribute to the existing routing policy, or whenever I apply any weight attribute, or whenever I apply any other routing policies, like maybe I applied some filters as path filters, or maybe I have used some filled route filtering methods in the PGP. This is all comes under different kinds of routing policies which we can apply. So whenever you make any changes to the routing policy, the PGP session must be reset.

Now, this is something very important because PGP session must be reset before any new policy is going to take effect. So initially there was only one way to do that, that is by simply using a command called Clear IPGP. So whenever I’m going to give this command, we call this as hard reset, where it is going to completely disconnect the neighbors and reconnect and reconverge each and everything. So let us see how to do that. So in this section we’ll be getting into in detailed understanding of the different kinds of resets which we can use in order to ensure that the next second, immediately, whenever we make any changes in the policy, it has to immediately apply to all the prefixes and the past. There are two different methods to do that.

We can either use hard reset and we can use soft reset command. So if I use a simple command, clear IP, BGP Star or Asterisk which means we are going to use a hard reset, or we can also use a soft command added to that particular specific Clear IPGP command. Let us see what is the difference between these two. Now, when we talk about BGP, it’s going to maintain a huge volume of routing information because it’s going to maintain some thousands of routes on the internet and the ISP. So the first thing we’ll see, what is the difference between hard reset and soft reset? Now, initially the only way to apply any new routing policy is to use a command called Clear Ipbgp. So either I can use Clear IP BGP Asterisk which is going to apply for all the neighbors, or you can apply only for specific neighbor.

Let’s say my router is connecting to some file neighbors and if you want the routing policy changes to be applied for all the file neighbors, then simply I can simply use this command. If I want the changes to be applied only for one specific neighbor, we can simply specify the IP address of that neighbor. Now, we call this as hard reset and the reason for that is it is going to reset all the BGP connections with the router and it’s going to disconnect all the neighbors. Let me show you here. Whenever I try to use Clear IPGP, so I already have a router, one which is configured with neighbor ships. I already have two neighbors here. If I go and check with Show? IP BGP Summary Command Now, if I use a command called Clear IP BGP Star Asterisk without using any of the Soft commands okay, I’m not using Soft.

I’m simply using Clear IP BGP Star Asterisk. So what it is going to do, it is going to see, you can see the neighbor is down because the neighbor has, it has listed everything. And you see the routes will take converge from the beginning again, which means whenever you apply this command, it’s going to reset all your neighbors, it’s going to take down the session, which means the session will go down automatically and then it will be reestablished for each and every pin and all the routes are relearn. And the major drawback with this one is it’s going to use more and more processing. The more bigger the BGP table, the more time it is going to take for processing all the routes and it’s going to take a longer time. And also it’s going to use a lot of resources on the router and also the bandwidth for sending and receiving the updates.

Now, this is something not really recommended. And if you want to do for only a specific neighbor the same thing, we can use a command called Clear IPGP and we can specifically do it only for one neighbor. It’s going to reset only for one specific neighbor. And the BGP session is going to transition from established to Idle, which means it’s already established. Again, it’s going back to idle and then it’s going to reestablish again with a new update policies. So this is something very important because by default, BGP will not update the new policy changes. Let’s say I’ve applied a new route map, which is going to change the local preference. And if I want this changes to effect immediately, I need to update the policy. If you want to update the policy, then it’s mandatory for you to reset. Now, in today’s production networks, we generally don’t want this kind of reset.

So instead what I can do is I can go with some soft reset configurations. Now, in case of soft reset, whenever I’m going to use the same command Clear Ipbgp when I use Asterisk, it means for all the neighbors. Or you can either do for a specific neighbor. So I can simply say Clear Ipbgp Asterisk and I’m going to use a keyword called Soft at the end. So whenever I’m giving this keyword Soft, it means we are going to do some soft reset or soft reconfiguration. Now, what it is going to do, what is the difference between hard reset and soft reset? Now, soft reset is going to provide the changes to the new policy. Let’s say I applied a new policy which is going to change the local preference value, or which is going to change the weight, or which is going to change some routing policies or route filtering any changes you apply.

It’s going to apply the new changes without actually disconnecting the sessions, which means the neighborship will not be disconnected. It’s going to ensure that the neighborship is not down. It’s just going to resend or receive the new policy changes. Let me show you how to implement it. It’s a very simple and straightforward command. Now, you can see router one is converging all the routes. Now I’m going to say Clear IPGP Star and then I’m going to use a keyboard called Soft. So when I’m going to use Soft, it’s going to let me just quickly change it because what I’m going to do is I got my topology, something like this, it’s pre conference like this, and by default it’s using some default path. I’m going to apply the weight on this side some 4000.

That’s it. Let’s take an example for easy understanding. So I want to make sure that the router one should apply the weight on towards router four. So what I need to do, I need to simply go to router BGP 500 x and then I’m going to say neighbor four one, which is then applying the weight of 4000. So when I apply the weight of 4000, if I use Show IP BGP, I don’t see the weight change changes. The new policy has been applied, but I don’t see any changes in my BGP. So if you want to ensure that the weight should get applied, it becomes mandatory for you to clear the PGP sessions. The one option I’m using, Clear IPGP Star command without Soft, it will be a hard reset. I’m not going to do that. I’m simply giving Soft command and then I’m going to press Enter.

So when I give this, you can see the neighborship is not done because you don’t see any message like the neighborship has been resettled. But still, if I verify Show IP BGP, I can see the weight values are applied. You can see the weight values applied here for all the routes which are receiving from four one. If you want to do the same thing with specific route map, it’s the same way. So I’m just taking an example here. Now, this is what we call as Soft reset or Soft reconfiguration. So now which is more better? So it’s always recommended to go with soft reset because if I try to do hard reset, it’s going to disconnect all the neighbors and especially it’s going to add some more overhead on the routers. But again, in that software set, we got two kinds of software.

We may come across a scenario where we may want to reset only one specific neighbor. Let’s say I got three neighbors and I want to reset only one specific neighbor, then we can simply use this command clear IP BGP so you might be applying policy on this neighbor and I want to reset only that specific neighbor. I can simply say clear IP BGP neighbor neighbor means I need to define some IP address on that and then I can simply say soft. Okay, so this is the way it’s always recommended to be more specific when you are clearing the BHP sessions so that you don’t add extra hole on the other interfaces or other from other neighbors. A similar way we can either go with Asterisk for all the neighbors or a specific neighbor with Soft command. And at the end we can use two more options. We have soft inbound, Reset outbound reset.

Now, the difference between inbound and outsource reset is whenever you apply outbound soft reconfiguration, which means I’m going to use the same commands clear IPGP, neighbor address or Asterisk and I’m going to say soft out, I’m going to apply the policy outbound. So it’s more simple. It’s going to, whenever you apply outbound, let us say out, it’s going to resend all the BGP information on all the BGP routes to the neighbor without resetting the connection. That’s the meaning of soft. It’s going to resend everything by applying the new policies. By applying the new policies. So let’s say I have applied a local preference value on the outbound direction and then I want to ensure that these changes should affect outbound or maybe generally we use as path prepared or any other attributes which you apply outbound.

So if you want the changes to get affected, we can simply use out option so which is going to apply only for outbound and it’s going to reset all the BGP information again by applying the new attributes without resetting the connection. So it’s always enabled, we don’t need to confirm anything. Now, if I use inbound, if I use the same thing, clear IP BGP, neighbor or Neighbor IP address or Asterisk soft in when I apply inbound, it is used for generating the inbound updates from a specific neighbor. So it’s going to ensure that it’s going to update the neighbor or request neighbor about the changes in the policy. So it’s going to resend, it’s going to receive from the neighbor with the changes whatever we applied.

So now if you get back to previous versions like Dynamic, we call this as Dynamic inbound Reset. Now it is an enhanced or added feature starting from iOS twelve one. Okay? So if you go back to previous versions of inbound inbound reset, it’s one of the most complicated method because by default clear IP BGP asterisk soft in or the Soft inbound Reset was not enabled by default. If you go back, get back to you earlier version of iOS, if you want to enable this one, you need to compulsory add a command called Neighbor IP address soft Rift confirmation Inbound. Now this is something with earlier versions. So I don’t think so. You really need this command as of now. But it’s good to know. The major drawback with this one is it was earlier version.

What it is going to do is it is going to maintain so whenever you configure this command, once we configure this command for a specific neighbor, the routes are stored whatever the routes received from the neighbor are stored in a separate copy of memory.It’s going to maintain an extra copy of memory and in that extra copy of memory, it’s going to maintain all the routes and this copy is taken before you apply any changes. So whenever you apply any changes inbound, it’s going to apply that changes. This copy is taken before applying the changes. And this process is not enabled by default. If you want to enable this process, you need to add this command. And one more drawback. The major drawback is it’s going to use amount of memory requires so it requires more memories to store a separate copy of the information.

Now, this is something no more used, we can say but now the new enhanced feature in your BGP inbound policy, if I just use Clear IP BGP neighbor soft in, the major difference between these two is it’s not going to maintain a separate copy here. So there’s less memory utilization here, which is an enhanced feature in the inbound which was not there in the older version. So you might come across this kind of thing. So, Republican networks, we just go with soft N. Let me just quickly wrap up this series of Clear Ipbgp sessions. So initially we discussed the hard reset where we use simply a command called Clear Ipbgp. Either we can use Asterisk or a specific nailer. Now, the advantage of this is we don’t need any extra memory overhead on the router because it’s not going to but again, the disadvantage with this hard reset is it’s going to reset all your neighborhood.

It’s not recommended by because it’s going to invalid all the cache and it’s not going to forward the traffic unless and until it completely converts the BGP and all the neighborhood is up, it’s going to tear down your BGP session. Now, apart from that, we can also use soft reset where we can add a keyword called soft in that we have some soft in or soft out. We can say outbound soft reset, northbound soft reset. We don’t need any special configurations. It’s not going to store any copy of the information. But the desert, it’s not going to reset the inbound routing updates. So it’s going to apply only for outbound. But whereas we can use inbound as separate command, clear IP, PGP, Star and soft command in. So when I apply inbound, which is the most commonly used now inbound inbound reset, which is both the routers must support this feature.

Generally, in order to support this feature, it’s compulsory that both the routers must support this feature which is by default enabled from twelve one releases. Now it does not clear the BGP session. That is a good thing. And also it’s not going to store any of the routing information updates and in case if this feature is not enabled or if this feature is not supported and in case if your routers do not support this dynamic, inbound reset feature. If you’re using earlier version of iOS then probably to support inbound software configuration we need to add this command which is no more used but again, it’s generally used whenever you don’t have this feature. The route refresh capability which is not supported in these versions if you are using earlier version of this but again it requires some pre configuration because you need to add this command here at the same time it’s going to use some extra memory to store a copy of information.

Okay, so if I’m using, let’s say if I’m using Clear Ipbgp, it’s simple, just one command, clear Ipbgp. If I use a stick without a soft command it will be hard reset and if I’m using soft command it will be soft reset and if I don’t use any keyword with in or out it’s going to apply for inbound and outbound automatically. So if these two features are supported automatically then if I don’t use any keyword it is going to apply for inbound as well as or more and if you want to be more specific for inbound then we can specify in and if you want to be specific only for outbound, we can specify out. Okay, so these are the three different implementations we can generally use and it’s always recommended to use soft reset whether outboard or inbound more specifically or whether you can apply for all the neighbors or you can apply you can specify the neighbor IP address.