First Hop Redundancy Protocol Explained | Cisco CCNA 200-301

hey what's up guys welcome to surprise in this video we're going to be looking at first hop redundancy protocol or fhrp for short so we've already talked about the importance of redundancy in modern day networks we've looked at using multiple switches and how spanning tree protocol removes any switching loops we've even looked at using multiple links and grouping them into an ether channel but what about layer 3 communication what redundancy do we have there it's all well and good keeping local communication running but imagine if the internet connection goes down all hell would break loose by only having one router we have just introduced a single point of failure if this router or the link goes down then goodbye internet so to combat this we can introduce a second redundant router each router would then connect to the internet or even the corporate wan we add the second router to a different switch to remove a single point of failure meaning if one switch fails then the other still has layer 3 access there is a problem with this setup though let's clear this up and add some more information can you spot the problem here what does a host do if it's trying to send data to a device that's not in its own network it sends it to the default gateway this is the problem each host lists router 2 on the right as its default gateway so what happens if router 2 goes down well luckily we still have router 1. but the host still has router 2 listed as the default gateway this is the problem when a router fails there is no way to automatically change the default gateway for all hosts they will continue to try to send data to the router that failed this is where first hop redundancy protocols come in fhrps allow hosts to send data to the redundant router without needing to make any changes and the way it works is very very clever here is a very brief overview before we go into some of the finer details you configure your routers to be part of a group the group is then assigned a virtual ip address and a virtual mac address this is how it works let's say host b sends an app request for the default gateway this app request will be for the virtual ip that request is sent to all devices in the network both routers receive this request but only one responds but it doesn't respond with its own mac address no it responds with the virtual mac address now let's say our router goes down this is where the magic happens when the same request is sent our second router receives it and knowing the other router is down it responds to the request again it sends the virtual mac address the host is none the wiser about which router it's talking to it only knows the virtual ip and the virtual mac address but what happens if no requests are sent and the hosts just use their up cache well when the new router takes over it will start to send a special type of arp message this message is called a gratuitous arp this tells the switches to update their mac address tables this way all devices will now use the new router so that is a very general look at how fhrps work and to be honest that could be enough for the ccna as it only states you should be able to describe the purpose but let's look a little bit deeper so you can get a solid understanding the first thing to know is that first hop redundancy protocol isn't one protocol it's actually a term used to describe a family of protocols here we have hsrp or hot standby router protocol vrrp or virtual router redundancy protocol glbp or gateway load balancing protocol we're going to look at each of these in a bit more depth let's first look at hot standby router protocol or hsrp hsrp is a protocol designed by cisco and it works in very much the same way as we've just seen let's bring back our network now remove some switches to make it easier to see the first thing that's going to happen is the routers will start to send messages routers with hsrp will send and receive multicast hello messages every three seconds this is used to communicate with each other their multicast address depends on the version for version one the address is 224.0.0 and for version 2 the address is 224.0.0.102 they then decide which router will be the active one and which will become the standby the active router takes ownership of the virtual ip and mac address the standby router simply waits until the active fails the way an active router is chosen is based on the highest hsrp priority and if that ties then the router with the highest i p address when you configure hsrp you will select a virtual ip address to be used this is the ip address you'll configure as the default gateway the virtual mac address however is generated automatically and it follows a specific format the first part of the mac address always stays the same this is how you can identify a hsrp virtual mac the last two or three digits depending on the version shows the group id when you configure hsrp you configure a group of routers this part of the mac address represents that group i recommend you try to remember this format for the ccna exam and the last thing we need to do is assign the virtual ip address to the host's default gateway now that everything is set up normal operation can resume the routers will send hello messages every three seconds this lets them know that everything is a okay but let's say our active router goes down our standby router won't receive any hello messages from the active router the standby router then waits the whole time which is 10 seconds by default before leaping into action once the whole timer has expired the standby router will assume the active role and then announce himself to the rest of the network this is so the switches can update their mac address tables and list the correct port number by default if our router comes back up it won't automatically take over as the active router it will instead become the standby if you have a favorite router that you want to always be active then you can enable something called standby preempt okay so that is hsrp now let's move on to vrrp after cisco released hsrp and rfc was created for a similar protocol virtual router redundancy protocol vrrp works in a very similar way to hsrp there are only a few slight differences the first differences is that the router roles are called master and backup not active and standby how the routers choose a master is also different first we need to bring in the ip addresses in vrrp you can assign one of the routers the same ip as the virtual ip this makes that router something called an ip address owner when choosing which router should be the master the ip address owner is always preferred if that router goes down and there are other routers to choose from then it goes to the same tiebreakers as before the highest priority then the highest assigned ip address if we now take a look at the virtual mac address you will see a different format just like before the first part of the mac address represents a vrrp virtual mac and the last two digits represents the group id again i recommend you try to remember this format for the exam like hsrp vrrp sends messages to the network unlike hsrp the messages are only sent from the master master routers will send advertisements to a multicast address of 224.0.0.18 these advertisements are sent every one second by default if the master router fails there is a master down timer which is three times the advertisement timer plus a little bit more so just over three seconds by default after the master down timer the master router can be assumed dead and the backup router would take on the role of master just like before the new master will announce himself to the network one last note on vrrp if the master router comes back to life it will take over as the master once again okay one last protocol i want to talk about after vrrp was released cisco created a new protocol this is called gateway load balancing protocol because this protocol was designed by cisco it has quite a few similarities to hsrp but with some notable differences just like hsrp each router will send a hello message to communicate with each other the messages are sent every three seconds by default and are sent to a multicast address of 224.0.0.102 the routers will then choose which one will become the active router how they choose an active router is the same as hsrp the router with the highest priority and if that ties the router with the highest assigned ip address just like before glbp uses a virtual ip address however it does something different with the virtual mac addresses instead of having one virtual mac address shared between the routers glbp assigns a virtual mac address to each router and there is a very good reason for that first though let's look at the virtual mac format the first part represents the glbp virtual mac address the yellow x's show the group number and the red wise represents something called the avf id avf or active virtual folders are routers that have been assigned virtual mac addresses just think of avf as the router id so why does it assign different virtual mac addresses to each router well this is where the magic happens even though each host has the same default gateway ip address assigned the routers can respond with different mac addresses this means both routers can be used to load balance the traffic rather than having only one active router and one standby router for example our host on the left has a default gateway of 192.1 it also has the mac address of the active router so all traffic will go through that active router the host on the right also has the default gateway of 192.168 but this time it has the mac address of the standby router so all traffic will go through the standby router by using different mac addresses we get the benefit of using both routers okay let's move on as we saw earlier multicast hello messages are sent every three seconds by default if our active router fails then there is a hold timer which is 10 seconds by default once the active router is assumed dead the standby router will take over and also take ownership of the virtual mac addresses when the old router comes back up it will assume the role of standby but it will take back responsibility for its own virtual mac addresses keep in mind this is a high level overview of these protocols the ccna does not require you to be an expert in any of these but hopefully this has given you a good understanding to summarize the key points take a look at this table the first column shows us if the protocol is cisco proprietary or an open standard the second column shows us the terminology used to describe the router roles next it shows the multicast address used by each protocol then it shows the different mac address formats and finally the number of virtual mac addresses that can be used i recommend learning this table for the ccna if you get any questions on this this should hopefully cover it this video is part of the full ccna course which can be found in the description so please feel free to go and check that out that's it for first top redundancy protocols if you like this video don't forget to give it a thumbs up leave a comment and subscribe the support from you guys really helps this channel grow other than that thank you for watching you

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Length: 13min 54sec (834 seconds)

Published: Tue Apr 27 2021