Understanding Cisco Express forwarding

in this section we'll be focusing on Cisco Express forwarding method of switching the packets now what is exactly safe as the name itself it says Cisco Express forwarding means in Cisco proprietary method of forwarding the packets now normally if you take an example and there were any packet let's say there is a red book called 192 168 1 or network want to communicate with some other network that say what I to do 168 would often work now whenever this is my source address and this is my destination address so by default whenever a PC realizes that the destination is on a different subnet it is going to forward the packet to the Gateway now the routers job is to find the destination network ID so it is going to verify the routing table it says show IP route and it will ensure that there is a destination network ID present in the routing table and based on that if it is present then it will 4 see what is the next hop address and based on that next hop address again it is going to forward why a specific exit interface and then finally it is going to forward the packet this is our traditional layer 3 lookup which generally happens in our routers or if you are doing any routing process this is a default process now this safe is something an advanced to this process so safe is going to ensure that this process can happen much faster than a default process let us try to see what are the different types of methods first before we get into the safe concept now before safe we have some two initial methods which was generally used to forward your packets the first one is a process switching now in case of process switching whenever any packet enters the router so router is going to do some layer 3 lookup which is a more like a software-based it's going to check the distillation network ID and then it is going to check the next hop IP address and then it will see what is the exit interface for that next hop IP address and then finally it is going to form the packet now similarly for any any other packet comes again the same process happens one more time so which means every and whenever any packet comes even if it is for the same destination so it is still going to do the lookup that is something what happens in case of passive switching it requires a CPU the router CPU has to be personally involved in each and every forwarding decision so which means in each and every packet entering the router it is going to do the layer 3 lookup and form the packet now in this way it is going to lead add some more overhead on the router or a multi-layer switches you can say so whatever routing process now then came we have something called fast switching method now in case of fast switching method what it is going to do is it is going to do the same job what generally processing is going to do like whenever any packet comes it is going to see the destination network ID and then see the next hop IP errors and then what is exit interface and then forward the packet and after that it is going to cache this information in the router cache it is going to maintain that information in the cache so that if any other user want to go for the same destination probably it doesn't need to do the again that I'll take lookup it will simply cache this information and forward out of this specific interface so now in this the main advantage we get here is we we are little bit reducing the processing utilization here by not looking up each and every time whenever a packet comes it's not going to look up every time so if the entry is present in the cache it is going to use that cache information and in case if there is no entry in that cache probably it is going to use the normal lay through lookup and it is going to cache again so that next time for the same destination if anything comes again it is going to use that cash so now these two methods are no more use we can say now Cisco introduced something called a new method Cisco Express forwarding method now in this method what is going to happen here is before a packet comes into your network before a packet arrives so it is going to already the router is going to maintain 1 layer 3 routing table now this layer 3 routing table is taken in your hardware which means this layer 3 table is being downloaded to the hardware and the processing is done at the hardware level rather than doing at the software level which means the processing is not going to happen your routing table instead it is going to done on the hardware so generally we have something called data plane control plane I'll get into that a technical terminology now that is the main thing here so when it is doing on the hardware it can provide wide speed of information at the same time this information whatever is downloaded from the routing table it is done on the hardware providing the wide speed at the same time it is proactively doing these things which means before the packet arrives it is proactively downloading that information and then keeping that information your hardware so that the entire packet switches through your hardware and it's going to provide a wide speed performance so ACEF is something by default enabling most of the iris in today's networks it is going to optimize the router to make it able to forward the packets much faster than normal so let's try to get into some more in detail about this safe Cisco express forwarding in most of the layer 3 switches we have latest features as well as the routers freaking same if I specifically says features means it it also applies for the routers here now majorly we have two planes here we have something called control plane and your data plane so the controller I can say it's more like a software information which is going to build a routing table we call as our IP routing information base table and based on that routing table now safe is going to copy this routing table information is carried into your data plane so now in the data plane you are going to have two tables we have something called fib forwarding information base and then we have something called edge SNC table so I have that in my next slide here so this is your fib table now this your layer 3 engine which is your normal a control plane and this is your data plane here ok so now this layer companies responsible for building routing information just like a normal router which is going to do and based on that this fib table is going to have each and every destination network ID and it is built proactively before a packet arrives a router so which means it let's say if I say 192 168 to dot network is the destination network ID and to reach that destination we have a next stop of let's say 10.1.1.1 now this information is maintained in your fib table the layer 3 forwarding engine here and then it is also going to maintain one more table cut edges in the table in that it is in stable it is going to maintain the exit interface information so which means whenever any packet at us for this destination it's not going to do the lookup again so the lookup is pre build it's going to simply forward the packet out of that eggs interface without actually doing the routing lookup without the software processing going on here because it is something pre-built already it is going to forward the eggs interface and this is going to provide a wide speed performance and it's going to ensure that your packets travel a moves through your layer 3 device much faster than a normal routing generally not to verify I got I got my four routers which I generally use in all my CMP routing labs you can see I got four routers connected and all the four routers are already pre confident with your routing protocol if you verify YPO SPF neighbor I did that already just for this a basic thing which I did and to verify the safe it's by default enabled in most of the highways and if you want to verify whether it is enabled or not when you show IP safe command you can see this is your forwarding information base table where you have a specific network entry for specific networks let's say if I take an example of any one network let's say to 12.00 one it's going to send where next hop address and what is exit interface that is something pre-built and this is done based on your routing table again it is not going to decide any best routes all the best routes whatever is given by the routing table and based on that is going to maintain that in so that if any packet come for these specific destinations it's going to simply for with the packet out of s1 by one interface so it's going to make your switching routing process much faster than normal so if you want to disable generally not recommended we can use something called Y piece F this command is going to disable it and when it is able if I give SH Y piece if it simply says safe is not running on your router so if you if you whenever you see this information you just need to say IP self-command IP safe command again reenable son so it is generally not recommended to disable it but when whenever safe is enabled it's going to ensure that you're fast switching is something happens automatically now even in the switches latest features also we have a same kind of process there's no much difference now in your latest features here I take an example I have a source IP I want to send a packet to B now your packet is going between two different VLANs VLAN 10 to VLAN 20 now this F is very much useful especially in you and layer 2 network switch environment where you want to for the traffic between the two different VLANs probably they are on the same LAN segment but we need to ensure that they need to communicate a much faster than a normal routing there's a reason we have some multi-layer switches involved here and if in the send your information at a wide speed and that is possible because of safe that is going to forward the packet now the switch is going to maintain that layer 3 information the software information it this will be forwarded to your layer - that is your hardware with FRB table and also it is going to maintain some edges in the table and based on that it is going to see the destination network ID and then forwards the packet to that interface and then that's it that's how that's how the safe is going to work you

Channel: Sikandar Shaik

Views: 66,845

Rating: 4.6666665 out of 5

Keywords: CEF, cisco express forwarding, switching, free CCNA, free CCNP

Id: rwKtGVLZEfg

Length: 10min 57sec (657 seconds)

Published: Sat Apr 19 2014