BGP MED - BGP In Depth 10

hello this is Joe Neville and I'm back for another BGP in-depth video coming up we're going to be looking at vgp made what is it and how is it used then I've got a demo for you then that's it ok b DP made med stands for a multi exit discriminator and it's a BGP path attribute so this is the tribute that's included in BGP update so when bgp sends prefixes to neighbors it can include this information it was originally called into a s metric and as we go into the details that name should become more apparent to you the attribute has a value which is called the metric and sometimes med is also referred to as just metric ok the reason for using meg is to determine the best path if multiple links to neighboring autonomous systems exist and lower is best so a lower Meg value so a lower metric is considered the best route here's a simple illustration of how made easy use so I've got two autonomous systems here we've got a s 6500 2 and a s 6500 one down here as you can see this two paths between the AAS so this left path and this right path both of the devices in AAS 6500 2 are advertising the 10 9900 slash 24 network down into a s 6500 1 via evey GP here you can see an update with a made value of 200 and over here we've got a BGP update with a mint value of 150 all other things being equal the right path will be the best path because it has the lower med I'm going to think of med as the opposite of weight and local preference this is because weight and local preference our local attributes that are set say in this example they would be set on AAS 6500 one for deciding the best exit from the AAS over the left or the right path but med works in a different direction so it's an attribute to influence the incoming traffic from a remote a s so we were in this example we would set made in a s 6500 to to try to influence the way that the ingress traffic was entering the a s 6500 to coming from the neighboring a s so it's an attempt to influence the best path that's used by a neighboring a s rather than locally okay let's have a look at a few more details about made then so it is an optional non-transitive BGP path attribute now what does that mean optional in this case doesn't mean that it is optional whether it is included it actually means that it's optional that the BGP speaker understands the attribute so an implementation of BGP doesn't actually have to understand med that's opposed to the well-known attribute such as next hop and a s path that must be known by a BGP speaker and non-transitive so what does that actually mean well looking at the RFC for BGP 42 71 the quote is unrecognized non-transitive optional attributes must be quietly ignored and not passed along to other BGP peers so this is essentially saying that BGP speakers do not need to understand BGP made but if they don't understand it and they receive an update that includes made they must drop it the opposite of this being an optional transitive which means that if they do not understand the attribute they just pass it along non-transitive if you don't understand it then you receive this attribute drop it okay some further details from the RFC about met and how it is propagated so the quote is if received over ebgp the mega tribute may be propagated over ibgp remember this is between a SS so we're going to be sending it across ebgp and if it's received across an e BGP link and it can be sent over to an IB GP neighbor and the next point is the made attribute received from a neighboring a s must not be propagated to another neighboring a s okay so to illustrate that I've drawn some diagrams here now this is the first point so 6500 to sending made down to another a s and neighbouring ASAS 6500 one that's okay because that's coming across ebgp if that is then sent across an eye BGP session to appear that's okay still but if this update from 6500 to that med value was then sent on to another a a that's not okay so this behavior is forbidden BGP speakers must not do this that's not to say that the devices within a s 6500 one can't send med down to this other a s they can do that but it would be a new Meg value it so it's not the same med value that Dain gets forwarded on from 6500 - it's one that would originate at 6500 one let's have a look where this fits in the BGP route selection table this is the con where one again that I'm looking at and it's down here at point six so it's not as important as weight or local preference or the shortest a s path but it is more important than whether a route is learned from ebgp or ibgp which is important to remember for the demo that's coming up and talking of demo let's get right into it here's my setup for consistency I'm going to use come where V s ours again I've got four vs ours much like my previous video about weight and local preference we've got the two a s 65 zero zero two and a s 6500 one these vs is 201 and 202 are going to be advertising this prefix 1099 0 0 is flush 24 down to a s 65 0 0 1 and what we're going to do is we're going to use made to manipulate the exit from a s 6500 1 in to 6500 to he'll have SSH sessions in 2/3 of the vs Rs 1 0 1 2 0 1 and 2 0 2 let's have a look at the VTP configuration on vsr-10 1 so we've just got a couple of bgp sessions once a DB TV session and one is an eye bgp session which you can see over in this diagram here EVP and ibgp so let's have a look at the BGP table and we're getting that $10.99 plus 24 prefix from ebgp and ib DP and notice here this is what we're interested in so this is the med value here and the default is 0 so it doesn't come into play now let's alter that so what we're going to do is we are going to add a mid value to this update that's the best pass at the moment so you can see the arrow there against this prefix coming from 2.1 and that is v sr to 0 1 so we go on to that device now and configure a mid value the mid can be configured using a route map for a specific prefixes or you can just put in a default value and that's the one I'm going to do here and that is with the default mid command let's give it 50 check back down on for your cell 1 0 1 so previously the best pun off was via 172 16 2.1 when both panels had 0 as their mid value the default value now if we look the 170 16 2.1 has got the made value of 50 and we can see that the best path is now a different path it's the one going 5 4.1 so it's this one across here remember when we were looking at the route selection and mid is more important than whether it's ebgp or ibgp so in this case it doesn't matter that we have to go across an eye bgp session to get to here so this is actually in the diagram it looks longer the higher mid value that's coming from 1 7 to 16 2.1 means that this is a less preferred path now so we go across this way to 1099 plus 24 so let's manipulate that again by adding a higher made on VSR to 0-2 again using default meds to make it 100 go back to vsr-10 one there we go the next top has changed we've come back across there the lower med 172 16 2.1 so I hope there you can see the ways of making the changes on this 6500 to is you're actually encouraging or manipulating the best path for ingress traffic rather than with weight and local preference where it's egress traffic there you're manipulating okay but there's an important additional point which I think that I should highlight which is that by default made from different a s is not compared so this is something that's quite easy to forget and I know that I've made this mistake in the past where I've wondered why made coming from two different a SS is not affecting the best path option it's because by default a BGP speaker will only compare meds if they come from the same AAS but this behavior is configurable so whatever network devices you're using whatever OS there will most probably be a command that allows you to override this and make a BGP speaker compare made from different a SS we've Cisco it's the BGP always compare med command I believe we've come where we've got a similar command so to illustrate this I've got another demo what I've done is the AF 6500 - I split that up so we've got vs r2 0 1 in 1a s and we've got B sr20 - in another is new a s 6500 3 same thing again though where this prefix we're going to be injecting 1099 / 24 and we'll have a look down on the vsr-10 1 about how it reacts to MIT ok going back to our devices then VSR 2 0 2 is now in a s 6500 3 i'll just show you that here's the configuration and I've put a default made on of 100 already so if we look down on the BGP routing table obvious are 1 0 1 here we are what we've got 10 99 / 24 it's being learnt from dot 2 so 2 0 1 and across the ibgp session from PS l 1 0 2 and at the moment we've got a mid of 50 coming from 2.1 and a mid of 100 coming from 4.1 so this is 4.1 with a mid 100 there so remember that point that by default and this is how it's set up by default made from two different AACC's not compared so what I do here is up the mid on to 0 1 to prove that so let's make that higher than the myth it's coming from 2 0 - I'll give it 1 0 1 as you can see there - 0 1 is still the preferred mixed hop 2.1 still preferred even though the maybes higher that's because we're not comparing we're coming from two different AAS now let's override that default behavior and we've come where it's the compared different a s made command I enter that look at the routing table again and you can see that Meg is now being compared so the one with the lowest med here across the 4.1 linked to 2-0 - that's the preferred path no the mid value coming from two different AES is being compared okay so that was a closer look at the BGP path for tribute made I hope you found that useful please like if you did or dislike if you didn't tell me your thoughts in the comment section and please do subscribe to the channel feel free to check me out on Twitter that's all for now thanks for watching my name's Joe Neville and goodbye

Channel: Airheads Broadcasting

Views: 15,937

Keywords: Aruba, Hewlett Packard Enterprise, Aruba Networks, HPE, Networks, Networking, HP, Switching, Routing, How to, Software, Configuration, HPN, Comware, Router, BGP, Technology, IT, BGP configuration, BGP MED, MED, BGP path attributes, BGP tutorial

Id: XgM4Fm-lNcw

Length: 15min 45sec (945 seconds)

Published: Wed Mar 01 2017