BGP Configuration on Cisco IOS.

you hello and welcome to this quick BGP tutorial on setting up BGP as well as troubleshooting what are probably two of the most common problems when setting it up so for a little experiment we're going to set up an external bgp neighbor ship between r6 and r8 the red line will represent the external neighbor ship and we'll also do an internal BGP neighbor ship between r5 and r6 the network segment here between our 5 and r6 is 10 dot 5600 / 24 and the network segment between r6 and r8 is the red line which is 192 168 668 0 so up here as we peer between our 6 and our 8 because we don't have knowledge of their internal network we're probably going to using these IP addresses to pair with an external neighbor with the internal neighbor ship we're probably going to use we could use loop backs we could use 6 6 6 6 which is the loopback interface of our 6 + 5 5 5 5 as long as we had reach ability to those addresses so that's what we're going to do as our 8 is already configured we'll set up our 6 with an external and internal neighbor ship and then our 5 with the internal neighbor ship up to our 6 so let's get to it on the routers themselves on our 6 let's just verify and I move my cursor up here let's just verify that our 6 doesn't have BGP already running do a show IP protocols easy for me to type and BGP is not running so let's set it up so we'll go to configuration mode router bgp 3 by the way that'll be the autonomous systems we'll use so we'll use a ton of system 3 over here and a s89 over here those would be our ton of the system numbers so back to the interface so router bgp 3 and we'll simply say neighbor and this can be one ninety two dot one sixty eight dot sixty eight dot eight that's the IP address of our eight of his serial interface and will say that device is in the remote a s of eighty nine because it is at the same time we can set up our neighbor for five dot v dot v dot v remote as3 there we go we've got our two neighbor statements in tow and the difference is that because we are in a s three anybody who's not in Ras like this guy right here would be considered an external bgp neighbor anybody in the same a s would be considered an internal BGP neighbor now we've configured that let's do a do show I get my mouse in the right place let's do a do show IP BGP summary and see what we have so it shows us that for the neighbor r8 that he is set up that we have prefixes on the far right hand side that we've received which is great news and we have a little bit of a problem with r5 now the problem with r5 is we've done the three-way handshake for the TCP but because our five isn't configured at all he's not responding to us with BGP so we're active meaning we're actively trying to get a BGP connection going with that device let's go configure BGP on our five so on our five we'll go ahead and do a router bgp three same autonomous system is our six neighbor six six six six and we'll say he's a remote as3 I'll put remote - a s because that's the syntax three now one of the things we'd want to verify is do we have layer three connectivity to this guy so we need to do ping of six six six six source it from loopback zero just to verify that we've got connectivity so that looks good and if we do a do a show IP BGP summary you'll notice that our five thinks he's in an active state meaning actively trying to set up bgp with our five so let's take a look at the first common problem and that's this when we use our loopback interfaces which we already verified are reachable between our five and our six by default they're sourcing those bgp sessions from those like those interfaces right here which is that 1056 network interfaces our five is looking for a peer who has the source IP address of 666 not ten fifty six zero six so what we need to do on one or both of these devices is tell them that they're going to source their BGP logical sessions from their loop backs which will match what the peer has in their configuration and as a result the BGP neighbor ship should come up so that's probably one of the biggest faults that people fail to do is in order to put in the update source for BGP so let's fix that on both sides so on router 5 go into router bgp 3 and we'll go to our neighbor statement I'm just going to say update source loop x0 just like that I'll do the same thing on our 6 so there's my neighbor I'm gonna say update source loop x0 and now I'm saying the updates from the appropriate interface and there the neighbor ship is coming up in reality we only had to do that on one based on the way BGP works but just for general good practice I did on both sides so if we do a show IP BGP summary now you'll notice that we have a 0 here which is actually a good sign this only means that our 5 doesn't have anything to share with us but it does indicate that we have an established state for BGP which is great news so let's take a look at the BGP table now show IP BGP it says that we've learned three routes from our good buddy at 68 if we do a show IP BGP we can see what those are and there's the three routes that we've learned from that neighbor super this little symbol here represents their the best route so greater than symbol says best route in case they get multiple routes from multiple BGP speakers and if I do a show IP route focus just on the BGP routes in the routing table it's going to show me that those BGP routes from the BGP table are also in the routing table great news let's go to make a road trip down to our five and see if we fared so well down there we should be getting those same routes from our six advertised our five let's go see if they're there so down on our five the show IP let's get out of this ish show IP PGP and there's the routes but notice we're missing the little greater than symbol the greater than symbol that indicates is the best route if it's not the best route there's no possible way it's going to get into the routing table from BGP anyway so if we do a show IP route for BGP there's no in the routing people no BGP source routes bummer why is that take a look at this next top this next hop is the next hop that r8 assigned to this those networks as he sent them so take a look at the diagram with me when r8 sends these updates to our six this way right here he's putting the next hop of his own IP address which is right there as a result r6 because it received it via external BGP passes that same next hop to our five who simply doesn't have a route to it and the first one of the rules not the first rule but one of the rules is if you don't have a route to the next hop don't put that route in the routing table because you can't get to it it's like your but your egos writing checks your body can't cash so to fix that what we could do is we could have our six as it sends those updates down to our five right here we can simply say our six please change the next hop to make it you so he'll take the next top of 192 168 68 8 and right here he'll change it to be 6 6 6 6 which is reachable by our five life is good so let's make that quick change actually does that be a quick change take your time but I want to make this video under 10 minutes so on our six going back into configuration mode for router bgp and we'll simply say neighbor 5 5 5 and they say next hop self I'm also going to do a clear IP BGP 4 5 5 5 5 and I'll do it outbound I'll do the soft outbound and that way I don't have to clear the entire BGP session but I'm going to send the new updates which will have the new next hop as a result if we go back to our 5 and do a show same command we did before show IP BGP take a look the next hop has changed the next stop is now six 666 because that's reachable that would now enable this to be considered a best route and if we do a show IP route those routes also migrated their way to the top of the stand which is the actual IP routing table so those are the two most common mistakes that I have noticed that when people are sitting at BGP they fail to correct one is they don'ts to the next hop from our sixth our five and when we have loop backs we forget to do the update source thanks everybody for watching and have a great great day you

Channel: Keith Barker

Views: 205,733

Rating: 4.8961449 out of 5

Keywords: cisco, ccnp, bgp, update-source, next-hop-self

Id: zqTFyuiq9bg

Length: 9min 54sec (594 seconds)

Published: Tue Aug 24 2010