![]() Underlying Code MUST be Doing Something Additional When we Set TTL of 2 If I literally needto change the TTL from the eBGP default of 1 to a TTL of 2 for the two IP addresses to even reach each other, then they must not be directly connected. Question: If I needa TTL of 2 for successful eBGP peering between R1 and R3 whether via loopback or physical then can it even be in the realm of possibility that they are directly connected? If I NEED to Set TTL of 2 for R1 and R3 to BGP Ping or Even Ping, Can They Be Directly Connected? A TTL of 2 would be needed to eBGP peer between R1 and R3.A TTL of 2 would be needed to ping R3 from R1.For our eBGP peering between R1 and R3 back in beginning we knew two things Well it basically seems to indicate, in the above documentation, that the default behavior is to check to see if the neighbors are directly connected. So if it isn’t TTL that “fails,” then what? If the default behavior is that only directly connected neighbors are “ allowed,” this would mean that some type of check happens that realizes that R1’s loopback and R2’s loopback are not directly connected to each other and the attempted eBGP connection must, then, by default fail. The documentation says that, by default, without this command ( ebgp-multihop) that for eBGP “ only directly connected neighbors are allowed.” By Default, Only Directed Connected Neighbors are Allowed Question: Is R1’s loopback directly connected to R2’s loopback?Īnswer: No. So this command says that it will help connect eBGP peers “ residing on networks that are not directly connected.” Let’s do a quick google search for neighbor ebgp-multihop. The truth is we actually don’t “need” to. Since R1 and R2 only need a TTL of 1 to get between their respective loopbacks, why do we “ need” to set eBGP multihop to 2 for R1 and R2 for eBGP to work? TTL of 3 is sufficient to reach R3’s loopback addressĪs you can see below… all those pings were successful.TTL of 2 is sufficient to reach R2’s loopback address.TTL of 1 is sufficient to reach R1’s loopback address.Let’s have the PC ping all 3 of the Loopbacks while setting TTL. ![]() So if the TTL actually were getting additionally decremented going to the loopback… then how exactly can R1 and R3 peer with each other AND also peer THRU another router? Taking a Step Back So as we can see, R1 and R3 can indeed eBGP peer loopback to loopback with a TTL of 2 and with R2 in the middle! Let’s check R1 and see if it actually has an established BGP session and let’s look at those configs. What if I told you that I can eBGP peer between R1 and R3 with a TTL of 2?ĭon’t take my word for it. A TTL of 1 isn’t going to make it between R1 and R3. The default TTL for an eBGP session is a TTL of 1. R1 and R3 BGP Peering via Loopbacksīefore we begin… yes… the routing connectivity between R1’s loopback and R3’s loopback is already set up via statics. Let’s BGP Peer between R1 and R3 and have them use their Loopbacks for the BGP session. the eBGP (which has a default TTL of 1) will come up with no playing or tweaking of the TTL.įor grins and giggles… let me show you something else. R1 and R2 BGP Peering via Subnet 10.1.2.0/24Īs you may already know, if we peer R1 and R2 together using the directly connected subnet (10.1.2.0) that connects them together…. In the picture above we have 3 Routers in 3 different BGP Autonomous Systems. I find pictures and seeing the flow helps it all sink in. Let’s go play in the lab and I’ll explain. was cause “ the TTL gets decremented on the way to the loopback” Why? Well cause what they said was that the reason one must set the TTL to 2 for eBGP peers that are peering with their loopbacks but also directly connected…. □ So to say I was confused, when I read what I did, would be an understatement. Back then I pretty much believed that if it made it into a book it must be true and my knowledge had to be in error. Years and years ago I was trying to learn more about BGP and I was reading some book with a chapter on the topic. Myth: You have to set ttl to 2 because it is decremented on the way to the loopback.
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