On Thu, Oct 5, 2023 at 9:42 AM Javier Gutierrez <GutierrezJ@westmancom.com> wrote:
the loopback of the core network devices is being set from RFC1918 while on the global routing table. I'm sure this is not a major issue but I have mostly seen that ISPs use global IPs for loopbacks on devices that would and hold global routing.
Hi Javier, It depends. If the loopback is used as the address source for unnumbered interfaces and any of the router's interfaces have differing MTUs then you have a red-alarm fire: you've broken path MTU discovery which breaks TCP. The problem is that the router will originate ICMP destination unreachable, fragmentation needed messages from that address. Those packets will then be filtered entering other networks. Without those messages, the client TCP stack doesn't know to reduce its packet size. It fails with the symptom that the initial connection succeeds but then the first large data stream immediately times out and the connection aborts after a couple minutes. Even if you have the same MTU on all interfaces, you've still broken traceroute since the TTL exceeded messages don't get through. On the other hand, if the loopback is only used to anchor BGP, you select the BGP router ID from a different address and all your network-facing interfaces have global IP addresses then everything should work fine. As you change the configuration over time you'll have to be mindful that you're riding a knife edge, but nothing will actually break. Regards, Bill Herrin -- William Herrin bill@herrin.us https://bill.herrin.us/