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The switches mention below are all Cisco.

I got sw01 and sw02 running BGP peering with multiple service provider for our internal infrastructure, between sw01 and sw02 have a port-channel group 100.

Currently, we are running iBGP between sw01 and sw02 to exchange route via VLAN900 interface which I assigned a /28 IP, then allow it to trunk over the port-channel group 100, they are reachable and iBGP between sw01 and sw02 is working fine.

If I want to use loopback0 interface, I will put additional config as interface loopback0, then add IP address on that loopback0 interface, then put a static route on top of VLAN900 interface, and finally peering iBGP via loopback0 interface IP instead of VLAN900 interface IP.

Question: is it still worth to use loopback0 interface IP for peering instead of VLAN900 interface? what's the downside without using loopback0 interface as I don't expect the port-channel would have any issue, hence the VLAN 900 interface should always up.

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  • If you peer iBGP on the loopback, then the routers can exchange routing information on any interface that is up between the routers, including any other VLANs, and you do not need a dedicated VLAN for that. Normally, you would use the loopback as the management address of the router, and it simplifies documentation and troubleshooting for the management and routing protocol to use the same address. The loopback should be used for all your services (logging, SNMP, TACACS, SSH, NTP, DNS, management, routing protocols, etc.). Then you have a single address to deal with for everything.
    – Ron Maupin ♦
    Dec 11, 2020 at 2:57
  • i thought about using management interface IP, but does it introduce traffic on the management interface? or it will just doing route exchange? Also, sometimes the management interface might go under maintenance, maybe repatch or something, that will cause the route to flap, what's your take on this?
    – Thomas G. Lau
    Dec 11, 2020 at 3:07
  • "Also, sometimes the management interface might go under maintenance, maybe repatch or something, that will cause the route to flap, what's your take on this?" No, that would affect the entire device, not specifically the loopback interface. The loopback interface does not get overloaded or go down. It should be used for all types of things because if there are multiple ways into the router, the interface can always be reached as long as one of the ways is up.
    – Ron Maupin ♦
    Dec 11, 2020 at 3:10
  • if I stick with loopback0 VS VLAN interface, does it still make sense to have loopback0 interface?
    – Thomas G. Lau
    Dec 11, 2020 at 5:26

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It is always recommended in BGP best practices to peer with loopback addresses for iBGP peers as we no longer have to rely on a single physical interface (or in your case a VLAN SVI) to be up/up. As Ron mentioned, using a loopback address should always be used for device services (SNMP, NTP, Remote access etc) and to assist with identifying the device when using SYSLOG, TACACs, Routing Protocols (peer IPs & Router IDs), etc.

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