Disadvantage of Multiple iBGP neighborship in same subnet

rueltabarangao@gmail.com · ‎08-10-2020

Hi Guys, Would like to ask for experts advise what are the Disadvantage of implementing Multiple iBGP neighborship in same subnet like flat network? Let's say 4 iBGP routers that are all connected in a switch and routers iBGP neighborship interfaces are in same subnet. Compare to using common point-to-point connection each or using route-reflector for mesh connection. TIA

Giuseppe Larosa · ‎08-11-2020

Hello rueltabarangao@gmail.com ,

clearly from your network diagram there is a single point of falure that is the LAN switch where every router connects to.

This is the main issue of the proposed topology.

In the past service providers POP (Point of Presence) solved this issue using two indipendent LAN switches and two different VLANs/IP subnets to have the required fault tolerance.

iBGP sessions at that point should be terminated on the loopback interfaces so that if at least one LAN interface is alive the router is reachable.

Using Route Reflector servers is compatible with this topology and allows to solve the full mesh requirement of iBGP sessions caused by iBGP split horizon rule.

Hope to help

Giuseppe

rueltabarangao@gmail.com · ‎08-11-2020

Hi Giuseppe,

Thanks a lot for your inputs and I really appreciate it responding to my curiosity.

Noted on the single point of failure actually the end state of that switch is it will become a 2 NX OS device say N7k multi VDC edge router with vPC peer-link and a vPC connections to each border routers with HSRP.

So these kind of topology that all iBGP routers belongs to single broadcast domain/same subnet peering are just fine? Though I tried to simulate this and everything works just fine, I'm just making sure if there's any other issues i'm not aware of like in actual operations or scalability issue.

As I'm always seeing on labs topology maybe for best practice are point-to-point topology from Router to router or with underlying IGP like OSPF for reachability purposes and performing route-reflectors to solve the mesh requirement of ibgp.