EBGP Multipath Load Sharing and CEF
When I was discussing the details of the BGP troubleshooting video with one of my readers, he pointed out that I should mention the need for CEF switching in EBGP multipath scenario. My initial response was “Why would you need CEF? EBGP multipath is older than CEF” and his answer told me I should turn on my gray cells before responding to emails: “Your video as well as Cisco’s web site recommends CEF for EBGP multipath design… but interestingly, it does work without CEF”.
The real reason we need CEF in EBGP load sharing designs is the efficacy of load distribution. Without CEF, the router will send all traffic toward a single BGP prefix over one of the links (fast switching performs per-destination-prefix load sharing). With CEF, the load is distributed based on the source-destination IP address pair combinations. Even if multiple clients send the traffic toward the same server, the load is spread across available links.
So every of the CEs only installs the eBGP learned prefixes in the RIB. Assuming the CEs are running HSRP so all traffic which is arriving at HSRP active router will just be send to the eBPG neighbor of this HSRP-active router. The other CE-uplink will not be utilized.
The only idea i had to make Ad of eBGP and iBGP equal on the CE-routers, but this is not really a good idea , is it ?