Auto Last Hop
--> In the case of the F5 BIG IP system, there is a concept of Auto Last Hop which does not use the routing table to forward the traffic.
--> By default, the Auto Last Hop feature is enabled on Big-IP System globally.
--> Auto Last Hop creates a connection table with the following information in it:
1) The interface on the F5 BIG-IP System the traffic has been received
2) MAC address of the upstream device which sent the traffic to the F5 Big-IP System
--> Auto Last Hop Concept/feature only applied to reply traffic which is sent from F5 to the requested device.
Auto Last Hop is not enabled
S2) The F5 BIG IP LTM Changes the destination IP address from the virtual server IP address to pool member IP address selected based upon load balancing algorithm.
S3) The F5 BIG IP LTM checks the routing table to forward the traffic to backend servers.
S4) Backend servers responds and sends the reply traffic to F5.
S5) Now F5 BIG IP LTM uses the Routing table for the reply traffic to be sent to the client.
Auto Last Hop is enabled
S1) The F5 BIG IP system receives traffic from the client.
S2) The F5 BIG IP LTM Changes the destination IP address from the virtual server IP address to pool member IP address selected based upon load balancing algorithm.
S3) The F5 BIG IP LTM checks the routing table to forward the traffic to backend servers.
S4) Backend servers responds and sends the reply traffic to F5.
S5) Now F5 BIG IP LTM uses the connection table created after enabling Auto last hop to send the reply traffic to the client, It does not use the Routing table for reply traffic to be sent to the client.
--> The BIG-IP system's auto last hop feature removes both latency and network congestion by eliminating a large percentage of routing table lookups and ARP requests.
--> In some situations, The Auto Last Hop feature going to create some issues in the network when the upstream devices are in the cluster does not use the virtual MAC address for sending the traffic.
--> In the above case, You can use Last Hop Pool which will override the default behavior of Auto Last Hop.
--> The Last Hop Pool contains the Pool of Firewalls or Routers which can be assigned to the virtual server.
--> If one of the Firewall/Router goes down in the Pool then the traffic will be automatically shifted to another device.
--> It is recommended to configure Last Hop Pool in the case of the virtual MAC address is not being used by Upstream devices.
--> Auto Last Hop feature can be enabled/disabled by navigating to system > configuration > Local Traffic > General
Reference: F5.com
Md.Kareemoddin
CCIE #54759
Super Duper Explanation
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