What are two characteristics of Server Load Balancing router mode? (Choose two.)
A.
The design supports multiple server subnets
B.
An end-user sees the IP address of the real server
C.
SLB routes between the outside and inside subnets
D.
The source or destination MAC address is rewritten, but the IP addresses left alone
E.
SLB acts as a “bump in the wire” between servers and upstream firewall or Layer 3 devices
Explanation:
The basic load-balancer scenarios are:
Two-Arm (or sometimes called In-Line)
One-Arm
Direct Server Response
Two-Arm (Routed mode) is basic scenario where you have a server farm in one side of the network (Back End) and the load-balancer is essentially the default
gateway router for the physical servers in the Back End network.
One-Arm Load-Balancer
One-Arm means that the Load-Balancer is not physically “in-line” of the traffic, but as you might understand right now, it has to get into the way of traffic somehow,
to have control over all the Client to Server connections going in both ways.
It is not important how far away the Client worksations are, they can be behind internet or in the same LAN and the load-balancing would be the same. However, the
Load-Balancer is using only one interface and this interface is on the same L2 network with all the servers.
The traffic that the client initializes will get to the Load-Balancer that has the virtual load-balanced IP. The load-sharing algorithm will pick a physical server to which
the Load-Balancer will forward the traffic with destination IP NATed to the physical IP of the server and forward it out the same interface towards the physical server.
BUT the Load-balancer also needs to do source IP nat so that the server reply will go back from the server to the Load-Balancer and not directly back to
the Client, who is not expecting a reply directly from physical server IP. From the physical servers perspective, all the traffic is coming from LoadBalancer.
Direct Server Response (or sometimes called Direct Server Return)Last basic Load-Balancer scenario is Direct Server Response, to understand this scenario, we need to bring a switch into the topology. As we hopefully all know,
switches learn about MAC addresses as they see frames coming on ports with source MACs. Also imagine that we have a router that has to know the MAC address
of the Load-Balanced IP on the last L3 hop. With the picture below, you can already spot the “trick” this scenario tries to present here once you notice the disabled
ARP on physical servers.
In this scenario, Load-balancer only sees the incoming part of client-server traffic and all the returning traffic from physical servers is coming directly back to the
client IP. The biggest advantages of this solution is that there is no NAT and the Load-Balancer throughput is only used in one way, so less performance impact for
the Load-Balancer system. Disabling ARP on a physical server is not a difficult task.
Disadvantages however are that you have to manually configure the Load-Balancer with all the server MAC addresses and might be more difficult to troubleshoot
with only one way traffic seen by the Load-Balancer on the whole L2 segment.
