IP Routing/Switching

Aspackets are received by the router, it must decide which route is the best for the given destination. For both of these decisions, three values are compared: prefix length, administrative distance, and metric values, in order of importance

Route type Administrative distance

Static 1

EIGRP 90
OSPF 110
RIP 120

Every routing protocol has an administrative distance, and this is included in the routing table entries. Lower values are preferred and so, given two routes with an equal prefix length, the lower administrative distance will be chosen.

Metric is used to compare routes that are learned via the same routing protocol when they have the same prefix

length. The metric values are dependent upon the routing protocol—RIP uses hopcount,  while OSPF uses a formula(Cost) to derive its dimensionless metric.

Link state protocols utilize greater detail about the links or connections between routers in order to make more informed decisions. For example, while two pathways might cover the same distance in order to get to the destination, if one path is based on 1Gbps Ethernet and the other is based on slower Frame Relay.

If the address is known, the frame is forwarded to the correct port. If the address is unknown, the frame is sent to every port except the source port. This is called flooding. 

If the destination MAC address is a broadcast address (in the form ff:ff:ff:ff:ff:ff), the frame is again sent everywhere except the original source port

Per-Destination and Per-Packet Load Balancing

Per-packet load-balancing means that the router sends one packet for destination1 over the first path, the second packet for (the same) destination1 over the second path, and so on. Per-packet load balancing guarantees equal load across all links. However, there is potential that the packets may arrive out of order at the destination because differential delay may exist within the network.

This form of per-packet load balancing is not well suited for higher speed interfaces.

Per-destination load balancing means the router distributes the packets based on the destination address. Given two paths to the same network, all packets for destination1 on that network go over the first path, all packets for destination2 on that network go over the second path, and so on. This preserves packet order, with potential unequal usage of the links.

Per-destination or per-packet load-balancing depends on the type of switching scheme used for IP packets

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VLANs can reduce the effect of flooding or broadcasting because they can be used to break the switch into smaller logical segments.

Mirroring copies the traffic from one port and sends it to another. With mirroring, a management host can be installed and collect traffic from any port or VLAN.


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When it comes to Layer 3 switching, there are two kinds: hardware and software. With a hardware-based solution, the device is using an ASIC (a dedicated chip) to perform the function. With the software implementation, the device is using a computer processor and software to perform the function. Generally, Layer 3 switches and high-end routers route packets using hardware (ASICs) and general-purpose routers use software to perform routing functions.