Chapter 1 - Packet Forwarding - Day 3

 

Packets must be routed when devices are on different networks.

Next-hop IP address is identified  by a static entry, default gateway or routing protocol

Device looks for next hop IP address in the ARP table and uses the MAC address from the next-hop IP as the destination MAC address.  Next-hop router receives the packet based on the destination MAC address then analyzes the destination IP address, finds its the appropriate entry in the routing table, looks for the outbound interface then finds the destination MAC address looking to see if it needs to be routed further. The router then changes the source MAC address to the MAC address of the router's outbound interface while also changing the destination MAC address to the destination device or next-hop router.

This process continues on and on as needed to get the packet from the source device to the destination device

With either IPv4 or IPv6 an IP address mus be assigned to an interface for a router or L3 switch to route packets.

IPv4 addresses are assigned with the interface configuration command ip address (ip address/subnet mask)

An interface with an IP address and up state injects the network into the  RIB (Routing Information Database)

Connected networks or routes have an AD of 0

Connected routes cannot be preempted in the RIB

a secondary IP can be assigned to an interface using ip address (ip address/subnet mask) secondary

When VLANs require routing but there are not enough ports on a switch then logical subinterfaces need to be created on a router, This is done by placing a period at the end of the interface followed by a number perferably the VLAN that will be routed.

 ie.... interface g0/0/1.10

These must be encapsulated with command encapsulation dot1q (vlan #)

IP address can be assigned to SVI or VLAN interface. There must be an interface associated to the VLAN.

On L3 switches SVIs can be used for routing packets between VLANS without the need for an external router.

Networks designs can include P2P (Point to Point) links between switches for routing. 

L3 switch ports can be converts from an L2 switch port to a routed switch port by using command no switchport then assigning it an IP address.

IPv4 addresses can be viewed using command show ip interface. This can provide useful info for troubleshooting.

Using optional keywords can limit the output in condensed format

IPv6 address can be view with a similar command of show ipv6 interface

Routers have streamlined the forwarding process by no longer remove and add L2 info now they simply rewrite the addresses. IP packet switching or IP Packet forwarding is a faster process for receiving an IP packet on an inbound interface and deciding whether it forwards it or drops it. 

Routers used to us a mechanism called Process switching to switch the packets through the routers, then evolved to fast switching or Cisco Express forwarding (CEF) 

Process switching also referred to as software switching or slow path in where the CPU on the router is in charge of packet switching.

Process switching is the fallback for CEF because it is dedicated to processing punted IP Packets when they cannot be switched by the CEF.



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