The Routing Process

rob42

I’m trying to get the Routing Process straight in my head and commit it to memory in a simplistic way, so it’s easier to remember. To that end I’ve come up with the process set out below. Would someone be good enough to have a look at it and let me know if and where I’ve slipped up?

I understand that IP Networks, are grouped by the value in the Network part of the IP Address and are not separated from each other by a Router, but I'm a little fuzzy in so much as, if the hosts ARE on the same Network and connected via a Switch, do the frames still go to the destination host via the default gateway, or directly to the destination host via the connected Switch?

Thanks.


Sending Host:

• At the Network Layer form an IP Packet
  
• Find the destination host's MAC Address by way of a ARP table or ARP Request
• At the Link Layer use the destination MAC Address in the formation of the Ethernet Data Frame
• At the Physical Layer send the frame to the destination MAC Address
  
  

Forwarding Router:
Check the FCS to determine if the Frame has errors

If CRC error:

• Drop the Frame

Else:

• Determine the destination MAC Address

If MAC Address does not match own:

• Drop the Frame

Else:

• Extract the IP Packet from the Frame to determine the destination IP Address
• Find the destination host's MAC Address by way of a ARP table or ARP Request
• Use Routing Table to determine Subnet/Interface/Next Hop
• At the Link Layer use destination MAC Address in the formation of the Ethernet Data Frame
• At the Physical Layer send the frame to the destination MAC Address

Welly_59

Hosts on the same switch and subnet will send directly to the other host without going through the default gateway.

If you think about the arp request, host a will go 'hey who is 11.22.33.44, send me your mac'. If the ip is on a different network the default-router will say 'hey it's me my mac is......' and so the frame will get to switch which will send it to corresponding interface for the default router.

If destination is on same switch and subnet then it still follows the same process but goes direct to host b. Host a arps 'hey who is 11.22.33.44, send me your mac'. Host b responds to arp request and frame gets made accordingly. When switch receives frame it will send it direct to host b. Switches don't care about ip address, all they care about is mac addresses and they forward frames accordingly.

rob42

Welly_59 wrote: »

Hosts on the same switch and subnet will send directly to the other host without going through the default gateway.

If you think about the arp request, host a will go 'hey who is 11.22.33.44, send me your mac'. If the ip is on a different network the default-router will say 'hey it's me my mac is......' and so the frame will get to switch which will send it to corresponding interface for the default router.

If destination is on same switch and subnet then it still follows the same process but goes direct to host b. Host a arps 'hey who is 11.22.33.44, send me your mac'. Host b responds to arp request and frame gets made accordingly. When switch receives frame it will send it direct to host b. Switches don't care about ip address, all they care about is mac addresses and they forward frames accordingly.

Thanks for the clarification; yes, that makes sense.

clarson

what the host does is:
1) gets the destination's ip address
2) using it's ip address and subnet masks determines if the destination's ip address are on the same network.
3a) on the same network
checks arp cache for destination's mac address
if not in the arp cache sends an arp request for the destination's mac address
4) sends frame with the destination's mac address

3b) not on the same network
checks arp cache for the default gateway's mac address
if not in the arp cache sends an arp request for the default gateway's mac address
4) sends frame with the default gateways's mac address to the default gateway, but uses the destination's ip address
5) default gateway computes the next hop from the destination's ip address.
6) assigns new mac addresses to send the frame on to the destination's nework.

rob42

Thanks for your input. That has helped me get things straight in my head.