Introduction
7
Ping
The “ping” message is actu ally a query status message that may be sent to devices on the LAN to query their
operation status. The ping message is basically a message asking “Are you alive?” The LAN device will reply with a
message if it is active.
Time and Mask server
Two other ICMP messages are used to query the time and/or subnet mask from a particu lar LAN device. A message
is sent to a LAN device as king for the time or mask, and the devic e replies appropriately.
RIP—Routing Information Protocol
The most important function of the IP protocol is routing. IP routers constantly e xchange information keeping their routing
tables up to date. A method of commun ication is required to ensure compatibility between all IP routers in the network. RIP
is the portion of the IP protocol that is used for router communication.
Route Tables
Each router will maintain a table of network addresses and the appropriate action to take with an IP frame it receives.
A routing table entry will usually consist of the following items:
•Network or sub-network address
•IP address of the next hop router
•Network interface to use to get to the n ext hop router
•Subnet mask for this network interface
•Number of hops to reach the destination net work
•Number of seconds since this route was updated
When a router receives an IP frame, the router will examine it to determine the destination network address. The
router will then look in the routing table, determine the next router to send the IP frame to, and send the frame to
that router.
The selection of the best route path is based solely on the number of hops to the destination network.
Update Mechanism
In order to ensure that the routing tables of all routers in the network are kept up to date, each router will broadcast
its routing table onto each of its locally connected networks. The broadcast of the routing tables occurs every 30
seconds.
The process of updating a routing ta ble with current information, and deciding which router to use to reach a
destination network, creates a ripple effect of changes through the network. When a router goes down and an
adjacent router determines that the path has disappeared, the remaining adjacent routers on that network mus t
determine the next path to use to reach the destination network. Each router will now broadcast its new routing table
with the updated information. The updated information will propagate through the network until all routing tables
have been brought up to date. This process is called convergence.
The broadcast of the routing tables is also used as a method of determining whether a router is still alive or has been
removed from the network. If a router has not heard from an adjacent router in 180 seconds, the local router will
mark the adjacent router as unreachable and start to adjust the routing table, if necessary.