ONL NPR Tutorial

The ONL NPR Tutorial

NPR Tutorial >> The Remote Laboratory Interface

TOC

Configuring Routes

If we try to send traffic from n1p3 to n2p2 using the configuration we just committed in the preceding page The Remote Laboratory Interface => Configuring Topology, NPR 1 (the left router) will drop the packets because no routes have been installed; i.e., all of its route tables are empty. This page explains conceptually how an NPR route table is used to forward packets from an input port to an output port and shows you how to install route tables.

An example based on some of the concepts discussed on this page is given in NPR Tutorial => Examples => The_Remote_Laboratory_Interface. The sections in this page are:

The NPR Route Table
Generating Default Routes
The Route Table Edit Menu
Verifing Connectivity Using Ping

The NPR Route Table

The most basic activity of a router is to transmit incoming packets out the output port designated by the best matching entry in its route (or forwarding) table. Conceptually, each port of an NPR has a route table in which each entry has three fields:

The Prefix/mask is a subnet address specified in Classless Inter-Domain Routing (CIDR) format. A CIDR address has the form a.b.c.d/x where a.b.c.d is an IP address prefix in dotted decimal notation (e.g., 192.168.1.0) and where x (the mask) indicates the number of bits in the network part of the IP address. For example, an x of 24 means that the leftmost 24 bits of the prefix is the network part of the address.
The Next hop indicates the output port associated with the table entry.
The Stats is the statistics index associated with the table entry and will be described later.

For now, suppose that we are using only a basic NPR that uses only route tables for packet forwarding decisions. (Later, we will see how NPR filters can alter packet forwarding.) When a packet arrives to port P, the NPR picks from port P's route table the best matching (P, N, S) entry using the longest prefix match (LPM) algorithm and forwards the packet knowing that the packet should go out port N.

Generating Default Routes

In order for both NPRs to forward traffic between all hosts and between the two NPRs, we need to install route tables at the inputs that have attached links; i.e., ports 1.2, 1.3, 1.4, 2.1, 2.2, and 2.3 (the notation X.Y refers to NPR X, port Y). [[ route-tables-2way-resize.png Figure ]]

The figure (right) shows route tables that could be used at ports 1.3, 2.1, 2.2 and 1.4 to forward packets flowing from n1p3 to n2p2 and back to n1p3. They have been arranged to show which entries would be used to forward the packets.

Recall: The nXpY network interface is assigned the IP address 192.168.X.Z where Z = 16 x (Y+1); e.g., n2p2 has the IP address 192.168.2.48.
So, packets flowing to n2p2 will have a destination IP address of 192.168.2.48 in its header.

The packets flowing from n1p3 to n2p2 (192.168.2.48) will match the prefix/mask fields 192.168.2.0/24 and 192.168.2.48/28 at ports 1.3 and 2.1 respectively. The packets flowing in the reverse direction from n2p2 to n1p3 (192.168.1.64) will match the prefix/mask fields 192.168.1.0/24 and 192.168.1.64/28 at ports 2.2 and 1.4 respectively.
[[ Generate Default Routes Figure ]]

The easiest way to generate the route tables we need is to generate default route tables at every port (including ones without attached hosts):

RLI Window:
Select Topology => Generate Default Routes
Select File => Commit

You can verify that the route tables are correct. For example, look at the routing table at port 1.3 by enterring (not shown):

Select NPR.1:Port 3 => Configuration => Route Table

The route table at port 1.3 will appear. It should look like the figure (right).

You should see a route table similar to the one shown right. The first five entries reflect the NPR's standard IP addressing where hosts on subnet 192.168.X.Z/28 are assumed to be attached to port Y and Z = 16 x (Y+1). For example, the third (P, N, S) entry is (192.168.1.48/28, 2, 46), and means that packets destined for a host in the subnet 192.168.1.48/28 (e.g., 192.168.1.48) should be forwarded to output port (next hop) 2.

The last (P, N, S) entry (192.168.2.0/24, 4, 49) forwards packets destined to hosts connected to NPR 2 and will forward those packets out port 4. The RLI generated that entry automatically because it could tell that there is only one route to the hosts attached to NPR 2.

But even when if there were multiple paths to NPR 2, the RLI would have inserted an arbitrary entry for packets destined for NPR 2.
Watch Out!!! If there are multiple paths, the output port chosen by the RLI may not be the one you want. So, check router-to-router entries in route tables.

The Route Table Edit Menu

You can examine the route table at a single port. In the figure (right), we are about to open the NPR.1:port 3 (port 1.3) route table window. The route table window has a Route Table => Edit menu that allows the user to add a route, delete a route, stop statistics and generate local default routes. Also, you can modify a route table entry by selecting an entry field and changing only that field (e.g., next hop). You must select File => Commit before the changes take effect.
[[ delete-route-block-resize.png Figure ]]

The Add Route and Delete Route menu items are straightforward to use. Note also that you can delete a block of entries (shown right):

Select the first item in the block
Then, select the last item in a block while holding down the SHIFT key to highlight the block of entries
Then, select Edit => Delete Route

[[ local-default-routes-resize.png Figure ]]

The Generate Local Default Routes menu item allows you to generate a route table that assumes the 192.168.X.Z/28 subnetting described earlier. Here is how you would use this menu item to install a route table at port 1.3 that is identical to the one generated earlier by the Topology => Generate Default Routes menu item in the main RLI window:

Select Edit => Generate Local Default Routes

The route table will be populated with the five entries for ports 0 to 4 for the NPR. But we need another entry for packets going to NPR 2.

Select Edit => Add Route

A default entry with prefix/mask 0.0.0.0/0 and next hop 0 will be added.

Change the new route entry so that the prefix/mask and next hop fields contain the values 192.168.2.0/24 and 4 respectively by selecting each field and keying in the correct values.
RLI Window:
Select File => Commit

All of the route entries have used non-overlapping prefix/mask entries. But we can construct a route table using the fact that route lookup uses the LPM algorithm.

Verifing Connectivity Using Ping

You can verify that packets from one host can reach another one by using the ping command. For example, if you want to see if packets can be sent from n1p3 to n2p2 (and back), ping n2p2 from host n1p3 by enterring:

onlusr> source ~onl/.topology	# add control network IP addresses to env
onlusr> ssh $n1p3		# ssh to host n1p3 through control network 
$n1p3>  ping n2p2		# send ping packets to n2p2 from $n1p3
        ^C			# enter ctrl-c to terminate the ping command

This provides a simple test that there is network connectivity between the sender and receiver.

Watch Out!! A common mistake is to enter "ping $n2p2" when you meant "ping n2p2". The first form attempts to send five ping packets to the control network interface, not the data network interface, attached to the n2p2 host.

The figure above shows that we have used ping to send five ping packets to n2p2 from n1p3 (onl029). The -c 5 limits ping to sending five packets instead of a continuous stream of packets.

When a user enters the ping command, the sending host sends one ICMP echo request packet to the destination once per second. When the receiver gets the ICMP echo request packet, it sends back an ICMP echo reply packet to the original sender. As each response is received, ping outputs information indicating the sequence number and the round-trip time (RTT). After ping terminates, it outputs a report indicating packet loss and RTT statistics (minimum, average, maximum, standard deviation). The ping command is a simple tool for general network troubleshooting. And a successful test (right) is a good indicator that you will likely be able to send packets between the two hosts participating in the ping.

The figure above shows how you can use the "netstat -i" Unix command to verify that the proper number of ICMP echo request packets were received at and ICMP echo reply packets sent from n2p3. The netstat command window shows that we ran netstat twice on the the receiver host onl074 &mdash once before pinging from n1p3 and once after ping finished. It shows that the receiver received six (6) packets and sent six (6) packets even though we sent only five (5) ping packets. The extra packet is an ARP packet which shows up if we wait long enough between network usage.

Recap

Route Table:
When a packet arrives to a port, the NPR uses that port's route table to determine which output port (next hop) to send the packet based on the destination IP address in the packet's IP header; i.e., where to forward the incoming packet.
LPM Route Lookup Algorithm:
The NPR uses the longest prefix match (LPM) algorithm when searching for the best matching entry in a route table.
Adding Routes:
All route tables start out empty. You can generate default route entries using either Topology => Generate Default Routes in the main RLI window or Edit => Generate Local Default Routes in the Route Table window of an input port.
ping and netstat:
The ping command is often used for testing connectivity between hosts and general troubleshooting. The "netstat -i" command can be used to verify packet count activity at a host's network interfaces.

Revised: Wed, Aug 6, 2008

NPR Tutorial >> The Remote Laboratory Interface	TOC