Kotebe University College (KUC)Department of Computer Science and Technology

Network & System Administration (CoSc3063)Lab Three – Routers, Static Routing and Default Routing

What is a router?A router is a specialized computer which can connect multiple networks to allow exchange of packets between them. Since a router uses IP header information (layer 3 protocol data unit), to transmit the packets between networks, this ability makes it a layer 3 device. Like switches, routers build a special database which serves as the source of information on what to do with incoming packets. This database is formally called Routing Information Base (RIB). But most often people call it a routing table. How a router constructs a routing table and maintains information in it, will be the topics of quite a few upcoming posts.

A router is in many ways similar to a regular PC. It has RAM and ROM memory chips as well as CPU and motherboard etc. But instead of using hard drive, it uses a flash memory to store files such as the operating system (IOS). Also, what makes it distinct, the operating system and hardware are optimized for fast packet transmissions. Typically the router uses at least two interfaces but more often than not, it has a greater number of them. Cisco operating system is called Internetwork Operating System (IOS). The same name is given to OS used by many Cisco Catalyst switches. Although some of them may also use CatOS.

There is a great variety of interfaces routers can use. For instance, they can connect few Ethernet networks together, but also Ethernet with Wide Area Networks (WANs) such as ATM, Frame-Relay, X.25, ISDN, Broadband etc.

Router FunctionsIn order for the routers to connect multiple layer 3 networks together, they must be able to do the following:

Learn which networks/subnets are available In case there are multiple paths, choose the the best one Keep (routing table) must be up-to-date Translate layer 2 headers (disparate network connections) Keep loop-free paths Make forwarding decisions based on layer 3 headers

Routing is primarily based on hop-by-hop paradigm. This means that if there are multiple routers in the path, a router must find the outbound interface and forward the packet to a next hop router. A router could not care less as to what happens to the packet after it has been expedited.

In order to find the outgoing interface an IP destination address of a packet and a routing table are used. The process whereby destination IP address is the key information to find the outbound interface for a packet is called destination-based routing. However, it is possible to influence that decision making

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process and choose other criteria such as source of IP transmission, size of the packet, importance of the packet in relation to others or some other parameters rather than destination IP address. When used it is referred to as the traffic engineering. For now though, I am going to focus in on the default behavior.

Before I describe the router's principles of operation, I must make sure we are on the same page with the fundamentals related to the traffic flow.

Let's recall what happens with the packets sent between computers residing in different networks. I will use a simple topology (pic. 1) to review a few facts. Please, get familiar with the picture below first. Pay a special attention to the three headers depicted and numbers in green circles. The numbers refer to the steps below. Of course, this is only a ten thousand foot view of what happens here. Before we jump into the deep water we need to warm up a bit by looking at the process from a high perspective.

Pic. 1 - Traffic Flow and Layer 2/layer 3 Encapsulation/Dencapsulation

The numbers in green circles mark the important points of the traffic sent from PC1 (left hand side) to the PC2 (on the right side of the picture).

In the explanation presented below I assume that SW1 and SW2 have populated their CAM tables (learned all MAC addresses on the appropriate ports). Here's how it goes.

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Step1 PC1 sends a packet destined to PC2. Since, PC1 has the IP address 192.168.1.1/24, it realizes that the first 24 bits of the destination IP address are different than its own (source: 192.168.1.1, destination: 192.168.3.1). Conclusion: PC2 is NOT in the same layer 3 network, so default gateway (192.168.1.254) must be used to forward the packet to PC2. Knowing it, the IP header is going to use:

Src IP = 192.168.1.1 Dst IP = 192.168.3.1 TTL = 32 (ttl is set by the application, here I use 32 as an example)

IP packet is encapsulation in Ethernet (layer 2) header in order to be put onto the wire. Ethernet header contains source MAC address of the sender, and destination MAC address of R1's F1/0 interface obtained from the computer's arp cache (if not found in the arp cache, arp request is sent):

Src MAC: 0000.1111.1111 Dst MAC: 0000.2222.2222

Step 2SW1 receives the frame on its port F0/1. It locates the outbound port (f0/2) for destination 0000.2222.2222. It sends the frame out towards F1/0 port of R1. Neither of layer 2 or layer 3 headers presented in the pic. 1 change during this transmission (parameters depicted).

Step 3R1 receives frame on F1/0 port. Layer 2 header is inspected by R1. Since the destination MAC address (0000.2222.2222) is the address of F1/0, R1 concludes it is the destination for the frame. Layer 2 header is removed and the content of the message (packet) is processed by the router. R1 processes IP header, reads the destination IP address (192.168.3.1) and compares it with the entries in its routing table trying to find the longest match. More on this in the upcoming post. Once the best path has been found, the routing table points to the outbound interface (F1/1) and the next-hop router's IP address (192.168.2.2) that should be used to expedite the packet.

Step 4The packet is moved to F1/1 port and the TTL number is decremented by 1 (now TTL=31). Then, the packet is encapsulated in the layer 2 header.The following source and destination MAC addresses are used now:

Scr MAC: 0000.3333.3333 Dst MAC: 0000.44444.4444

The destination MAC address is obtained from R1's arp cache. If R1 does not know the MAC address for 192.168.2.2 (next-hop router), arp request is sent asking for its MAC address.

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Step 5R2 receives the frame on F1/0 port. It performs the same job R1 has done. It reads the destination MAC address. Since it is the recipient (0000.4444.4444), it dumps the layer 2 frame and processes IP header. It performs layer 3 lookup in its routing table and finds the outbound interface for destination 192.168.3.1. In our example it turns out that the destination network is connected directly to F1/1 interface. In such case, R2 checks arp cache for MAC address of the destination (192.168.3.1) if one is not found, arp request is sent (who's 192.16.8.3.1 ?, I need you MAC address!).

Step 6The packet is moved to F1/1 interface and before it gets encapsulated, the TTL number is decremented by 1 (TTL=30). The packet is encapsulated in an Ethernet frame header using the following addresses:

Src MAC: 0000.5555.55555 Dst MAC: 0000.6666.66666

The frame is sent out F1/1 interface.

Step 7SW2 receives the frame and finds the outbound interface for the MAC address: 0000.6666.6666 int its MAC address table. It is port is F0/2.

Step 8The frame is sent out F0/2 towards PC2. Fields in the layer 2 and layer 3’s headers remain the same.The above is just a quick review in case you've forgotten that.The interesting bit for us now is the router's process of finding the outgoing interface and layer 2 addresses of the next hop device. I'm going to elaborate on this in my next post. Now, let me quickly present the routing table components that are essential in this traffic flow.

Pic. 2 - Routing Table Components.

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Components of Routing Table: C and S - point how a router obtained the information (C = connected, S=static route) 192.168.3.0/24 - Example of prefix (destination network/subnet) [1/0] - Square brackets show two numbers. First (1) is Administrative Distance, second (0) is

Metric via 192.168.2. - the next-hop-router address

How router populates the routing table, what these terms in red mean and how router uses these parameters to pick the longest match and as a result of that the best path, are going to be covered in class.

Static Routing In the previous post I attempted to explain how a router selects the best route if there are multiple paths available. In this lesson, I'm going to show you how you can use static routes effectively in two different topologies (the second one uses backup links). You'll see how basic knowledge on route selection can come in handy if you plan on using primary and backup connections.

All Cisco routers have the routing capability turned ON by default. The command responsible for this is:router(config)#ip routing

This allows a router to create and use the routing table the moment we enable and configure at least two interfaces.

NOTICE!Some subnets and networks are simulated by means of creating and configuring virtual interfaces (Loopback) in my topology.

Pic. 1 - Routing Topology 1

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Directly Connected NetworksIn the topology used (pic. 1), the routers have been assigned IP addresses and the interfaces are up. Since the routing process is enabled (ip routing) the directly connected subnets/networks show in the routing table immediately. Look at R1's routing table:

Pic. 2 - Directly Connected Networks.

The problem is with the destinations that are NOT connected directly to a router (remote). A router does not know anything about these by default. There are two ways of "teaching" a router about remote networks or subnets:

1. Applying static routing (manual method)2. Applying dynamic routing (a routing protocol that distributes information automatically)

Static RoutingThere are pros and cons of using manual method. In complex scenarios (with redundant connections), more often than not, we use dynamic routing protocols. But there are situations in which static routing is good or perhaps the best solution.

Consider our example. R4 and R5 are connected to the so called stub networks. A stub network has only one way in and out (one path). Some routers used in such designs are relatively cheap and may not even have enough hardware resources to run a dynamic routing protocol (such as OSPF or EIGRP). Then, installing static routes is the only option possible. Also, imagine your broadband router (your home network is also the stub-like if you're connected to one ISP). This router does not have the paths to each and every destination on the Internet. It uses a form of static route instead known as: default route. More on the default route later in the post.

Let's look at the syntax which allows us to instruct a router about remote networks and subnets manually.

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Pic. 3 - Static Route Command Version 1.

Let's read what this command does. "IP route towards class C network 192.168.1.0/24 can be reached by sending packets to a next-hop router out the serial0/2 interface."

The last parameter used shows the router which interface should be used to send the packets out. If you configure the outbound interface instead of the IP address of the next-hop router in the path, this connection must be point-to-point (not multiaccess).

In case, the router's egress (outbound) interface is multiaccess link (Ethernet, Frame-Relay, ATM etc.), we must NEVER use local interface but IP address of the next-hop router instead. If you do not follow this recommendation, the router will try to resolve the layer 3 to layer 2 address for every destination out that interface. This leads to serious inefficiency and shows little understanding of routing operation of a person who used it.

If the router must send the packet to the next router in order to get to the destination (egress interface is multiaccess), the 'ip route' command should look like the example below (pic. 4).

Pic. 4 - Static Route Command Version 2.

Let's configure our routers so they can reach all networks int the topology used (pic. 1).

NOTICE!The routing works in both directions. This means that the router receiving packet to its directly connected network/subnet must know the returning path to the sender of the packet (source).

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Configuration on R1Step 1Reachability towards 172.31.2.0/24. The next-hop router is R2. The outbound interface is multiaccess link (F1/0). The order of statements does not matter. Configuring the remaining routers I will use a more logical approach than on R1.

R1#configure terminalR1(config)#ip route 172.31.2.0 255.255.255.0 172.31.123.2R1(config)#

Step 2Reachability towards 172.31.3.0/28 and 172.31.16.0/28. The same egress interface (F1/0).

R1(config)#ip route 172.31.3.0 255.255.255.240 172.31.123.3R1(config)#ip route 172.31.3.16 255.255.255.240 172.31.123.3R1(config)#

Step 3Reachability towards 192.168.4.0/24. The egress interface is point-to-point (S0/2 running HDLC protocol). I can use either the next-hop IP address or the local interface s0/2.

R1(config)#ip route 192.168.4.0 255.255.255.0 s0/2R1(config)#

Step 4In order to reach Branch2 network 192.168.5.0/24, R1 must use R2 as the gateway. Even though R2 does not know how to get there now, I will configure it and then configure R2 to reach all networks and subnets (including 192.168.5.0/24).

R1(config)#ip route 192.168.5.0 255.255.255.0 172.31.123.2R1(config)#

Step 5Reachability to the point-to-point subnet between R2 and R5 (172.31.25.0/24).

R1(config)#ip route 172.31.25.0 255.255.255.0 172.31.123.2R1(config)#

Now, let's see what the routing table reveals:

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Pic. 5 - Routing Table of R1.

 Before I proceed with the configuration of the other routers let's consider a few things.

Look at the R1's routing table and the topology carefully, and try to answer the following questions before you test the reachability using 'ping'. If you have problems answering the questions 1, the remaining ones (2-4) should give you a few hints.Question 1How many IP addresses presented in the topology (pic. 1) will respond to ping from R1 after you have configured static routes so far (only R1 is configured with static routes; all other routers have IP addresses and interfaces enabled)?Question 2R1 sends ping (echo request) towards 192.168.4.1. What is going to be the source IP address of this request?Question 3R1 sends ping (echo request) towards 172.31.25.2. Is R1 going to receive reply (echo reply)? Why?Question 4R1 sends ping (echo request) towards 172.31.25.5. Is R1 going to receive reply (echo reply)? Why?The answers are as follows.Answer 1There are 11 IP addresses to respond to the ping sent by R1. These are:

172.31.1.1 - reason: directly connected subnet (Loopback 1). 172.31.123.1 through 3 - reason: directly connected subnet (F1/0). 172.31.3.1 and 172.31.3.17 - reason: source IP address is the 172.31.123.1. It's the egress

interface to reach these two addresses (via F1/0). R3 knows how to get back to R1's F1/0 interface (R3's F1/0 is connected to 172.31.123.0/24 too).

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172.31.2.1 and 172.31.25.2 - reason: R1 will use F1/0 (egress interface) to reach these IP addresses according to our 'ip route' statements. The source IP address is going to be the address of F1/0. R2 knows its way back to 172.31.123.0/24 subnet (directly connected to F1/0).

172.31.14.1, 172.31.14.4 and 192.168.4.1 - reason: R4 knows how to get back to the source IP address R1 uses for these destination. R1 uses 172.31.14.1 as the source IP address. This source (subnet 172.31.14.0/24) is shared between R1 and R4 on their Serial0/2 interfaces.

The reason I ask this question is to draw your attention to two important facts:

A router is going to find the best match in the routing table for each destination. If not found, of course the packet is dropped. If found though, a router will not change the source and destination addresses in packets TRAVERSING it. If the packet is ORIGINATED by the router (here: ping), the source of IP address used is going to be the address of its egress (outbound) interface by default.

Sending a packet out is one job, but the destination will try to send a response back to the source. The remote router which is going to respond, must know how to reach the source of the transmission as well (valid path back to the source in its routing table).

Answer 2Ping from R1 towards 192.168.4.1 is going to use 172.31.14.1 as its source address since according to the routing table Serial0/2 is the outbound interface.

Destination 192.168.4.1 shows the following detailed output on R1:

Pic. 6 - R1's Route Towards 192.168.4.1.

The route shows that the longest match for 192.168.4.1 is: 192.168.4.0/24. This routing table entry points to Serial0/2 as an egress interface.Answer 3R1 sends the ping (echo request) packet towards 172.31.25.2. Like explained in the answer 2, the source IP address for this echo request is going to be the address of the outbound interface (FastEthernet1/0). R2 knows how to reply back to 172.31.123.1 since R2 is directly connected to the subnet 172.31.123.0/24 with its FastEthernet1/0 interface.Answer 4R1 sends the ping (echo request) packet towards 172.31.25.5. It is NOT going to get the reply from R5. The reason is that R5 does not know how to reply back to the source (172.31.123.1). It has not been configured to reach remote subnets and networks yet.I hope you have found this little quiz entertaining and informative enough.

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Would you know how to configure R2 and R3 using R1's configuration as an example? Give it a try. If you can't do it yet, just follow the configuration presented below.

Configuration on R2Step 1Reachability to networks/subnets via R1.

R2#configure terminalR2(config)#ip route 172.31.1.0 255.255.255.0 172.31.123.1R2(config)#ip route 172.31.14.0 255.255.255.0 172.31.123.1R2(config)#ip route 192.168.4.0 255.255.255.0 172.31.123.1R2(config)#

Step 2Reachability to networks/subnets via R3.

R2(config)#ip route 172.31.3.0 255.255.255.240 172.31.123.3R2(config)#ip route 172.31.3.16 255.255.255.240 172.31.123.3R2(config)#

Step 3Reachability to network via R5.

R2(config)#ip route 192.168.5.0 255.255.255.0 s0/2R2(config)#

Configuration on R3Step 1Reachability to networks/subnets via R1.

R3#configure terminalR3(config)#ip route 172.31.1.0 255.255.255.0 172.31.123.1R3(config)#ip route 172.31.14.0 255.255.255.0 172.31.123.1R3(config)#ip route 192.168.4.0 255.255.255.0 172.31.123.1

Step 2Reachability to networks/subnets via R2.

R3(config)#ip route 172.31.2.0 255.255.255.0 172.31.123.2R3(config)#ip route 172.31.25.0 255.255.255.0 172.31.123.2R3(config)#ip route 192.168.5.0 255.255.255.0 172.31.123.2R3(config)#

As for the routers R4 and R5 they connect stub networks. In order to simplify the configuration on these and reduce the number of entries on them, I am going to use a special type of static route called: the default route.

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Pic. 7 - Default Route Example.

The destination IP address 0.0.0.0 (unknown) represents all destinations which cannot be found in the routing table. This address uses the network mask of all zeros (0.0.0.0). As long as the router does not have the best match in the routing table for a given destination ('subnet not in table') the default route is going to be used instead. It is the 'gateway of last resort'. Like previously explained, on point-to-point links you can use the outbound interface instead of the address of the next-hop router.

Applying default routes is going to be easy.

Configuration on R4Step 1Packets for all unknown destinations send via R1.

R4(config)#ip route 0.0.0.0 0.0.0.0 s0/2R4(config)#

Configuration on R5Step 1Packets for all unknown destinations send via R2.

R5(config)#ip route 0.0.0.0 0.0.0.0 s0/2R5(config)#

Simple test will prove the default route operation:

Pic. 8 - Default Route Test.

Even though the routing table does not have the route towards 192.168.5.1, the packets are delivered using the default route (via R1 which knows how to get there).

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Look what the routing table shows when default route has been added (pic. 9).

Pic. 9 - Routing Table with Default Route.

Static Routing with Primary and Backup LinksIn order to spice things up, I am going to configure two additional connections from HQ to our branches using Frame-Relay. These redundant paths must be used as the backup links. They should be used in the case of losing main path via Serial0/2 interfaces (down).

Pic. 10 - Routing Topology with Redundant Paths.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Please, disregard my configuration of Frame-Relay links for now. I'm going to address WAN

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protocols in the upcoming posts. I only need the extra connectivity to show you how to handle the primary and backup scenario using static routing.

Frame-Relay Configuration is going to look like this in order to reflect the topology in the pic. 10.

Circuit between R1 and R5

R1 Configuration:R1(config)#interface serial0/0R1(config-if)#encapsulation frame-relayR1(config-if)#no frame-relay inverse-arpR1(config-if)#ip address 172.31.15.1 255.255.255.0R1(config-if)#frame-relay map ip 172.31.15.5 105 broadcastR1(config-if)#no shutdownR1(config-if)#

R5 Configuration:R5(config)#interface serial0/0R5(config-if)#encapsulation frame-relayR5(config-if)#no frame-relay inverse-arpR5(config-if)#ip address 172.31.15.5 255.255.255.0R5(config-if)#frame-relay map ip 172.31.15.1 501 broadcastR5(config-if)#no shutdownR5(config-if)#

Circuit Between R2 and R4

R2 Configuration:R2(config)#interface serial0/0R2(config-if)#encapsulation frame-relayR2(config-if)#no frame-relay inverse-arpR2(config-if)#ip address 172.31.24.2 255.255.255.0R2(config-if)#frame-relay map ip 172.31.24.4 204 broadcastR2(config-if)#no shutdownR2(config-if)#

R4 Configuration:R4(config)#interface serial0/0R4(config-if)#encapsulation frame-relay R4(config-if)#no frame-relay inverse-arpR4(config-if)#ip address 172.31.24.4 255.255.255.0R4(config-if)#frame-relay map ip 172.31.24.2 402 broadcastR4(config-if)#no shutdownR4(config-if)#

Now let's get back to the business. If I add two static route entries using the newly created paths, the metric of each of them is going to be identical with the metric used by the primary link

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(Serial0/2). This way, load balancing (traffic sharing) is going to be used since two equal cost paths exist. Our design stipulates that Frame-Relay circuits should be used as the backup links only (Serial0/2 down).

In order to accomplish this, I should change either the metric or administrative distance of the backup path. Unfortunately, we cannot change the metric (no command available) on static routes, but we can easily increase the value of administrative distance to make the backup path less preferred. The default AD for static routes is 1, so I will make the backup route less trusted by using the value of, say, 3.

Backup Link Between R1 and R5R1 Configuration:R1(config)#ip route 192.168.5.0 255.255.255.0 172.31.15.5 3R1(config)#

R5 Configuration:

R5(config)#ip route 0.0.0.0 0.0.0.0 172.31.15.1 3R5(config)#

Backup Link Between R2 and R4R2 Configuration:R2(config)#ip route 192.168.4.0 255.255.255.0 172.31.24.4 3R2(config)#R4 Configuration:R4(config)#ip route 0.0.0.0 0.0.0.0 172.31.24.2 3R4(config)#This way, the primary link (via Serial0/2) is preferred due to the lower administrative distance (AD=1). Look at R4 now:

Pic. 11 - R4's Routing Table with Primary Link UP.

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In case the primary link goes down, the extra ip route (using Frame-Relay link) kicks in like shown in the pic. 12.

Pic. 12. - R4's Routing Table with Primary Link DOWN.

In the few upcoming posts, I will focus in on dynamic routing protocols.

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