Build a network router on Linux

Build a network router on Linux

Zebra offers a competent substitute for dedicated Cisco routers

Dominique Cimafranca, IT Specialist, IBM

Rex Young, Network Manager, C-Cube

Summary:  Zebra is open source TCP/IP routing software that is similar to Cisco's Internetworking Operating System (IOS). Flexible and powerful, it can handle routing protocols such as Routing Information Protocol (RIP), Open Shortest Path First (OSPF), Border Gateway Protocol (BGP), and all of their various flavors. This article shows how our authors set up Zebra and used it to manage routes dynamically in conjunction with real Cisco hardware.

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Date:  08 Oct 2003
Level:  Intermediate
Also available in:   Portuguese

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Dynamic and robust routing is so essential to the workings of the Internet that any fledgling internetworking engineer must not only understand its concepts but also be able put it into practice in real situations. At the same time, this part of the routing scene is dominated by products from high-end network equipment providers such as Cisco, well outside the means of most individuals: learning would otherwise be limited to school or laboratory environments, with time and availability of the resources as perennial adversaries.

We were faced with such a dilemma when organizing a class on TCP/IP routing. Working with a small test network, we wanted to demonstrate various load-balancing scenarios with Routing Information Protocol (RIP) and Open Shortest Path First (OSPF). However, we were limited by the number of Cisco routers on hand. We did have some PCs available, so we began looking for alternative means to simulate Cisco routers using Linux.

Initially, we attempted to set up our test network using the traditional routed and gated daemons, but we quickly realized that their awkward configuration and limited capabilities created more hindrance than value. We decided to look for more modern means of completing our network and fortunately came upon Zebra.

Zebra is TCP/IP routing software that supports BGP-4, BGP-4+, OSPFv2, OSPFv3, RIPv1, RIPv2, and RIPng. It is distributed under the GNU General Public License and runs on Linux as well as other UNIX variants. Zebra is included in most modern distributions as routing software. The latest version, along with documentation, is available at the GNU Zebra Web site (see Resources for a link).

The original Zebra package was written by Kunihiro Ishiguro and Yoshinari Yoshikawa back in 1996. Today, the package is maintained primarily by IP Infusion -- of which Mr. Ishiguro is the CTO -- with the assistance of networking engineers and open source volunteers.

Zebra is unique in its design because it takes a modular approach to the protocols that it manages. Protocols can be enabled or disabled as network requirements dictate.

By far the most useful feature we found with Zebra was its close similarity to the Cisco IOS configuration formats. While there are some differences from IOS, the feel is close enough that network engineers already familiar with IOS will feel very comfortable in the environment.

While Zebra has not yet hit version 1.0 -- the version at the time of this writing is 0.93b -- the quality of the product is adequate for small networks that need a core router. [Please note that this article was written using version 0.93b. Later versions may need to be installed and configured differently. -Ed.]

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Our test platform for Zebra was an old but sturdy ThinkPad X20 running Red Hat Linux 9. The ThinkPad has a built-in Ethernet port, and we added another Ethernet PCMCIA card so it could act as a router. Before proceeding with the Zebra installation, we made sure that both network cards were recognized by Linux and were confirmed to be working.

An RPM for Zebra-0.93b already ships with Red Hat 9. As this was the same version available on the Zebra Web site, we decided to use this instead of downloading and compiling our own. The Zebra RPM installs binaries, scripts, and configuration files as well as the requisite manuals, examples, and documentation files.

The zebra daemon is the actual routing manager that controls the other modules; it also provides the primary point of interaction with the user. This was the first thing we needed to configure, and we did it through the /etc/zebra/zebra.conf file.

The Zebra RPM package includes a complete sample configuration file. However, at a minimum, we really only needed to create a /etc/zebra/zebra.conf file containing the following lines:

hostname speedmetal password zebra enable password zebra

The hostname directive specifies the name of the router whenever you enter interactive configuration mode. It can be any label and does not necessarily have to correspond to the hostname of the machine.

The password directive specifies the password for logging into the interactive Zebra terminal.

The enable password directive specifies the password for a higher level of access to Zebra, when you want to make configuration changes.

Having created the /etc/zebra/zebra.conf file, we could now start the zebra daemon by executing:

# service zebra start

We were now able to enter the Zebra interactive session by telnetting into port 2601 of our machine.

[root@speedmetal zebra]# telnet 127.0.0.1 2601 Trying 127.0.0.1... Connected to 127.0.0.1. Escape character is '^]'. Hello, this is zebra (version 0.93b). Copyright 1996-2002 Kunihiro Ishiguro. User Access Verification Password: zebra speedmetal> enable Password: zebra speedmetal# ? configure Configuration from vty interface copy Copy configuration debug Debugging functions (see also 'undebug') disable Turn off privileged mode command end End current mode and change to enable mode. exit Exit current mode and down to previous mode help Description of the interactive help system list Print command list no Negate a command or set its defaults quit Exit current mode and down to previous mode show Show running system information terminal Set terminal line parameters who Display who is on vty write Write running configuration to memory, network, or terminal speedmetal#

Navigating within the interactive terminal is easy. For hints about the available commands, you can press ? at any time and the options will show on screen. If you're setting up your own Zebra router, this configuration should be very familiar if you have Cisco experience.

At this point, only Zebra was configured and running; none of the other protocols were, as yet. Later, when we get to the meat of the configuration, we'll show you how we did that.

The Multi-Router Looking Glass, or MRLG, written by John Frazier of EnterZone, is a Web-based utility that can be used to display the interfaces and routes recognized by Zebra. MRLG is really nothing more than a Web interface to the Zebra shell with a limited set of commands, but in the course of our testing, we found it to be a quick and useful way to display routes. So, before getting to the configuration of the Zebra protocols, we'll show you how we installed it.

MRLG requires the Net::Telnet Perl package so that it can communicate with the Zebra shell. Unfortunately, this package is not included as part of the stock Red Hat 9 distribution, so we had to download it (see Resources for a link).

Since MRLG runs as a CGI application, we also needed to install a Web server. If you're trying this yourself, you can use the stock httpd RPM that comes with Red Hat 9.

We copied the mrlg.cgi file from /usr/share/doc/zebra-0.93b/tools to /var/www/cgi-bin. Then, we modified line 36 of mrlg.cgi, changing it from:

$url="";

to:

$url="";

We also modified the block from lines 168 to 174 so that it read as follows:

if ($Form{'router'} eq 'router1') { $server = '127.0.0.1'; $login_pass = 'zebra'; $bgpd = "2605"; $zebra = "2601"; $full_tables=1;

To access MRLG, point the browser to .

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Our lab configuration consisted of two Cisco 3620 routers and a ThinkPad X20 with a built-in Ethernet port and a Home-and-Away PCMCIA Ethernet card. The two routers communicated with each other via serial connection, and each router connected to the ThinkPad through Ethernet. This is shown in our connection diagram:

Our first foray into routing with Zebra started with RIP. We installed Zebra on the ThinkPad as described above. Since we needed another network interface on the ThinkPad, we installed a dummy network device like so:

# modprobe dummy
# ifconfig dummy0

We telnetted into the Zebra port to begin configuration. Our dialog with Zebra followed this sequence:

User Access Verification Password: zebra speedmetal> enable Password: zebra speedmetal# configure terminal speedmetal(config)# interface eth0 speedmetal(config-if)# ip address 192.168.2.1/30 speedmetal(config-if)# quit speedmetal(config)# interface eth1 speedmetal(config-if)# ip address 192.168.1.1/30 speedmetal(config-if)# quit speedmetal(config)# interface dummy0 speedmetal(config-if)# ip address 10.0.2.1/24 speedmetal(config-if)# write Configuration saved to /etc/zebra/zebra.conf speedmetal(config-if)# end speedmetal# show run Current configuration: ! hostname speedmetal password zebra enable password zebra ! interface lo ! interface eth0 ip address 192.168.2.1/30 ! interface dummy0 ip address 10.0.2.1/24 ! interface eth1 ip address 192.168.1.1/30 ! ! line vty ! end

Take note that we did not set the IP addresses on the ThinkPad using the ordinary means; instead, we configured them through Zebra. The settings are kept in the configuration file, /etc/zebra/zebra.conf, so every time the Zebra service starts up, these settings will take effect.

The contents of our zebra.conf file, as modified by Zebra, are:

! ! Zebra configuration saved from vty ! 2003/08/20 00:07:51 ! hostname speedmetal password zebra enable password zebra ! interface lo ! interface eth0 ip address 192.168.2.1/30 ! interface dummy0 ip address 10.0.2.1/24 ! interface eth1 ip address 192.168.1.1/30 ! ! line vty !

We are also able to check the status of the interfaces using MRLG by selecting the default, "router1," choosing the radio button "show interface," and clicking "Execute."

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Now that we had set up the network interfaces on our ThinkPad/router, we configured it to work with RIP updates. As we've already mentioned, Zebra implements the routing protocols using separate daemons, so we first had to create a simple configuration file, ripd.conf, for the RIP daemon in /etc/zebra.

hostname speedmetal-rip password zebra enable password zebra

Then we started the ripd daemon:

# service ripd start

That done, we were able to configure the RIP daemon by telnetting into port 2602 of our Zebra router.

User Access Verification Password: zebra speedmetal-rip> enable Password: zebra speedmetal-rip# configure terminal speedmetal-rip(config)# router rip speedmetal-rip(config-router)# network 10.0.0.0/8 speedmetal-rip(config-router)# network 192.168.0.0/16 speedmetal-rip(config-router)# end speedmetal-rip# show run Current configuration: ! hostname speedmetal-rip password zebra enable password zebra ! interface lo ! interface eth0 ! interface dummy0 ! router rip network 0.0.0.0/0 network 192.168.0.0/16 ! line vty ! end speedmetal-rip# write Configuration saved to /etc/zebra/ripd.conf speedmetal-rip#

The resulting ripd.conf configuration file is:

! ! Zebra configuration saved from vty ! 2003/08/19 13:50:30 ! hostname speedmetal-rip password zebra enable password zebra ! interface lo ! interface eth0 ! interface eth1 ! interface dummy0 ! router rip network 10.0.0.0/8 network 192.168.0.0/16 ! line vty !

To facilitate configuration of the two Cisco 3620 routers, which we named "A" and "B," we configured only the basic settings needed to make the routers run properly. This included setting up the interface IP addresses, the loopback address, and the serial clockrates for proper serial port communication.

Router#config terminal Router(config)#hostname RouterA RouterA(config)#int s0/0 RouterA(config-if)#ip address 192.168.0.1 255.255.255.252 RouterA(config-if)#no shut RouterA(config-if)# interface fastEthernet 0/0 RouterA(config-if)#ip address 192.168.2.2 255.255.255.252 RouterA(config-if)#no shut RouterA(config-if)#int loopback 0 RouterA(config-if)#ip address 10.0.0.1 255.255.255.0 RouterA(config-if)#end RouterA#write

We followed a similar procedure for router B.

Router#configure terminal Router(config)#hostname RouterB RouterB(config)#int s0/0 RouterB(config-if)#ip address 192.168.0.2 255.255.255.252 RouterB(config-if)#no shut RouterB(config-if)#int fastEthernet0/0 RouterB(config-if)#ip address 192.168.1.2 255.255.255.252 RouterB(config-if)#no shut RouterB(config-if)#int loopback 0 RouterB(config-if)#ip address 10.0.1.1 255.255.255.0 RouterB(config-router)#end RouterB#write

Setting up RIP on the 3620 routers is very similar to the commands in Zebra. We accessed both of the 3620s through the console cables and issued the following commands:

RouterA#conf t Enter configuration commands, one per line. End with CNTL/Z. RouterA(config)#router rip RouterA(config-router)#network 10.0.0.0 RouterA(config-router)#network 192.168.0.0 RouterA(config-router)#network 192.168.2.0 RouterA(config-router)#version 2 RouterA(config-router)#end RouterA#write

And, on router B:

RouterB#conf t Enter configuration commands, one per line. End with CNTL/Z. RouterB(config)#router rip RouterB(config-router)#network 10.0.1.0 RouterB(config-router)#network 192.168.0.0 RouterB(config-router)#network 192.168.1.0 RouterB(config-router)#version 2 RouterB(config-router)#end RouterB#write

The router rip command turns on the RIP process. The network command tells the router which networks will be propagated by RIP.

Now that the Cisco routers and Zebra were all configured, we checked that the routes were being propagated. From MRLG, we selected "show ip route" and clicked "Execute." This generated the following report:

Codes: K - kernel route, C - connected, S - static, R - RIP, O - OSPF, B - BGP, > - selected route, * - FIB route R>* 10.0.0.0/24 [120/2] via 192.168.2.2, eth0, 00:11:05 R>* 10.0.1.0/24 [120/2] via 192.168.1.2, eth1, 00:02:08 C>* 10.0.2.0/24 is directly connected, dummy0 K * 127.0.0.0/8 is directly connected, lo C>* 127.0.0.0/8 is directly connected, lo R>* 192.168.0.0/30 [120/2] via 192.168.2.2, eth0, 00:11:05 C>* 192.168.1.0/30 is directly connected, eth1 C>* 192.168.2.0/30 is directly connected, eth0

The routes learned through RIP are marked with an R.

Note that Zebra now knew about the networks 10.0.0.0/24 and 10.0.1.0/24, as broadcast by router A and router B. We tested this by pinging 10.0.0.1 and 10.0.1.1 from the ThinkPad Zebra router, and by pinging 10.0.2.1 (the ThinkPad dummy interface) from either router.

To test for route failover, we disconnected the network connection from router A, which led to network 10.0.0.0/24. After a total timeout of about two minutes, Zebra learned about the alternate route to network 10.0.0.0/24, going through router B. Note that in the listing below, Zebra reached 10.0.0.0/24 through 192.168.1.2 instead of its previous path.

Codes: K - kernel route, C - connected, S - static, R - RIP, O - OSPF, B - BGP, > - selected route, * - FIB route R>* 10.0.0.0/24 [120/3] via 192.168.1.2, eth0, 00:00:26 R>* 10.0.1.0/24 [120/2] via 192.168.1.2, eth1, 00:06:02 C>* 10.0.2.0/24 is directly connected, dummy0 K * 127.0.0.0/8 is directly connected, lo C>* 127.0.0.0/8 is directly connected, lo R>* 192.168.0.0/30 [120/2] via 192.168.1.2, eth1, 00:00:26 C>* 192.168.1.0/30 is directly connected, eth1 C>* 192.168.2.0/30 is directly connected, eth0

Why was the total timeout more than two minutes? The default timeout for RIP is 30 seconds, but the RIP protocol specifies three retries (total of 90 seconds) before it determines a route is invalid and another period for flushing the invalid route (another 240 seconds). RIP has generally been known to be slow to responding to connection failures, and this behavior is clearly demonstrated here.

Here is the output showing router A's routing table before failover occurred.

RouterA#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set 10.0.0.0/24 is subnetted, 3 subnets R 10.0.2.0 [120/1] via 192.168.2.1, 00:00:11, FastEthernet0/0 C 10.0.0.0 is directly connected, Loopback0 R 10.0.1.0 [120/1] via 192.168.0.2, 00:00:18, Serial0/0 192.168.0.0/30 is subnetted, 1 subnets C 192.168.0.0 is directly connected, Serial0/0 192.168.1.0/30 is subnetted, 1 subnets R 192.168.1.0 [120/1] via 192.168.0.2, 00:00:18, Serial0/0 [120/1] via 192.168.2.1, 00:00:11, FastEthernet0/0 192.168.2.0/30 is subnetted, 1 subnets C 192.168.2.0 is directly connected, FastEthernet0/0

And after failover:

RouterA#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set 10.0.0.0/24 is subnetted, 3 subnets R 10.0.2.0 [120/2] via 192.168.0.2, 00:00:09, Serial0/0 C 10.0.0.0 is directly connected, Loopback0 R 10.0.1.0 [120/1] via 192.168.0.2, 00:00:09, Serial0/0 192.168.0.0/30 is subnetted, 1 subnets C 192.168.0.0 is directly connected, Serial0/0 192.168.1.0/30 is subnetted, 1 subnets R 192.168.1.0 [120/1] via 192.168.0.2, 00:00:09, Serial0/0 192.168.2.0/30 is subnetted, 1 subnets R 192.168.2.0 [120/2] via 192.168.0.2, 00:00:10, Serial0/0

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Having done RIP, we moved on to OSPF routing. Although OSPF and RIP can be used together, we preferred to work with a simpler configuration and used OSPF exclusively. To disable RIP from Zebra, we simply shut down the ripd service.

# service ripd stop

As before, we started with a basic configuration file for OSPF, in this case, ospfd.conf, still in /etc/zebra.

hostname speedmetal-ospf password zebra enable password zebra

Then, we started the OSPF service:

# service ospfd start

Configuring OSPF is actually simpler than configuring RIP: at the most basic, we only need to tell OSPF to broadcast all the routes it knows.

The port for OSPF configuration is 2604.

Here was our dialog for OSPF configuration.

[root@speedmetal zebra]# telnet 127.0.0.1 2604 User Access Verification Password: zebra speedmetal-ospf> enable Password: zebra speedmetal-ospf# configure terminal speedmetal-ospf(config)# router ospf speedmetal-ospf(config-router)# network 0.0.0.0/0 area 0 speedmetal-ospf(config-router)# end speedmetal-ospf# write Configuration saved to /etc/zebra/ospfd.conf speedmetal-ospf# show run Current configuration: ! hostname speedmetal-ospf password zebra enable password zebra ! ! router ospf network 0.0.0.0/0 area 0 ! line vty ! end speedmetal-ospf#

The resulting configuration file, ospfd.conf, was:

! ! Zebra configuration saved from vty ! 2003/08/19 14:22:17 ! hostname speedmetal-ospf password zebra enable password zebra ! ! ! interface lo ! interface eth0 ! interface eth1 ! interface dummy0 ! router ospf network 0.0.0.0/0 area 0 ! line vty !

To remove RIP from the routers and add OSPF, we executed the following commands:

RouterA#conf term RouterA(config)no router rip RouterA(config)#router ospf 100 RouterA(config-router)#network 0.0.0.0 255.255.255.255 area 0 RouterA(config-router)end

We performed the same steps for both router A and router B.

Our MRLG report looked like this:

Codes: K - kernel route, C - connected, S - static, R - RIP, O - OSPF, B - BGP, > - selected route, * - FIB route O>* 10.0.0.0/32 [110/11] via 192.168.2.2, eth0, 00:00:01 O>* 10.0.1.1/32 [110/11] via 192.168.1.2, eth1, 00:02:53 O 10.0.2.0/24 [110/10] is directly connected, dummy0, 00:03:31 C>* 10.0.2.0/24 is directly connected, dummy0 K * 127.0.0.0/8 is directly connected, lo C>* 127.0.0.0/8 is directly connected, lo O>* 192.168.0.0/30 [110/58] via 192.168.2.2, eth0, 00:00:01 via 192.168.1.2, eth1, 00:00:01 O 192.168.1.0/30 [110/10] is directly connected, eth1, 00:03:21 C>* 192.168.1.0/30 is directly connected, eth1 O 192.168.2.0/30 [110/10] is directly connected, eth0, 00:03:31 C>* 192.168.2.0/30 is directly connected, eth0

Note that the routes to 10.0.0.1/32 and 10.0.1.1/32 are marked with O, indicating that they were learned through OSPF.

When we disconnected the connection from the Zebra router to router A, the routes were automatically updated. MRLG reported the following:

Codes: K - kernel route, C - connected, S - static, R - RIP, O - OSPF, B - BGP, > - selected route, * - FIB route O>* 10.0.0.1/32 [110/59] via 192.168.2.2, eth0, 00:01:10 O>* 10.0.1.1/32 [110/11] via 192.168.1.2, eth1, 00:09:46 O 10.0.2.0/24 [110/10] is directly connected, dummy0, 00:10:24 C>* 10.0.2.0/24 is directly connected, dummy0 K * 127.0.0.0/8 is directly connected, lo C>* 127.0.0.0/8 is directly connected, lo O>* 192.168.0.0/30 [110/58] via 192.168.1.2, eth1, 00:01:10 O 192.168.1.0/30 [110/10[ is directly connected, eth1, 00:10:14 C>* 192.168.1.0/30 is directly connected, eth1 O 192.168.2.0/30 [110/10] is directly connected, eth0, 00:10:24 C>* 192.168.2.0/30 is directly connected, eth0

Due to the link-state nature of OSPF, failover was much faster: the new routes were propagated in under 30 seconds.

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We started out with a simple need to find a substitute Cisco router for our equipment-strapped networking class. Zebra was an intriguing option, one that had received good recommendations from reviewers on the Internet. As our testing shows, Zebra makes an adequate replacement for Cisco routers in simple networking environments, and possibly for some more complex installations as well.

Admittedly, there was some degree of learning in going to Zebra. The use of separate routing daemons and separate configuration files was somewhat confusing at first, but after we had sorted that out, its general affinity to Cisco IOS immediately shone through.

Overall, Zebra makes dynamic routing in a Linux environment a much simpler task. If you're ever in a situation where you need to set up a router quickly on a limited budget, you should give Zebra a try.

• The is the ideal place to locate resources having to do with Zebra.
  
  
• The Multi-Router Looking Glass requires the Perl package, available from CPAN.
  
  
• Iljitsch van Beijnum, author of the O'Reilly BGP book, gives an overview of Zebra in his article "".
  
  
• If you need to learn the basics of building a Linux LAN, read "Setting up a Local Area Network" (developerWorks, February 2001).
  
  
• Going wireless? Read "Building a wireless access point on Linux" (developerWorks, July 2003).
  
  
• More on IBM's networking software can be found in our listing.
  
  
• Find more resources for Linux developers in the developerWorks Linux zone.