Chapter 17. Dynamic routing - OSPF and BGP

Once your network starts to get really big, or you start to consider 'the internet' as your network, you need tools which dynamically route your data. Sites are often connected to each other with multiple links, and more are popping up all the time.

The Internet has mostly standardized on OSPF (RFC 2328) and BGP4 (RFC 1771). Linux supports both, by way of gated and zebra.

While currently not within the scope of this document, we would like to point you to the definitive works:


Cisco Systems Designing large-scale IP Internetworks


Moy, John T. "OSPF. The anatomy of an Internet routing protocol" Addison Wesley. Reading, MA. 1998.

Halabi has also written a good guide to OSPF routing design, but this appears to have been dropped from the Cisco web site.

For BGP:

Halabi, Bassam "Internet routing architectures" Cisco Press (New Riders Publishing). Indianapolis, IN. 1997.


Cisco Systems

Using the Border Gateway Protocol for interdomain routing

Although the examples are Cisco-specific, they are remarkably similar to the configuration language in Zebra :-)

17.1. Setting up OSPF with Zebra

Please, let me know if any of the following information is not accurate or if you have any suggestions. Zebra is a great dynamic routing software written by Kunihiro Ishiguro, Toshiaki Takada and Yasuhiro Ohara. With Zebra, setting up OSPF is fast an simple, but in practice there's a lot of parameters to tune if you have very specific needs. OSPF stands for Open Shortest Path First, and some of its principal features are:


Networks are grouped by areas, which are interconnected by a backbone area which will be designated as area 0. All traffic goes through area 0, and all the routers in area 0 have routing information about all the other areas.

Short convergence

Routes are propagated very fast, compared with RIP, for example.

Bandwith efficient

Uses multicasting instead of broadcasting, so it doesn't flood other hosts with routing information that may not be of interest for them, thus reducing network overhead. Also, Internal Routers (those which only have interfaces in one area) don't have routing information about other areas. Routers with interfaces in more than one area are called Area Border Routers, and hold topological information about the areas they are connected to.

Cpu intensive

OSPF is based on Dijkstra's Shortest Path First algorithm, which is expensive compared to other routing algorithms. But really is not that bad, since the Shortest Path is only calculated for each area, also for small to medium sized networks this won't be an issue, and you won't even notice.

Link state

OSPF counts with the special characteristics of networks and interfaces, such as bandwith, link failures, and monetary cost.

Open protocol and GPLed software

OSPF is an open protocol, and Zebra is GPL software, which has obvious advantages over propietary software and protocols.

17.1.2. Configuring Zebra

Let's take this network as an example:

|                                   |
|                                                  |
|      Area 0    100BaseTX Switched                |
|     Backbone     Ethernet                        |
  |           |                |              |
  |           |                |              |
  |eth1       |eth1            |eth0          |
  |100BaseTX  |100BaseTX       |100BaseTX     |100BaseTX
  |.1         |.2              |.253          |
 ---------   ------------   -----------      ----------------
 |R Omega|   |R Atlantis|   |R Legolas|      |R Frodo       |
 ---------   ------------   -----------      ----------------
  |eth0         |eth0             |             |          |
  |             |                 |             |          |
  |2MbDSL/ATM   |100BaseTX        |10BaseT      |10BaseT   |10BaseT
------------   ------------------------------------       -------------------------------
| Internet |   |        Area 1      |       | wlan  Area 2|
------------   |         Student network (dorm)   |       |       barcelonawireless     |
               ------------------------------------       -------------------------------
Don't be afraid by this diagram, zebra does most of the work automatically, so it won't take any work to put all the routes up with zebra. It would be painful to maintain all those routes by hand in a day to day basis. The most important thing you must make clear, is the network topology. And take special care with Area 0, since it's the most important. First configure zebra, editing zebra.conf and adapt it to your needs:
hostname omega
password xxx 
enable password xxx
! Interface's description.
!interface lo
! description test of desc.
interface eth1
! Static default route
ip route
log file /var/log/zebra/zebra.log
In Debian, I will also have to edit /etc/zebra/daemons so they start at boot:
Now we have to edit ospfd.conf if you are still running IPV4 or ospf6d.conf if you run IPV6. My ospfd.conf looks like:
hostname omega
password xxx
enable password xxx
router ospf
  network area 0
  network area 1
! log stdout
log file /var/log/zebra/ospfd.log
Here we instruct ospf about our network topology.

17.1.3. Running Zebra

Now, we have to start Zebra; either by hand by typing "zebra -d" or with some script like "/etc/init.d/zebra start". Then carefully watching the ospdfd logs we should see something like:

2002/12/13 22:46:24 OSPF: interface join AllSPFRouters Multicast group.
2002/12/13 22:46:34 OSPF: SMUX_CLOSE with reason: 5   
2002/12/13 22:46:44 OSPF: SMUX_CLOSE with reason: 5
2002/12/13 22:46:54 OSPF: SMUX_CLOSE with reason: 5   
2002/12/13 22:47:04 OSPF: SMUX_CLOSE with reason: 5   
2002/12/13 22:47:04 OSPF: DR-Election[1st]: Backup
2002/12/13 22:47:04 OSPF: DR-Election[1st]: DR
2002/12/13 22:47:04 OSPF: DR-Election[2nd]: Backup
2002/12/13 22:47:04 OSPF: DR-Election[2nd]: DR
2002/12/13 22:47:04 OSPF: interface join AllDRouters Multicast group.
2002/12/13 22:47:06 OSPF: DR-Election[1st]: Backup
2002/12/13 22:47:06 OSPF: DR-Election[1st]: DR
2002/12/13 22:47:06 OSPF: Packet[DD]: Negotiation done (Slave).
2002/12/13 22:47:06 OSPF: nsm_change_status(): scheduling new router-LSA origination
2002/12/13 22:47:11 OSPF: ospf_intra_add_router: Start
Ignore the SMUX_CLOSE message by now, since it's about SNMP. We can see that is the Designated Router and is the Backup Designated Router

We can also interact with the zebra or the ospfd interface by executing:

$ telnet localhost zebra
$ telnet localhost ospfd
Let's see how to view if the routes are propagating, log into zebra and type:
root@atlantis:~# telnet localhost zebra
Connected to atlantis.
Escape character is '^]'.

Hello, this is zebra (version 0.92a).
Copyright 1996-2001 Kunihiro Ishiguro.

User Access Verification

atlantis> show ip route
Codes: K - kernel route, C - connected, S - static, R - RIP, O - OSPF,
       B - BGP, > - selected route, * - FIB route

K>* via, eth1
C>* is directly connected, lo
O [110/10] is directly connected, eth0, 06:21:53
C>* is directly connected, eth0
O [110/10] is directly connected, eth1, 06:21:53
C>* is directly connected, eth1
atlantis> show ip ospf border-routers
============ OSPF router routing table =============
R         [10] area: (, ABR
			   via, eth1
				 [10] area: (, ABR
			   via, eth0
Or with iproute directly:
root@omega:~# ip route dev eth0  proto kernel  scope link  src dev eth1  proto kernel  scope link  src via dev eth1  proto zebra  metric 20 
default via dev eth0  proto zebra 
We can see the zebra routes, that weren't there before. It's really nice to see routes appearing just a few seconds after you start zebra and ospfd. You can check connectivity to other hosts with ping. Zebra routes are automatic, you can just add another router to the network, configure zebra, and voila!

Hint: You can use:

tcpdump -i eth1 ip[9] == 89
To capture OSPF packets for analysis. OSPF ip protocol number is 89, and the protocol field is the 9th octet on the ip header.

OSPF has a lot of tunable parameters, specially for large networks. In further ampliations of the howto we will show some methodologies for fine tunning OSPF.