thePacketGeek

a developing networker

ExaBGP and python: Getting Started

21 May 2015 » ExaBGP

I’m really excited about these next few posts. I’ve been doing some research on BGP and automating routing decisions with python, which led to my discovery of ExaBGP. ExaBGP is dubbed “The BGP swiss army knife”, and I’m early in my experimentation with this tool, but it seems to be a very easy way to peer with your BGP routers and control the advertisement of networks.

This post will cover basic setup of ExaBGP and peering with a router, as well as how you can tie in python to present control for the advertisement of routes. The next post will use the Flask web framework to offer a simple HTTP API for adding/removing routes. I hope the following posts will be along the lines of receiving the BGP UPDATE messages from peered routers to monitor and analyze advertised networks.

Step 1: Install ExaBGP

The installation is very simple, you just install via pip in your global python packages, or preferably in a virtualenv (or similar) to not interfere with your other projects:

$ pip3 install exabgp

This should work on OS X and *nix, but I did have some problems trying to run ExaBGP on Windows.

Step 2: Configure your Router

I’m using a Cisco CSR-1000V virtual router instance on the same machine I’m running python/ExaBGP on, but you can also use any router that has IP connectivity to the machine you are running ExaBGP with. Note that my CSR-1000V router has the IP address of 172.16.2.128/24, and my ExaBGP machine has an IP address of 172.16.2.1/24. Here’s my router’s very basic BGP config and local IP addresses.

CRS1#sh run | s i bgp
router bgp 65000
  bgp log-neighbor-changes
  network 10.10.0.0 mask 255.255.255.0
  neighbor 172.16.2.1 remote-as 65000
  neighbor 172.16.2.1 update-source GigabitEthernet1
CRS1#sh ip int bri
Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet1       172.16.2.128   YES NVRAM  up                    up
Loopback0              1.1.1.1         YES NVRAM  up                    up
Loopback10             10.10.0.1       YES NVRAM  up                    up

If you run a `show bgp summary` command, you will see the neighbor in BGP Idle state since our ExaBGP side hasn’t tried peering yet.  I also recommend using `debug bgp all` (only in a lab of course) to monitor the connection status and updates from ExaBGP:

CRS1#show bgp summary | i 172.16.2.1
172.16.2.1      4        65000       0       0        1    0    0 00:01:53 Idle
CRS1#debug bgp all
BGP debugging is on for all address families

Step 3: Configure ExaBGP

Now we can configure ExaBGP to peer with our router using a `conf.ini` file. Explanations for each directive are commented inline:

neighbor 172.16.2.128 {                 # Remote neighbor to peer with
    router-id 172.16.2.1;              # Our local router-id
    local-address 172.16.2.1;          # Our local update-source
    local-as 65000;                    # Our local AS
    peer-as 65000;                     # Peer's AS
}

If we start ExaBGP now, we’ll see the neighbors peer successfully and the ExaBGP process will receive BGP UPDATE messages for any prefixes the CSR router is currently advertising. Here’s what we see when starting ExaBGP:

(exabgp)packetgeek:exabgp mat$ exabgp ./conf.ini
....(configuration load info truncated)
17:01:37 | 27403  | welcome       | Thank you for using ExaBGP
17:01:37 | 27403  | version       | 4.0.2
17:01:37 | 27403  | configuration | performing reload of exabgp 4.0.2-1c737d99
17:01:37 | 27403  | reactor       | loaded new configuration successfully
17:01:37 | 27403  | reactor       | connected to peer-1 with outgoing-1 172.16.2.1-172.16.2.128

And running `show bgp summary` on the router also confirms that the peering is in an Established state:

CRS1#show bgp summary | i Neighbor|172.16.2.1
Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
172.16.2.1      4        65000       6       8       23    0    0 00:03:31        0

Well, that’s certainly a start!

Step 4: Write python Script to add/remove Routes

Using an example from ExaBGP, we’re going to create a very simple python script named `example.py` to advertise two prefixes to our CSR router:

#!/usr/bin/env python3

from __future__ import print_function

from sys import stdout
from time import sleep

messages = [
    'announce route 100.10.0.0/24 next-hop self',
    'announce route 200.20.0.0/24 next-hop self',
]

sleep(5)

#Iterate through messages
for message in messages:
    stdout.write(message + '\n')
    stdout.flush()
    sleep(1)

#Loop endlessly to allow ExaBGP to continue running
while True:
    sleep(1)

The reason this script works is that ExaBGP monitors STDOUT while running and parses for commands such as “announce route …” and “withdraw route …”. Check out this full list of API commands, I’ll be covering several of these in the upcoming articles. Now we just need to update our `conf.ini` file to run this python script upon startup of ExaBGP:

process announce-routes {
    run /path/to/python3 /path/to/example.py;
    encoder json;
}

neighbor 172.16.2.128 {                 # Remote neighbor to peer with
    router-id 172.16.2.1;              # Our local router-id
    local-address 172.16.2.1;          # Our local update-source
    local-as 65000;                    # Our local AS
    peer-as 65000;                     # Peer's AS

    api {
        processes [announce-routes];
    }
}

Here’s the output from exabgp:

(exabgp)packetgeek:exabgp mat$ exabgp ./conf.ini
....(configuration load info truncated)
17:11:52 | 27396  | welcome       | Thank you for using ExaBGP
17:11:52 | 27396  | version       | 4.0.2
17:11:52 | 27396  | configuration | performing reload of exabgp 4.0.2-1c737d99
17:11:52 | 27396  | reactor       | loaded new configuration successfully
17:11:52 | 27396  | reactor       | connected to peer-1 with outgoing-1 172.16.2.1-172.16.2.128
17:11:57 | 27396  | api           | route added to neighbor 172.16.2.128 local-ip 172.16.2.1 local-as 65000 peer-as 65000 router-id 172.16.2.1 family-allowed in-open : 100.10.0.0/24 next-hop self
17:11:58 | 27396  | api           | route added to neighbor 172.16.2.128 local-ip 172.16.2.1 local-as 65000 peer-as 65000 router-id 172.16.2.1 family-allowed in-open : 200.20.0.0/24 next-hop self

And after this finishes, we can confirm the new prefixes on our router by running `show bgp`:

CRS1#sh bgp | b Network
     Network          Next Hop            Metric LocPrf Weight Path
 *>  10.10.0.0/24     0.0.0.0                  0         32768 i
 *>i 100.10.0.0/24    172.16.2.1                    100      0 i
 *>i 200.20.0.0       172.16.2.1                    100      0 i

Congrats! We just used python to inject routes into the BGP and RIB tables on a router. I know this example is a little boring since you have to preconfigure the prefixes and use sleep timers, but I’ll cover how to make this a little more interactive in the next post.