Our company has two upstreams for internet access. Assume that the upstreams are ISP1 and ISP2. The router is using Linux running BGP (Quagga) for dynamic routing between the two upstreams. See the image below for the sample of source-based IP routing topology.
By default the uplink traffic is going through ISP1 for both Cust A and Cust B networks. I would like to set the traffic coming from network 123.45.67.0/24 is going through ISP2. I have set the local preference to make it going to ISP2 but it seems it doesn’t work in Quagga. In Cisco it works well.
To solve this issue, in Linux we can use iproute2 to set source-based IP routing. Here is the step-by-step:
1. Add a custom policy routing table in /etc/iproute2/rt_tables file
echo "200 isp2" >> /etc/iproute2/rt_tables
So the /etc/iproute2/rt_tables file is like below:
# # reserved values # 255 local 254 main 253 default 0 unspec # # local # #1 inr.ruhep 200 isp2
2. Create an IP policy rule for all traffics coming from 123.45.67.0/24
ip rule add from 123.45.67.0/24 lookup isp2
3. Add an ip route to populate the routing table.
According to the image above, the ISP2 IP address 34.56.78.1 is reachable via eth1, so we need to add route as below:
ip route add default via 34.56.78.1 dev eth1 table isp2
Verify the uplink traffics from 123.45.67.0/24 using ‘traceroute’.
4. To make it active on boot, add the ip rule and ip route above in /etc/rc.local file as below:
#!/bin/sh -e # # rc.local # # This script is executed at the end of each multiuser runlevel. # Make sure that the script will "exit 0" on success or any other # value on error. # # In order to enable or disable this script just change the execution # bits. # # By default this script does nothing. # force customer uplink traffic via ISP2 /sbin/ip rule add from 123.45.67.0/24 lookup isp2 /sbin/ip route add default via 34.56.78.1 dev eth1 table isp2 exit 0
Hopefully this post helps you understanding on how to configure source-based IP routing in Linux