Super-Networking Blog

So I have been doing BGP on Cisco routers for about 6 years or so, it is a really good way of having a redundant ISP connection. Sure it can be a pain to get it setup as some ISPs take a while to get the settings right but once it is setup and tested normally it works like a charm. Typically I run the 2 ISP connections in an active/standby fashion instead of load sharing. This allows you to get a better rate on the backup connection as long as you don’t use them on a regular basis. Not using both connections is easier said then done though, when you setup BGP by default it will choose the best route for the connection so it will use both connections. In the past I have done my best to fix this problem by weighting the connections both in and out so I can choose the best path for outgoing connections and advertise the best path for incoming connections. Doing it in this manner works pretty well but their is always some traffic on the incoming side of the redudnant ISP for destinations that are directly connected to that ISP.

I have never had a problem with this lower amount of traffic before from that 2nd ISP but I was tasked recently with finding a way to bring that traffic down to almost zero. What I found was BGP Conditional Advertisement Feature that Cisco has. I had never heard of this before but sounds like the perfect solution to my problem. Basically what this feature does is allow you to setup a conditional BGP advertisement to the 2nd ISP connection. In this condition you put a route or some other dynamic information you recieve from ISP 1 and your normal advertisments for ISP 2. If this condition sees the information you specified from ISP 1 it will withhold the advertisements from ISP 2. If the information from ISP 1 goes away the condition will then advertise to ISP 2. I have not put this in place yet as I am still looking into aspects of putting it in place. This might cause a longer outage when ISP 1 fails but will solve the problem of incoming traffic on ISP 2 because if I am not advertising at all out ISP 2 I can’t get any traffic from them.

Here is a document on how to configure this feature on you Cisco Router.

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080094309.shtml

I am testing a sweet little UMTS/HSDPA/HSUPA router this week called the Option Globalsurfer III. This is a wi-fi enabled router that will be available on the NCW networks. This is one of the best cellular routers I have ever seen.

Speeds (Maximums not offered everywhere) : Upload 5.76Mbps, Download 7.2Mbps

Features:

Wi-Fi - Supports WPA2

2 Ethernet Ports - 2 Port Ethernet Switch

USB for Storage Server or Printer Server - Can connect a USB hard drive and use as file server or a USB printer for printer sharing

SMS - Send and receive SMS messages from your desktop

Phone - Plug in any landline phone and you are ready to start sending and receiving phone calls

Firewall - Built in firewall

External Antenna Port - Can add external antenna in low signal areas

The best part is it works anywhere you have signal from a NCW network and soon nationwide

So if you are multi-homed on BGP and you want to be able to decide yourself which path incoming and outgoing your traffic will take you will need to make a few changes. By default BGP will try to make the best decision on what path to take. Problem is a lot of time it will just choose the lowest AS number which might not be the best route.

router bgp 11111
neighbor 2.2.2.2 weight 500
neighbor 3.3.3.3 weight 400
neighbor 3.3.3.3 route-map prepend out

ip prefix-list aggregate seq 5 permit 1.1.1.0/24

route-map prepend permit 10
match ip address prefix-list aggregate
set as-path prepend 11111 11111 11111 11111 11111

Under your BGP config on your router add some weight statements, the higher the weight the more preferred route for outgoing traffic. The route-map prepend out statement you will want to put on the neighbor that you don’t want traffic coming in on. Incoming BGP traffic will always try to take the smallest AS number first so if you prepend your AS number onto your neighbors AS path it will seem like the worst path. This will give you the effect of incoming traffic on the neighbor without the prepend router-map.