Border Gateway Protocol, The Routing Protocol of the Internet - page 2
IGP, OSPF, BGP, As, Oh My!
A great deal of routing on the Internet is said to be policy-based. Sometimes you’ll have an expensive link that you only want to use when necessary, or perhaps you’ll have a link that you can use to send traffic only to certain parties. Many times the BGP attribute “Community” will be used to identify a set of routes. If you want to let your neighbor know some secret information about a route, you can set a community number before you export those routes. These numbers are completely arbitrary, so whatever you send must be agreed upon a priori to have some sort of meaning.
Another important BGP attribute is the Multi-Exit Discriminator (MED). This is used to tell a remote AS that we prefer a specific exit point, even though we may have many. This is very important in iBGP, the IGP version of BGP that will be covered next time.
To get a true sense of how BGP works, it’s important to spend some time talking about the issues that plague the Internet.
First, we have a very big problem with routing table growth. If someone decides to deaggregate a network that used to be a single /16 network, they could potentially start advertising hundreds of new routes. Every router on the Internet will get every new route when this happens. People are constantly pressured to aggregate, or combine multiple routes into a single advertisement. Aggregation isn’t always possible, especially if you want to break up a /19 into two /20’s that will be geographically separate. Routing tables are approaching 200,000 routes now, and for a time they were appearing to grow exponentially.
Second, there is always a concern that someone will “advertise the Internet.” If some large ISP’s customer suddenly decides to advertise everything, and the ISP accepts the routes, all of the Internet’s traffic will be sent to the small customer’s AS. There’s a simple solution to this, and it’s called route filtering. It’s quite simple to set up filters so that your routers won’t accept routes from customers that you aren’t expecting, but many large ISPs will still accept the equivalent of “default” from peers that have no likelihood of being able to provide transit.
Finally, we come to flapping. BGP has a mechanism to “hold down” routes that appear to be flaky. Routes that flap, or come and go, usually aren’t reliable enough to send traffic to. If routes flap frequently, the load on all Internet routes will increase due to the processing of updates every time someone disappears and reappears. Dampening will prevent BGP peers from listening to all routing updates from flapping peers. The amount of time one is in hold-down increases exponentially with every flap. It’s annoying when you have a faulty link, since it can be more than an hour before you can get to many Internet sites, but it is very necessary.
This has been a very quick discussion of BGP; enough to get you thinking the right way about the protocol but is by no means comprehensive. Spend some time reading the RFCs if you’re tasked with operating a BGP router: your peers will appreciate it.
When he's not writing for Enterprise Networking Planet or riding his motorcycle, Charlie Schluting is the Associate Director of Computing Infrastructure at Portland State University. Charlie also operates OmniTraining.net, and recently finished Network Ninja, a must-read for every network engineer.
Article courtesy of Enterprise Networking Planet, originally published June 23, 2006
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