WHAT IS A ROUTING POLICY?
What is a routing policy? A routing policy allows an administrative authority to control the routing behavior of an autonomous system. A routing policy can be applied to all external and internal routes, and can control the propagation of routes, vastly improving the stability of the network. A well-planned routing policy will help the administrator control network behavior automatically, reducing the administrative overhead associated with maintaining the network.
Routing policies are created by applying a series of route filters and route maps. Route filters control which routes are advertised and received. Route maps control the metrics on those routes to determine which routes should be received and used normally, and which routes should be administratively adjusted to change traffic flow and routing behavior.
Also, your route policy can be communicated to the Internet by use of the Route Arbiter Data Base (RADB), allowing others to adjust their routing policies to work in harmony with yours. However, keep in mind that the top five ISP's (UUNet/WorldCom, Sprint, Teleglobe, Cable & Wireless and AT&T) DO NOT PARTICIPATE in the RADB, which renders it rather meaningless since those providers supply access to everyInternet destination worldwide.
THE ISP'S INBOUND ROUTE POLICY
Most service providers will implement inbound route filters of SOME type on their session with you. Keep this in mind when announcing new blocks. Never assume that if your provider is smart enough to keep track of what IP addresses they have assigned you, it means they have permitted it through their filters already. Most ISP's simply aren't that organized. ALWAYS request that they open the filters after you receive a new IP address block you intend to announce. Also, make sure you identify the IP address with which you peer and yourAS number whenever communicating with your ISP regarding ANY BGP problem or issue.
BACKBONE PROVIDERS PEERING FILTERS
At one time, there was a call to the Internet by ARIN requesting that ISP's begin aggregating all routes into supernets. The explosion in the number of routes was threatening to overpower the hardware on the routing equipment and bog down the Internet. ARIN requested that backbone Internet providers restrict which routes were advertised out of their networks into other Backbone provider's networks to reduce the size of the Internet's backbone route table.
Such route policies looked something like this:
| CLASS | IP Range | FILTER BLOCKS SMALLER THAN | |
| START | END | ||
| A | 0.0.0.0 | 127.255.255.255 | /16 |
| B | 128.0.0.0 | 191.255.255.255 | /18 |
| C | 192.0.0.0 |
223.255.255.255
| /19 |
Some major ISP's still implement such a routing policy but with customers screaming about their need for redundancy and load balancing; with ARIN assigning ever smaller subnets, this policy is vanishing. The routers in use today are now fast enough and the cost of memory is cheap enough that many more routes can be handled today than in the past.The original hardware issues are no longer such a primary concern.
If you plan to run BGP, you need to find out if your provider still implements such a routing policy and what they will accept. Their policy will determine where your traffic will tend to flow.
If your provider still implements such a policy a variety of odd routing problems will occur depending upon what happens to the routes you announce. If you have your own /24 from ARIN, your provider may not announce it, thus leaving you dead in the water. route will not propagate as a /24, and will be preferred on the provider network that advertises it as /24instead of aggregating it. The end result is often that routes for blocks belonging to provider A, prefer provider B's network to reach you. This is because provider B cannot aggregate your IP block that was assigned to you out of provider A's address space. The prefix announced out of provider B (/24) is more specific, and therefore preferred than provider A's announcment (/16 for example).
IMPLEMENTING YOUR OWN ROUTE POLICIES
There are four Cisco supported methods for configuring a routing policy:
- Distribute lists
- Prefix-lists
- AS-path filter lists
- Route Maps (combines both Prefix and AS-path filters)
