There can also be a series of route map statements (3) that are identified by their route map name. The statements include line numbers that determine the order in which those statements are read. Each statement has a condition established by the "match" command and/or an action established by the "set" command. The match command can refer to an access list (2) to identify a route based on its network address. A special "AS path access list" can be used to make a match based on the AS number. The AS path access list can use regular expressions to search for patterns.

Route map SET-LOCAL-PREF, highlighted in red, is applied to routing updates coming in from 10.1.4.2. Line 10 looks to the AS path access list to find updates coming from AS 20. If a match is found, a Local Preference of 300 is applied. If no match is found, however, the process proceeds to Line 20, which applies a Local Preference of 300 to the remaining routes.

Two route Maps and all their associated commands.

The NON-TRANSIT-FILTER route map, highlighted in blue, refers to access List 20 to permit networks that start with 172.25 (the internal network) and deny everything else. It prevents Internet advertisements from being readvertised back to the peer at the ISP.

Preparing for Multihoming

Step 2: Design: It's important for you to give some thought to how you'll integrate BGP with the rest of your network. Keep in mind that you'll want to run BGP only at the edge of your network. If you have multiple routers that are connected to the Internet, you can run BGP between them. This is commonly known as iBGP. However, whenever possible, you'll want to isolate your network from the large routing tables and consequently the instability that's common to the Internet. Instead, use default routes that send traffic destined for the Internet to one of your routers running BGP. To direct external traffic from your BGP routers back into your network, run your IGP (interior gateway protocol) on your BGP routers. Just don't redistribute the BGP routes into your IGP.

Step 3: Configuration: BGP is enabled with the "router" command, which references the ASN. The recommended "no synchronization" command lets BGP advertise the internal network without having to wait to hear updates from the internal network. The "network" command indicates which networks on the router are participating in BGP. The "neighbor" command establishes peering with a router in another AS. It's also used to activate other BGP options that are relevant to that neighbor.

Step 4: Basic BGP configuration on Cisco router:

• router bgp 5
• no synchronization
• network 172.25.0.0
• neighbor 172.25.4.2 remote-as 20

What About Quality of Service?

You can set up such assurances on a per-interface basis for traffic leaving your network. For example, with Cisco routers you can set up a "priority list" that will give preferences to different applications based on requested TCP ports. Other vendors have similar features in their equipment.

Traffic entering your network is another story. Your ISP controls such traffic, and unfortunately, most ISPs don't offer such services. But there are a few notable exceptions. Concentric Network Corp. (www.concentric.com) has announced it will provide varying levels of Quality of Service (QoS) with its VPN service. And Concert (www.concert.com), the AT&T and BT joint venture, has announced it will provide three priority levels using the emerging DiffServ standard for classifying packets based on the first six bits of the ToS (type of service) field found in the IP header.

DiffServ, outlined in RFC 2474 and 2475, is backward-compatible with an earlier scheme that uses the first three bits of the ToS field and offers 64 levels of service. However, DiffServ won't guarantee bandwidth. Another standard, RSVP (Resource Reservation Protocol), attempts to reserve a specific level of bandwidth from end to end. If the bandwidth is unavailable, it tries again later. RSVP's advantage is that once the bandwidth is reserved, there's no danger additional traffic will cause problems, because it will be simply refused. On the down side, RSVP doesn't scale very well with a large number of connections because the participating routers must set up and track the state of every connection. DiffServ probably will become the preferred ISP QoS implementation because it can aggregate all connections into the predefined levels of QoS.

Unfortunately, if you think it's taking a long time for ISPs to step up to the plate with QoS services, it's going to take a lot longer to get end-to-end QoS when crossing ISPs. The services mentioned above only will work for traffic that is kept within an ISP's network. An ISP cannot map QoS levels to another ISP and guarantee that they will be followed.