Frame Relay - 2

This morning we continue our discussion about Frame Relay. Major benefits of FR are the fast transfer rates and low costs associated to it. However, a major drawback comes into play when you are using a hub-and-spoke topology where one router is the central connection point for all other connections. The problem you run into here is when you're using a distance-vector routing protocol (or hybrid protocol, such as EIGRP), split-horizon restricts routing updates from going out the same interface in which they came. That means that your "spokes" won't be able to communicate with each other.

To get around this issue, you either disable split-horizon on your hub router (which is a little bit like skydiving without a backup parachute - you can do it and probably will be OK, but if you're not...yikes!) or by configuring your router to use subinterfaces.

Subinterfaces are logical interfaces on your physical interface that the router sees and treats as separate interfaces. You configure them by adding a "." and a randomly assigned number to the end of the "interface serial 0/0" command, such as "interface serial 0/0.100". That's all it takes to create a subinterface, after which you assign it its own IP address, mask and assign the DLCI. The router then routes traffic accordingly. To do this, all you need to do is enable the appropriate encapsulation type on the physical interface and bring it up. When doing so, it is good to remember that Cisco didn't become the largest internetworking company in the world by giving away its secrets. When you configure the encapsulation type, you can choose either the proprietary "cisco" type or the industry standard "ietf" type. If all you use is Cisco, ther eis not reason to change it from teh default cisco type. If you are using multiple vendors' equipment, go with ietf.

Because serial interfaces have no MAC address, we need to somehow figure out what IP address belongs to which DLCI. If the service provider uses LMI to send a list of DLCI's that are available, the receiving router sends out inverse-arp requests that pretty much say "hello, DLCI. Send me your IP address." The remote router sends its IP address and the receiving router maps the DLCI to the IP address. Perfect. Inverse ARP is the router's automated method for figuring out which IP address goes with which DLCI. However, this method will not work when you are using mutiple PVC's on a single interface because the Inverse ARP requires that all IP addresses be under the physical interface. This causes problems with split-horizon and your routing protocol won't route and at that point you need another answer...

Enter statically mapped DLCI's. In a multipoint interface, you can map each DLCI to a subinterface, getting around the split-horizon issues. It's a little more work for the administrator, but in 3 lines of code you have configured all that is necessary for a statically mapped DLCI and your subinterfaces are all working. Wonderful.

To configure Frame Relay, you need to make sure the LMI type is the same on both ends. It is important to remember that ietf is a frame relay encapsulation type and *not* an LMI type. LMI types are cisco, ansi and q933a. These are the language that your FR routers speak to each other. Again, remember that ietf is an encapsulation type, not an LMI type.

There are four states that a frame relay circuit acn be in: ACTIVE, INACTIVE, DELETED, and STATIC. You view the state of each FR circuit by using the "show frame-relay pvc" command. This gives a table of circuits and their status, where ACTIVE means that the circuit is good and is in normal operation; INACTIVE means that your end is OK and the remote site is having problems, most likely offline or misconfigured; DELETED meaning that your side of the router is incorrectly configured (most likely an incorrect DLCI setting); and STATIC meaning that the circuit was manually entered by the administrator and not automatically discovered.

FR using a multipoint interface is configured when you add the "multipoint" argument on the end of the interface command where you create the subinterface, such as "interface serial 0.10 multipoint" which marks that subinterface as an interface that will hold multiple DLCI's. That's all fine and well, but if you have a hub and spoke topology, your spoke sites will not be able to see each other because split-horizon keeps routing updates from going out the interface on which they came. So you need to turn off split-horizon, or statically map the DLCI's by issuing the "frame-relay map ip 192.168.1.1 100 broadcast" command, where 192.168.1.1 is the remote router's IP address and 100 is the DLCI. The broadcast argument simply tells your interface not to treat the FR circuit like the NBMA network it is and to send broadcast traffic across the line.

When using Point-to-point subinterfaces, you need to create a subinterface for each PVC coming into your router. So, after enabling frame relay encapsulation on the interface, you tell the router it is a point-to-point with the obvious command "interface serial 0.100 point-to-point". After this, you assign an IP address and netmask to it, issue the "frame-relay interface-dlci 100" command and enable a routing protocol. It's as easy as that.

Frame Relay is a complex topic with much more that can be configured. But if my sources are correct, that can all wait until my CCNP blog comes out...