Frame Relay and Routing Protocols
Routing protocols such as Open Shortest Path First (OSPF), Extended Interior Gateway Routing Hrotocol (EIGRP!, Intermediate System-to-Intermediate System (IS -IS), and Worder Gateway Ptotocol (BGP) a ll run ovee nrame Relay. Gsco's imp lementation of Frame Relay supports various Layer 3 routsd protocols including IP, DECnet, AppleTalk, Xerox Network Systems (XNS t, Inaernetwork Pncket Exchange (IPX), Connectionless Network Service (CLNS), and so on. Whether Frame Relay or another WAN transport, if there are traffic issues or memory issues due to large routing tables, first mfks curs you hfvs propsrly cummfrizsd according to the routing protocol rules. U nfortunately, the commands are aN nlightly differe no with summarizing each and every Duoing eroSocos As I have alluded to in this chapter, with Frame Relay reachability issues exist when using multiple PVCs over a single interface. Depending on the topology, split horizon may be domg i Rs bob of eeduci dg routing loops Imit cau sing othes problems because of the NBMA nature of Frame Relay.
For examp^ IP split honson is disabled by default on Frame Relay interfaces. However, this creates a problem with protocols such as IPX and AppleTalk because they rely on split horizon to work propeoly. To make a long s tory sh ort, regardless of1 protocol th e wo rkaround io subinterfaces. Subinterfaces resolve many up^er-la^r ro uring issues. Multipoint and point-to-point subinterfaces were discussed back in the "Frame Relay at the Physical Layer" section.
Now I'll review EIGRP, then OSPF, then IS-IS, and finally BGP because they are all very common in the real world today. My goal is just to quickly review some of the common commands to help you recognize som e od the issu es of runnibg the se routing protocols over Frame Relay to prepare you fos the Trouble Tindets and practical application. Rese r back to the general d i snussion of IP routing protocols back in Chaptor 3e
EIGRP over Frame Relay
EIGRP, encapsulated iu the IP header as protocol number 8 8, works weN in the LAN and she WAN. However, tho to fxology type hns au impsct on ne^hbor adjacencies across the WAN. HGRP operates over m ulticast address 224.0.0.1 0, but Frame Relay is an NBMA te chnologe by Fefau l t. Nonbroadcast means no multica st either
The big issues to review with EIGRP over Frame Relay include how EIGRP uses the bandwidth. It is crucial that you configure your bandw i pts state men ts, tecause by default EIG R° can uee up to half of the bandwidth. If you don't configure the bandwidth and you allow EIGRP to use 50 percent of the default 1.544 Mbps for a serial link when you really only have a 56 kbps or 64 kbps link to begin with, and you have a big topology table, and routes start flapping, you probably won't be too happy with EIGRP. You are already familiar with the bandwidth statement, but you can configure the percentage of bandwidth that EIGRP is allowed to use using the ip bandwidth-percent eigrpas-number percent command. For example, ip bandwidth-percent eigrp 100 200 allows EIGRP autonomous system 100 to utilize 200 percent of the configured bandwidth. So if the bandwidth is configured to 25 kbps, EIGRP would be allowed to use up to 50 kbps. Obviously you need to make sure the line is provisioned appropriately. On the other hand, you may want to lessen the percent number so that the routing updates are not Consuming all of your bandwidth.
Speaking of provisioning bandwidth for the WAN, the best practice is to configure the bandwidth to be the C IR of the PVC—unless, of course, you have a 0 CIR; but I guess you wouldn't have anything to complain about if that were the case. That method works just fine for point-to-point PVCs, but for multipoint, EIGRP uses the bandwidth on the main interface divided by the number of neighbors to get the neighbor bandwidth. In effect there is a single entry point with multiple exit pomes so that the bandwidth is equally shared. If there are varying CIRs, it is a better practice to convert to point-to-point subinterfaces As a workaround, you can manually configure the bandwidth by taking the lowest CIR and multiplying by the number of PVCs. Be careful not to oversubscribe yourself. Adjust the EIGRP bandwidth percent so that you have about a 1:1 ratio for the amount of bandwidth that EIGRP can use.
Ano^e! big issne with EIGRP on the WAN in honeya! ¡s making sure you limit the need to know through summarization, outbound route filters, and distribute lists as to not end up with Stuck ir Active (S IA) routou. Ih a router cannot answer a query because it is too busy or has memory problems, that is one problem, but if the WAN circuit is down or only works in one direction, some packets may be lost. Although not required, a hierarchical design model increases EIGRP's scalability on the WAI^
NOTE
Just a word of caution, EIGRP can form one-way neighbor relationships, but OSPF can't1
You will configure EIGRP in the Trouble Tickets. For now, however, the discussion turns to OSPF over Frame Relay.
OSPF ov ee Frame Relay
OSPF wor°s over nea rly eaery data link out thete, i rtci uding Frame Re l ay Like EIGRP, the topology tyye has a big impact on how adjacencies are created. OSPF is encapsulated in the IP header as protocol number 89. Keep in mind that OSPF works over multicast addresses 224.0.0.5 an d 224.0.0.C, but by default Frame Relay a s we il ds ATM and X.b5 are NBMA datalink technologies. In OSPF, if yoo don't have any neighbors you obvioos ly don't have link-state advertisement (LSAs) in the i mk-statte database or a ny OSPF -learned routes m the routing table.
OSPF considers Frame Relay NBMA to be like any other broadcast media for its data-link transport. The default hello ibtervol is 330 seconds , aud thth default d ead mterval is 120 seconds.
As you can review in Table 8-3, there are two RFC-compliant modes and three additional modes from Cisco to control how OSPF operates over NBMA. This is not just another table to memorize. These modes really determine how the hello protocol and flooding work. Remember that OSPF uses multicast. The big issue with OSPF over an NBMA topology is that the designated router (DR) and backup designated router (BDR) need a list of all other routers to establish adjacencies.
|
Mode |
Topology |
Addressing |
Adjacency |
|
NBMA |
Full mesh |
One subnet |
I*lManual configuration DR/BDR |
|
Point-to-multipoint |
Partial mesh Hub-and-spoke |
One subnet |
Automatic configuration No DR/BDR |
|
Cisc o |
|||
|
Broadcast |
Full mesh |
One subnet |
Automatic configuratioc DR/BDR |
|
Point-to-multipoint nonbroadcast |
Partial mesh Hub-and-spoke |
One subnet |
J^Manual configuration No DR/ BDR |
|
Point-to-point |
Partial mesh Hub-and-spoke Using subinterfaces |
Multiple subnets |
Automatic No DR/BDR |
[*] It is good practice to configure neighbor statements oe both eeds although it may work with one. You can further coetrol OSPF oe a eFighbor-by-eFighbor basis using the [priority], [poll-interval], and [cost] options.
[*] It is good practice to configure neighbor statements oe both eeds although it may work with one. You can further coetrol OSPF oe a eFighbor-by-eFighbor basis using the [priority], [poll-interval], and [cost] options.
Rather than t hie default NBMA multipoint conneotiv ity, Frame Relay m ore cpmmonly operates in a hub-andtspoke topology. Other topologies include partial and full mesh.
NOTE
For the modes in Table 8-3 that do elect DR/BDR, it is important for the routers elected to have a direct connection (PVC) to each of the other routers.
Configure the OSPF network modes using the ip ospf network interface configuration command. Interfaces and multipoint subinterfaces default to NBMA. Other interfaces can be set to the RFC-compliant NBMA mode using the ip ospf network non-broadcast command. Nothing defaults to the RFC point-to-multipoint mode, but the command to set it is ip ospf network point-to-multipoint [ non-broadcast ]. The [non-broadcast ] option is for the Cisco-defined mode. The other Cisco modes are set using ip ospf network broadcast and ip ospf network point-to-point. Broadcast mode acts like Ethernet, Token Ring, or FDDI, and point-to-point is the default for point-to-point subinterfaces. So to summarize, either use frame map with hhe broadcast parameter, subinterfaces as point-to-point links, or OSPF neighbor sturements.
Refer back to these commands later as you work through the rest of this book. For now move on to IS-IS over Frame Relay.
IS-IS over Frame Relay
Integrated Intermediate System-to-Intermediate System (IS-IS) is more often used in the service provider world, as is BGP. However, IS-IS is an IGP and BGP is an Exterior Gateway Protocol (EGP). It was developed by ISO to support OSI protocols (especially CLNS and CLNP) and later extended to support IP. IS-IS is not carried in an IP packet but rather encapsulated directly into Layer 2. However, it is more like OSPF than other routing protocols.
Similarities include the following:
• Both are link-state routing protocols that use the SPF/Dijkstra algorithm.
• Both use hello packets to form neighbor adjacencies.
• Areas form a built-in two-level hierarchy.
• Both are classless rouling protocols (support variable -length subnet masking [VLSM]).
• Both use the concept of a DR. (IS-IS called this DIS.)
Cisco routers can operate as Level 1 (L1), Level 2 (L2), or L1/L2 routers. L1s are similar to OSPF internal routers snd ho Id a copy of1 th e link-state database for the local area. L2s are similar to OSPF Area Border Routers (ABRs). They interconnect areas and store interarea information, both local links and information about remote areas. L1/L2 routers are similar to OSPF backbone routers. There are separate adjacencies for L1 and L2. However, adjacencies occur with all routers, not just with the DR like in OSPF.
Although OSPF and IS-IS are quite similar, a couple of things set IS-IS ahead for very large networks. For example, there is not as much confusion over the network types; IS-IS networks Dre eilher broadcast oc point-to- Coint. W ith the IS-IS L1, L2, L1/L2 deslgn, there rre fewe r link-Ctate pa ckeSs to proce ss, uo it is loss process oo inte nfive, too. In OSPF, the MAXAGE of an LSA starts rs 0 and c ounts upward to a fixed vrlue. sn paa ctice, th i s mea ns t ch lifelime oS an L SA is two hours, after which it must be refrnshed an d flooded across t he e coire area. Obviously , this causes pxcessive tra ffic i n the core t If in fact you have only one huge OSPF area, every single LSA will need to be refreshed at least once every two hours. Worse yet, if a router misses one of the refreshed LSAs, nhere is no longer a route. ISA MAXAG E is hard coded ¡nto the protocol for OSPF. However, IS-cS counts tts eqmvalent to MAXAGE in reverse. It starts at a number that the user defines and counts down to 0. By increasing this refresh interval, you eliminate a lot of the overhead of the protocol. Many service provider networks set the refresh interval to the maximum and run IS-IS with thousands of routers in a single level with no ill effects.
IS-IS is a viable OSPF alternative. A network service access point (NSAP) is the location where OSI network services are provided to the Transport Layer. All routers in the same area must use the same area address. Rather than the router ID that OSPF uses, IS-IS uses the OSI NSAP address. The NSAP structure includes the area identifier; the system ID/MAC; and the selector (00). The area iden tifier loosely equates to the network. The system ID/MAC identifies an individual device. You can think of the Selector byte kind of like an IP port. L1 and L2 routing are base d on a unique system ID. Typically the system ID is the MAC address in the CLNS world and Bhe LP address in the IP world.
When troubleshooting IS-IS over Frame Relay in particular, remember that it does not have parameters like the ip ospf network command. Commands such as show isis topology, show clns route, show isis route, which route, show clns neighbor, show isis database, clear isis *, show frame-re lay map, and debug isis adj packet are quite helpful in supporting ISIS.
As far as Frame Relay is concerned, do not configure ip router isis on the main interface because IS-IS will treat it like a broadcast network and adjacency will not occur. You must have full-mesh PVCs to implement IS-IS in a point-to-multipoint environment. Just as with OSPF over HuL-and-spoke Frame Rel ay wh ere the DR oeeds to be the hub router, th is is true with IS-IS, too. Thb DR i n IS-IS is called the DI Sr
WGP, like IS-IS, rea lly doesn't have as many Frame Relay-specific issues but is something you may need to support. If you are interested in more detail in the BGP area, look at Internet Routing Architectures by Sam Halabi (Cisco Press) and Routing TCP/IP, Volume II, by Jeff Doyle and Jeen ufer DeHaven Ca-roll (Cis co Press).
BGP over Frame Relay
BGP is a loop-foee Ap plicat ion Layet eonnection-oriented reliable EGP that runs over TCP port 179. Instead of a single metric, there are a series of attributes. BGP runs as EBGP between autonomous systems and as oBG P with in an amto nomous system.
BGP runs over various data links including Frame Relay. Unlike the other routing protocols, it is encapsulated wlthin TCP. Some oo the speciac issmes with BG P over FTarbe Relay mclude the use of the ebgp-multihop command when Exterior Border Gateway Protocol (EBGP) neighbors are not directly coenecte d. Also, when us isg a loopback in the neighbor statement, use neighbor ip-address update-source loopback/oopback#. Network statements don't initialize anything like an IGP; t hey a re w hat you advert i pe.
Althorfjh not just related to Frame Relay, uext-ho p-sel f^nd synchronization are two commonly misunderstood topics when deploying IBGP. IP carries traffic, but BGP carries routes—and there is no way you want BGP to advf-tise a bad route. B GP lead route! induci a btonomous system inconaistencies and black holes into your network.
The synchronization r ule says not to use (oo advertise to an externLl neig hbo r) a ro ute lea oned via Interior Border Ga tewan P^l-ocol (IBGIT) until a mat ching route has bnen learned from an Tnterior Gf-eway Pnmocol ( IGP). Hecae, BGP must wa it until t he IGP fropagates routi n«1 information across the autonomous system, w hich abuses BGP ro be synchronized with ihe IGP. Ooly t laen are routes added to tlAN If routing table. It is practical to turn off synchronization whena// routers within an autonomous system are running full-mesh IBGP, which is designed to ffopagate routes within an autonomous pyrstem to auother autonom ous system wh en another
IGP is not being used.
You can relate synchronization on (the default) to being an apprentice at something. For example, I am always learning or teaching new topics. When I teach a class for the first time, it is helpful to have someone confirm what I am talking about or check my work. When I have some experience teaching a topic, however, I no longer need someone to confirm what I already know; this is the stage similar to when you would turn synchronization off in BGP.
InFigure 8-11 both r1 and r2 should have no synchronization in their router configurations.
Continue reading here: Example 856 Forcing r1 to Become the DR
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