Ethernet Basics

It's no surprise that the concepts, protocols, and commands related to Ethernet are a key part of the CCIE Routing and Switching written exam. Almost all campus networks today are built using Ethernet technology. Also, Ethernet technology is moving into the WAN with the emergence of metro Ethernet. Even in an IT world, where technology changes rapidly, you can expect that ten years from now, Ethernet will still be an important part of the CCIE Routing and Switching written and lab exams. For...

ARP and Proxy ARP

You would imagine that anyone getting this far in their CCIE study would already have a solid understanding of the Address Resolution Protocol (ARP, RFC 826). However, proxy ARP (RFC 1027) is often ignored, in part because of its lack of use today. To see how they both work, Figure 5-1 shows an example of each, with Fred and Barney both trying to reach the web server at IP address 10.1.2.200. Figure 5-1 Comparing ARP and Proxy ARP Figure 5-1 Comparing ARP and Proxy ARP Fred follows a normal ARP...

Authentication and Encryption

One area in which OSPFv3 is simpler than OSPFv2, at the protocol operation level, is that it uses Key IPv6's native authentication support rather than implementing its own authentication mechanisms. TopiC OSPFv3 uses Authentication Header (AH), beginning with Cisco IOS Release 12.3(4)T, and Encapsulating Security Payload (ESP) protocols for authentication, beginning with Cisco IOS Release 12.4(9)T. Both of these features require a Crypto feature set in the router. To enable IPv6 OSPF...

Automatic 6to4 Tunnels

Unlike the previous two tunnel types we have discussed, automatic 6to4 tunnels are inherently point-to-multipoint in nature. These tunnels treat the underlying IPv4 network as an NBMA cloud. In automatic 6to4 tunnels, the tunnel operates on a per-packet basis to encapsulate traffic to the i c correct destination thus its point-to-multipoint nature. These tunnels determine the appropriate destination address by combining the IPv6 prefix with the globally unique destination 6to4 border router's...

BGP Messages and Neighbor States

The desired state for BGP neighbors is the established state. In that state, the routers have formed a TCP connection, and they have exchanged Open messages, with the parameter checks having passed. At this point, topology information can be exchanged using Update messages. Table 11-3 lists the BGP neighbor states, along with some of their characteristics. Note that if the IP addresses mismatch, the neighbors settle into an active state. BGP uses four basic messages. Table 11-4 lists the...

Blueprint topics covered in this chapter

This chapter covers the following subtopics from the Cisco CCIE Routing and Switching written exam blueprint. Refer to the full blueprint in Table I-1 in the Introduction for more details on the topics covered in each chapter and their context within the blueprint. Bridge Protocol Data Unit (BRDU) Guard STP Port Roles, Failure Propagation, and Loopguard Operation Rapid Spanning Tree Protocol (RSTP)

CB Policing Concepts

CB Policing is enabled for packets either entering or exiting an interface, or those entering or exiting a subinterface. It monitors, or meters, the bit rate of the combined packets when a packet pushes the metered rate past the configured policing rate, the policer takes action against that packet. The most aggressive action is to discard the packet. Alternately, the policer can simply re-mark a field in the packet. This second option allows the packets through, but if congestion occurs at...

CB Shaping to a Peak Rate

The shape average command has been used in all the examples so far. However, the command shape peak mean-rate is also allowed, which implements slightly different behavior as compared with shape average for the same configured rate. The key actions of the shape peak mean-rate command are summarized as follows It calculates (or defaults) Bc, Be, and Tc the same way as the shape average command. It refills Bc + Be tokens (instead of just Bc tokens) into the token bucket for each time interval....

Checks Before Becoming BGP Neighbors

Similar to IGPs, BGP checks certain requirements before another router may become a neighbor, reaching the BGP established state. Most of the settings are straightforward the only tricky part relates to the use of IP addresses. The following list describes the checks that BGP performs when forming neighbor relationships . 1. The router must receive a TCP connection request with a source address that the router finds Topic in a BGP neighbor command. 2. A router's ASN (on the router bgp asn...

Cisco 3550 and 3560 Switch Ingress Queueing

Cisco 3550 and 3560 switches perform both ingress and egress queuing. The 3550 uses a single FIFO ingress queue as a place to hold frames waiting to be forwarded to the egress interface, so the details are not terribly interesting. The 3560, on the other hand, has two ingress queues, one of which can be configured as a priority queue. This section addresses the details of these features. The 3560 packet scheduler uses a method called shared round-robin (SRR) to control the rates at Topic which...

Cisco 3550 Congestion Avoidance

Catalyst 3550 Gigabit interfaces support a mutually exclusive choice of either WRED or tail-drop logic for managing drops in egress queues. The 3550 Fast Ethernet interfaces do not use WRED or tail drop, but rather use a switch-specific method of managing internal buffers (which is not covered in this book). Cisco 3550 WRED has the same overall strategy as WRED as implemented in Cisco routers but with many differences in implementation details. The key features of Cisco 3550 WRED are as...

Cisco 3550 Switch Egress Queuing

For egress, the 3550 supports four queues per interface, with classification into the queues based on CoS. Scheduling is based on weighted round-robin (WRR) logic, with an optional expedited (priority) queue. The Cisco 3550 uses a relatively simple classification scheme, assuming you consider only what happens when the forwarding decision has been made. These switches make most internal QoS decisions based on an internal DSCP setting. The internal DSCP has been determined when the frame is...

Cisco Group Management Protocol

IGMP helps routers to determine how to distribute multicast traffic. However, IGMP works at Layer 3, and switches do not understand IGMP messages. Switches, by default, flood multicast traffic to all the hosts in a broadcast domain, which wastes bandwidth. Figure 16-15 illustrates the problem. Figure 16-15 Switches Flood Multicast Traffic Figure 16-15 Switches Flood Multicast Traffic -> - IGMP Join 226.6.6.6 -> - Multicast to 226.6.6.6 -> - IGMP Join 226.6.6.6 -> - Multicast to...

Classless and Classful Routing

So far this chapter has reviewed the basic forwarding process for IP packets in a Cisco router. The logic requires matching the packet destination with the routing table, or with the CEF FIB if CEF is enabled, or with other tables for the other options Cisco uses for route table lookup. (Those options include fast switching in routers and NetFlow switching in multilayer switches, both of which populate an optimized forwarding table based on flows, but not on the contents of the routing table.)...

Classless Interdomain Routing

CIDR is a convention defined in RFCs 1517 through 1520 that calls for aggregating routes for multiple classful network numbers into a single routing table entry. The primary goal of CIDR is to improve the scalability of Internet routers' routing tables. Imagine the implications of an Internet router being burdened by carrying a route to every class A, B, and C network on the planet CIDR uses both technical tools and administrative strategies to reduce the size of the Internet routing tables....

Comparing Queuing Tools

Cisco IOS provides a wide variety of queuing tools. The upcoming sections of this chapter describe several different IOS queuing tools, with a brief summary ending the section on queuing. Table 13-2 summarizes the main characteristics of different queuing tools that you will want to keep in mind while comparing each successive queuing tool. Table 13-2 Key Comparison Points for Queuing Tools Table 13-2 Key Comparison Points for Queuing Tools The ability to look at packet headers to choose the...

Comparison of IGMPv1 IGMPv2 and IGMPv3

Table 16-5 compares the important features of IGMPvl, IGMPv2, and IGMPv3. First Octet Value for the Query Message Destination Address for the General Query Joining multicast group address and source address Is Report Suppression Mechanism Available Can Maximum Response Time Be Configured Can a Host Send a Leave Group Message Destination Address for the Leave Group Message Can a Router Send a Group-Specific Query Can a Host Send Source-and Group-Specific Reports Can a Router Send Source- and...

Comparisons Between Cisco 3550 and 3560 Switches

Cisco includes the 3550 and 3560 series switches in the CCIE Routing and Switching lab exam. Cisco is not specific about any particular switch models to expect on the CCIE Routing and Switching written exam. As a result, it is useful to compare the QoS features of the two switches that you may encounter in the lab exam. Table 13-9 summarizes the key differences. (The comparisons listed here assume the Enhanced software image is used on both models of switches.) Table 13-9 Comparison of Cisco...

Configuring OSPFv3 over Frame Relay

In IPv4 Frame Relay networks, you are likely to be familiar with mapping IP addresses to DLCI numbers. i Topic The configuration of frame-relay map statements is much the same in IPv6, but there is a twist It requires two map statements instead ofjust one. One map statement points to the link-local address, and the other points to the unicast address of the next-hop interface. Only the link-local mapping statement requires the broadcast keyword (which actually permits multicast, as there is no...

Configuring Route Maps with the routemap Command

Route maps provide programming logic similar to the If Then Else logic seen in other programming languages. A single route map has one or more route-map commands in it, and routers process route-map commands in sequential order based on sequence numbers. Each route-map command has underlying matching parameters, configured with the aptly named match command. (To match all packets, the route-map clause simply omits the match command.) Each route-map command also has one or more optional set...

Contents

Do I Know This Already Quiz 5 Foundation Topics 8 Ethernet Layer 1 Wiring, Speed, and Duplex 8 RJ-45 Pinouts and Category 5 Wiring 8 Auto-negotiation, Speed, and Duplex 9 CSMA CD 10 Collision Domains and Switch Buffering 10 Basic Switch Port Configuration 12 Ethernet Layer 2 Framing and Addressing 14 Types of Ethernet Addresses 16 Ethernet Address Formats 17 Protocol Types and the 802.3 Length Field 18 Switching and Bridging Logic 19 Foundation Summary 22 Memory Builders 25 Fill in Key Tables...

Context Based Access Control

In some cases, access-list filtering may be enough to control and secure a router interface. However, as attackers have become more sophisticated, Cisco has developed better tools to deal with threats. The challenge, as always, is to make security features relatively transparent to network users while thwarting attackers. CBAC is one of those features. A function of the firewall feature set in Cisco IOS, CBAC takes access-list filtering a step or two farther by providing dynamic inspection of...

Converged Steady State Operation

Example 7-1 shows a few details of R1's operation while all interfaces in Figure 7-1 are up and working. The example lists the basic (and identical) RIP configuration on all four routers configuration will be covered in more detail later in the chapter. As configured, all four routers are using only RIPv2, on all interfaces shown in Figure 7-1. Read the comments in Example 7-1 for explanations of the output. Example 7-1 Steady-State RIP Operation in Figure 7-1 All routers use the same three...

Converging to a New STP Topology

STP logic monitors the normal ongoing Hello process when the network topology is stable when the Hello process changes, STP then needs to react and converge to a new STP topology. When STP has a stable topology, the following occurs 1. The root switch generates a Hello regularly based on the Hello timer. 2. Each non-root switch regularly (based on the Hello timer) receives a copy of the root's Hello on its RP. 3. Each switch updates and forwards the Hello out its Designated Ports. 4. For each...

D Spanning Tree Protocol

Although many CCIE candidates already know STP well, the details are easily forgotten. For instance, you can install a campus LAN, possibly turn on a few STP optimizations and security features out of habit, and have a working LAN using STP without ever really contemplating how STP does what it does. And in a network that makes good use of Layer 3 switching, each STP instance might span only three to four switches, making the STP issues much more manageable but more forgettable in terms of...

Decimal to Binary Conversion Table

This appendix provides a handy reference for converting between decimal and binary formats for the decimal numbers 0 through 255. Feel free to refer to this table when practicing the subnetting problems in Appendix D, IP Addressing Practice, which is on the CD. Although this appendix is useful as a reference tool, note that if you plan to convert values between decimal and binary when doing subnetting-related exam questions, instead of using the shortcut processes that mostly avoid binary math,...

Default Routes

Routers forward packets using a default route when there are no specific routes that match a packet's destination IP address in the IP routing table. Routing protocols can advertise default routes, with each router choosing the best default route to list as that router's gateway of last resort. This section covers how a router can create a default route and then cause an IGP to advertise the default route. In addition to the advertisement of default routes, each router may use one of two...

Definitions

Next, take a few moments to write down the definitions for the following terms subnet, prefix, classless IP addressing, classful IP addressing, CIDR, NAT, IPv4, subnet broadcast address, subnet number, subnet zero, broadcast subnet, subnet mask, private addresses, SLSM, VLSM, Inside Local address, Inside Global address, Outside Local address, Outside Global address, PAT, overloading, quartet Refer to the glossary to check your answers. Further Reading All topics in this chapter are covered to...

Designated Routers on WANs and OSPF Network Types

Using a DR makes good sense on a LAN because it improves LSA flooding efficiency. Likewise, not using a DR on a point-to-point WAN link also makes sense, because with only two routers on the subnet, there is no inefficiency upon which to improve. However, on nonbroadcast multiaccess (NBMA) networks, arguments can be made regarding whether a DR is helpful. So, OSPF includes several options that include a choice of whether to use a DR on WAN interfaces. Cisco router interfaces can be configured...

Do I Know This Already Quiz

Table 1-1 outlines the major headings in this chapter and the corresponding Do I Know This Already quiz questions. Table 1-1 Do I Know This Already Foundation Topics Section-to-Question Mapping_ Table 1-1 Do I Know This Already Foundation Topics Section-to-Question Mapping_ Ethernet Layer 1 Wiring, Speed, and Duplex Ethernet Layer 2 Framing and Addressing In order to best use this pre-chapter assessment, remember to score yourself strictly. You can find the answers in Appendix A, Answers to the...

Dynamically Finding RPs and Using Redundant RPs

In a PIM-SM network, every router must somehow learn the IP address of an RP. A PIM-SM router can use one of the following three methods to learn the IP address of an RP The RP address can be statically configured on all the PIM-SM routers with the Cisco IOS global command ip pim rp-addr topology shown in Figure 17-19. Topic global command ip pim rp-address address. This is the method used for the five-router The Cisco-proprietary Auto-RP protocol can be used to designate the RP and advertise...

EIGRP Configuration Example

Example 8-6 lists the configuration for R1, R2, R4, and R5 from Figure 8-4. The routers were configured based on the following design goals Configure K values to ignore bandwidth. Configure R5 as an EIGRP stub router. Ensure that R2's LAN interface uses a Hello and Hold time of 2 and 6, respectively. Configure R4 to allow 75 percent of interface bandwidth for EIGRP updates. Advertise R4's LAN subnet, but do not attempt to send or receive EIGRP updates on the LAN. Example 8-6 Basic EIGRP...

EIGRP Offset Lists

EIGRP offset lists allow EIGRP to add to a route's metric, either before sending an update, or for routes received in an update. The offset list refers to an ACL (standard, extended, or named) to match the routes any matched routes have the specified offset, or extra metric, added to their metrics. Any routes not matched by the offset list are unchanged. The offset list also specifies which routing updates to examine by specifying a direction (in or out) and, optionally, an interface. If the...

EIGRP Route Filtering

Outbound and inbound EIGRP updates can be filtered at any interface, or for the entire EIGRP process. To filter the routes, the distribute-list command is used under router eigrp asn, referencing an IP ACL. The generic command, when creating an EIGRP distribution list that uses an ACL, is distribute-list access-list-number I name in I out interface-type interface-number Example 8-8 shows an inbound distribution list on router R2 (in the example in Figure 8-1), filtering routes in the...

Enabling and Configuring OSPFv3

., Enabling OSPFv3 on a Cisco router is straightforward if you have a good grasp of OSPFv2. Once Kcv Topic basic IPv6 addressing and reachability are configured and working, the OSPFv3 configuration process includes these steps Step 1 Identify the desired links connected to each OSPFv3 router. Step 2 Determine the OSPF area design and the area to which each router link (interface) should belong. Step 3 Identify any special OSPF routing requirements, such as stub areas, address summarization,...

Enabling RIP and the Effects of Autosummarization

Example 7-4 covers basic RIP configuration, the meaning and implication of the RIP network command, and the effects of the default setting for autosummarization. To examine just those functions, Example 7-4 shows the related RIP configuration on R1, R2, and R6, along with some command output. Example 7-4 Basic RIP Configuration on R1, R2, R4, and S1 First, the three lines of configuration are the same on R1 and S1 (Point 1) the version 2 command tells R1 to send and receive only RIPv2 updates,...

External BGP Neighbors

The physical topology between eBGP peers is often a single link, mainly because the connection is between different companies in different autonomous systems. As a result, eBGP peering can simply use the interface IP addresses for redundancy, because if the link fails, the TCP connection will fail because there is no longer an IP route between the peers. For instance, in Figure 11-2, the R1-R6 eBGP peering uses interface IP addresses defined in the neighbor commands. When IP redundancy exists...

Foundation Summary

This section lists additional details and facts to round out the coverage of the topics in this chapter. Unlike most of the Cisco Press Exam Certification Guides, this Foundation Summary does not repeat information presented in the Foundation Topics section of the chapter. Please take the time to read and study the details in the Foundation Topics section of the chapter, as well as review items noted with a Key Topic icon. Table 1-8 lists the different types of Ethernet and some distinguishing...

Foundation Topics

You must know IPv4 addressing intimately to even reach this point in your CCIE study efforts. This chapter takes advantage of that fact to help you better learn about IPv6 addressing by making comparisons between IPv4 and IPv6. But first, you need to briefly explore why we need IPv6 or, more precisely perhaps, why we will need it in the future. IPv6 was created to meet the need for more host addresses than IPv4 can accommodate a lot more. In the early 1990s, when the number of...

Frame Relay Configuration Basics

Two of the most important details regarding Frame Relay configuration are the association of DLCIs with the correct interface or subinterface, and the mapping of L3 addresses to those DLCIs. Interesting, both features can be configured using the same two commands the frame-relay map and frame-relay interface-dlci commands. Chapter 6 already covered the details of mapping L3 addresses to DLCIs using InARP and static mapping. (If you have not reviewed those Table 15-3 summarizes some of the key...

Frame Relay Congestion De Becn and FECN

FR networks, like any other multiaccess network, create the possibility for congestion caused by speed mismatches. For instance, imagine an FR network with 20 remote sites with 256-kbps links, and one main site with a T1 link. If all 20 remote sites were to send continuous frames to the main site at the same time, about 5 Mbps of data would need to exit the FR switch over the 1.5-Mbps T1 connected to the main router, causing the output queue on the FR switch to grow. Similarly, when the main...

Frame Relay Fragmentation

Frame Relay Forum IA 12, or FRF.12, defines a standard method of performing LFI over a Frame Relay PVC. Cisco IOS supports two methods for configuring FRF.12. The legacy FRF.12 configuration requires FRTS to be configured, and requires a queuing tool to be applied to the shaped packets. (Example 14-7 in Chapter 14 shows an FRTS map-class shape-with-LLQ command that shapes and applies LLQ.) Figure 15-5 shows the overall logic of how FRF.12 interleaves packets using LFI, when configured using...

Frame Relay Payload Compression

Cisco IOS software supports three options for payload compression on Frame Relay VCs packet-by-packet, data-stream, and Frame Relay Forum Implementation Agreement 9 (FRF.9). FRF.9 is the only standardized protocol of the three options. FRF.9 compression and data-stream compression function basically the same way the only real difference is that FRF.9 implies compatibility with non-Cisco devices. All three FR compression options use LZS as the compression algorithm, but one key difference...

Frame Relay Traffic Shaping Configuration

Frame Relay Traffic Shaping (FRTS) differs from CB Shaping in several significant ways, although the underlying token-bucket mechanics are identical. The following list highlights some of the key similarities and differences FRTS can be used only on Frame Relay interfaces, whereas CB Shaping can be used with any Topic underlying data link protocol. Like CB Shaping, FRTS allows a large number of IOS queuing tools to be used instead of a single FIFO shaping queue. Unlike CB Shaping, FRTS does not...

Further Reading

The topics in this chapter tend to be covered in slightly more detail in CCNP Switching exam preparation books. For more details on these topics, refer to CCNP BCMSN Official Exam Certification Guide, Fourth Edition, and Authorized Self-Study Guide Building Cisco Multilayer Switched Networks (BCMSN), Fourth Edition. Cisco LAN Switching, by Kennedy Clark and Kevin Hamilton, covers STP logic and operations in detail. MSTP, PVST+, and Rapid PVST+ (RPVST+) configuration are covered in the...

General Layer 2 Security Recommendations

Recall that the beginning of the Layer 2 Security section outlined the Cisco SAFE Blueprint recommendations for user and unused ports and some general recommendations. The general recommendations include configuring VTP authentication globally on each switch, putting unused switch ports in an unused VLAN, and simply not using VLAN 1. The underlying configuration for each of these general recommendations is covered in Chapter 2. Additionally, Cisco recommends not using the native VLANs on...

Graceful Restart

In steady-state operation, OSPF can react to changes in the routing domain and reconverge quickly. This is one of OSPF's strengths as an IGP. However, what happens when something goes really wrong is just as important as how things work under relatively stable conditions. One of those really wrong things that sometimes happens is that a router requires a restart to its OSPF software process. To prevent various routing problems, including loops, that can take place when an OSPF router suddenly...

Hellos Neighbors and Adjacencies

After a router has been configured for EIGRP, and its interfaces come up, it attempts to find neighbors by sending EIGRP Hellos (destination 224.0.0.10). Once a pair of routers have heard each other say Hello, they become adjacent assuming several key conditions are met. Once neighbors pass the checks in the following list, they are considered to be adjacent. At that point, they can exchange routes and are listed in the output of the show ip eigrp neighbor command. Neighbors should always form...

How WRED Weights Packets

WRED gives preference to packets with certain IPP or DSCP values. To do so, WRED uses different traffic profiles for packets with different IPP and DSCP values. A WRED traffic profile consists of a setting for three key WRED variables the minimum threshold, the maximum threshold, and the MPD. Figure 13-6 shows just such a case, with two WRED traffic profiles (for IPP 0 and IPP 3). As Figure 13-6 illustrates, IPP 3's minimum threshold was higher than for IPP 0. As a result, IPP 0 traffic will be...

IGMP Version

In October 2002, RFC 3376 defined specifications for IGMPv3, which is a major revision of the protocol and is very complex. To use the new features of IGMPv3, last-hop routers have to be updated, host operating systems have to be modified, and applications have to be specially designed and written. At the time of this writing (mid-2007), a limited number of IGMPv3 applications are available. Therefore, this section does not examine IGMPv3 in detail instead, it summarizes IGMPv3's major...

IGMPvl and IGMPv2 Interoperability

IGMPv2 is designed to be backward compatible with IGMPv1. RFC 2236 defines some special interoperability rules. The next few sections explore the following interoperability scenarios IGMPv2 Host and IGMPv1 Routers Defines how an IGMPv2 host should behave in the presence of an IGMPv1 router on the same subnet. IGMPv1 Host and IGMPv2 Routers Defines how an IGMPv2 router should behave in the presence of an IGMPv1 host on the same subnet. When a host sends the IGMPv2 Report with the message type...

IGP Route Redistribution Route Summarization and Default Routing

This chapter covers several topics related to the use of multiple IGP routing protocols. IGPs can use default routes to pull packets toward a small set of routers, with those routers having learned routes from some external source. IGPs can use route summarization with a single routing protocol, but it is often used at redistribution points between IGPs as well. Finally, route redistribution by definition involves moving routes from one routing source to another. This chapter takes a look at...

Input Events and Local Computation

An EIGRP router needs to react when an input event occurs. The obvious input events are when a router learns of new prefixes via newly received routing updates, when an interface fails, or when a neighbor fails. Because EIGRP sends updates only as a result of changed or new topology information, a router must consider the update and decide if any of its routes have changed. When an input event implies that a route has failed, the router performs local computation, a fancy term for a process...

Internal BGP Neighbors

A BGP router considers each neighbor to be either an internal BGP (iBGP) peer or an external BGP (eBGP) peer. Each BGP router resides in a single AS, so neighbor relationships are either with other routers in the same AS (iBGP neighbors) or with routers in other autonomous systems (eBGP neighbors). The two types of neighbors differ only slightly in regard to forming neighbor relationships, with more significant differences in how the type of neighbor (iBGP or eBGP) impacts the BGP update...

Internet Group Management Protocol

IGMP has evolved from the Host Membership Protocol, described in Dr. Steve Deering's doctoral thesis, to IGMPvl (RFC 1112), to IGMPv2 (RFC 2236), to the latest, IGMPv3 (RFC 3376). IGMP messages are sent in IP datagrams with IP protocol number 2, with the IP Time-to-Live (TTL) field set to 1. IGMP packets pass only over a LAN and are not forwarded by routers, due to their TTL field values. The two most important goals of IGMP are as follows To inform a local multicast router that a host wants to...

IP Forwarding Routing

Chapter 6 begins the largest part of the book. This part of the book, containing Chapters 7 through 11, focuses on the topics that are the most important and popular for both the CCIE Routing and Switching written and practical (lab) exams. Chapter 6 begins with coverage of the details of the forwarding plane the actual forwarding of IP packets. This process of forwarding IP packets is often called IP routing, or simply routing. Also, many people also refer to IP routing as the data plane,...

IP Multicast Routing

In Chapter 16, Introduction to IP Multicasting, you learned how a multicast router communicates with hosts and then decides whether to forward or stop the multicast traffic on a subnet. But how does a multicast router receive the group traffic How is the multicast traffic forwarded from a source so that all the group users receive it This chapter provides answers to those questions. This chapter first defines the multicast routing problem by identifying the difference between unicast and...

IP Precedence and DSCP Compared

The IP header is defined in RFC 791, including a 1-byte field called the Type of Service (ToS) byte. The ToS byte was intended to be used as a field to mark a packet for treatment with QoS tools. The ToS byte itself was further subdivided, with the high-order 3 bits defined as the IP Precedence (IPP) field. The complete list of values from the ToS byte's original IPP 3-bit field, and the corresponding names, is provided in Table 12-2. Table 12-2 IP Precedence Values and Names Table 12-2 IP...

IP Prefix Lists

IP prefix lists provide mechanisms to match two components of an IP route The route prefix (the subnet number) The prefix length (the subnet mask) The redistribute command cannot directly reference a prefix list, but a route map can refer to a prefix list by using the match command. A prefix list itself has similar characteristics to a route map. The list consists of one or more statements with the same text name. Each statement has a sequence number to allow deletion of individual commands,...

Pv6 Address Notation

Because of the length of IPv6 addresses, it is impractical to represent them the same way as IPv4 addresses. At 128 bits, IPv6 addresses are four times the length of IPv4 addresses, so a more efficient way of representing them is called for. As a result, each of the eight groups of 16 bits in an IPv6 address is represented in hex, and these groups are separated by colons, as follows In IPv6, as in IPv4, unicast addresses have a two-level network host hierarchy (known in IPv6 as the prefix and...

ISL and 8021Q Concepts

If two devices are to perform trunking, they must agree to use either ISL or 802.1Q, because there are several differences between the two, as summarized in Table 2-7. 1ISL originally supported only normal-range VLANs, but was later improved to support extended-range VLANs as well. ISL and 802.1Q differ in how they add a header to the Ethernet frame before sending it over a trunk. ISL adds a new 26-byte header, plus a new trailer (to allow for the new FCS value), encapsulating the original...

LANSpecific Issues with PIMDM and PIMSM

This section covers three small topics related to operations that only matter when PIM is used on LANs Both PIM-DM and PIM-SM use these features in the same way. Prune Override In both PIM-DM and PIM-SM, the Prune process on multiaccess networks operates differently from how it operates on point-to-point links. The reason for this difference is that when one router sends a Prune message on a multiaccess network, other routers might not want the link pruned by the upstream router. Figure 17-11...

Local Management Interface

Local Management Interface (LMI) messages manage the local access link between the router and the Frame Relay switch. A Frame Relay DTE can send an LMI Status Enquiry message to the switch the switch then replies with an LMI Status message to inform the router about the DLCIs of the defined VCs, as well as the status of each VC. By default, the LMI messages flow every 10 seconds. Every sixth message carries a full Status message, which includes more complete status information about each VC....

Management and Avoidance

Congestion management, commonly called queuing, refers to how a router or switch manages packets or frames while they wait to exit a device. With routers, the waiting occurs when IP forwarding has been completed, so the queuing is always considered to be output queuing. LAN switches often support both output queuing and input queuing, where input queuing is used for received frames that are waiting to be switched to the switch's output interfaces. Congestion avoidance refers to the logic used...

Managing Distribution of Multicast Traffic with IGMP

NOTE The current CCIE Routing and Switching blueprint (v3) specifically includes IGMPv2 but not IGMPv1. For perspective, however, this section of the chapter touches on IGMPv1. Refer to Figure 16-4. Assume that R2 has started receiving multicast traffic from the server. R2 has to make a decision about forwarding this traffic on the Ethernet links. R2 needs to know the answers to the following questions Is there any host connected to any of my Ethernet links that has shown interest in receiving...

Manually Configured Tunnels

This tunnel type is point-to-point in nature. Cisco IOS requires statically configuring the destination addresses of these tunnels. Configuring a manual IPv6 over IPv4 tunnel is almost identical to configuring an IPv4 GRE tunnel the only difference is setting the tunnel mode. Example 20-14 and Figure 20-7 show a manually configured tunnel. IPv4 reachability has already been configured and verified, but is not shown. Figure 20-7 Manually Configured Tunnel Example 20-14 Manual Tunnel...

Marking Using Policers

Traffic policers measure the traffic rate for data entering or exiting an interface, with the goal of determining if a configured traffic contract has been exceeded. The contract has two components a traffic rate, configured in bits second, and a burst size, configured as a number of bytes. If the traffic is within the contract, all packets are considered to have conformed to the contract. However, if the rate or burst exceeds the contract, then some packets are considered to have exceeded the...

Mechanics of the redistribute Command

The redistribute router subcommand tells one routing protocol to take routes from another routing protocol. This command can simply redistribute all routes or, by using matching logic, redistribute only a subset of the routes. The redistribute command also supports actions for setting some parameters about the redistributed routes for example, the metric. The full syntax of the redistribute command is as follows redistribute protocol process-id level-1 I level-1-2 I level-2 as-number metric...

MLS Configuration

The upcoming MLS configuration example is designed to show all of the configuration options. The network design is shown in Figures 6-4 and 6-5. In Figure 6-4, the physical topology is shown, with routed ports, VLAN trunks, a routed PortChannel, and access links. Figure 6-5 shows the same network, with a Layer 3 view of the subnets used in the network. Figure 6-4 Physical Topology Example Using MLS Figure 6-4 Physical Topology Example Using MLS Figure 6-5 Layer 3 Topology View Example Using MLS...

Mpls Ip Forwarding Control Plane

For pure IP routing to work using the FIB, routers must use control plane protocols, like routing protocols, to first populate the IP routing table and then populate the CEF FIB. Similarly, for MPLS forwarding to work, MPLS relies on control plane protocols to learn which MPLS labels to use to reach each IP prefix, and then populate both the FIB and the LFIB with the correct labels. MPLS supports many different control plane protocols. However, an engineer's choice of which control plane...

Multicast Open Shortest Path First

MOSPF is defined in RFC 1584, Multicast Extensions to OSPF, which is an extension to the OSPFv2 unicast routing protocol. The basic operation of MOSPF is described here MOSPF uses the group membership LSA, Type 6, which it floods throughout the originating router's area. As with unicast OSPF, all MOSPF routers in an area must have identical linkstate databases so that every MOSPF router in an area can calculate the same SPT. The SPT is calculated on-demand, when the first multicast packet for...

Multiple Spanning Trees IEEE 8021s

IEEE 802.1s Multiple Spanning Trees (MST), sometimes referred to as Multiple Instance STP (MISTP) or Multiple STP (MSTP), defines a way to use multiple instances of STP in a network that uses 802.1Q trunking. The following are some of the main benefits of 802.1s Like PVST+, it allows the tuning of STP parameters so that while some ports block for one VLAN, the same port can forward in another VLAN. Always uses 802.1w RSTP, for faster convergence. Does not require an STP instance for each VLAN...

NonIP Header Marking Fields

As IP packets pass through an internetwork, the packet is encapsulated in a variety of other headers. In several cases, these other headers have QoS fields that can be used for classification and marking. Ethernet supports a 3-bit QoS marking field, but the field only exists when the Ethernet header includes either an 802.1Q or ISL trunking header. IEEE 802.1Q defines its QoS field as the 3 most-significant bits of the 2-byte Tag Control field, calling the field the user-priority bits. ISL...

Operation of Protocol Independent Multicast Dense Mode

Protocol Independent Multicast (PIM) defines a series of protocol messages and rules by which routers can provide efficient forwarding of multicast IP packets. PIM previously existed as a Cisco-proprietary protocol, although it has been offered as an experimental protocol via RFCs 2362, 3446, and 3973. The PIM specifications spell out the rules mentioned in the earlier examples in this chapter things like the RPF check, the PIM dense-mode logic of flooding multicasts until routers send Prune...

OSPF Configuration

This section covers the core OSPF configuration commands, along with the OSPF configuration topics not already covered previously in the chapter. (If you happened to skip the earlier parts of this chapter, planning to review OSPF configuration, make sure to go back and look at the earlier examples in the chapter. These examples cover OSPF stubby area configuration, OSPF network types, plus OSPF neighbor and priority commands.) Example 9-8 shows configuration for the routers in Figure 9-6, with...

OSPF Design Terms

OSPF design calls for grouping links into contiguous areas. Routers that connect to links in different areas are Area Border Routers (ABRs). ABRs must connect to area 0, the backbone area, and one or more other areas as well. Autonomous System Boundary Routers (ASBRs) inject routes external to OSPF into the OSPF domain, having learned those routes from wide-ranging sources from the Border Gateway Protocol (BGP) on down to simple redistribution of static routes. Figure 9-5 shows the terms in the...

OSPFv3 LSA Types

Most LSA functionality in OSPFv3 is the same as that in OSPFv2, with a few changes in the LSA names. In addition, OSPFv3 has two additional LSA types. Table 20-5 briefly describes each of the LSA types in OSPFv3. Compare this table to Table 9-4 for a better perspective on how OSPFv2 and OSPFv3 LSA types are similar to and different from each other. Note that OSPFv3 LSA types are basically the same as OSPFv2 LSAs, except for their slightly different names and the additions of type 8 and 9 LSAs...

Other MPLS Applications

This last relatively short section of the chapter introduces the general idea about the protocols used by several other MPLS applications. To that end, this section introduces and explains the concept of a Forwarding Equivalence Class (FEC) and summarizes the concept of an FEC as used by various MPLS applications. Frankly, this chapter has already covered all the concepts surrounding the term FEC. However, it Topic is helpful to know the term and the FEC concept as an end to itself, because it...

Overview of Multicast Routing Protocols

Routers can forward a multicast packet by using either a dense-mode multicast routing protocol or a sparse-mode multicast routing protocol. This section examines the basic concepts of multicast forwarding using dense mode, the Reverse Path Forwarding (RPF) check, and multicast forwarding using sparse mode, all of which help to solve the multicast routing problem. Multicast Forwarding Using Dense Mode Dense-mode routing protocols assume that the multicast group application is so popular that...

PerVLAN Spanning Tree and STP over Trunks

If only one instance of STP was used for a switched network with redundant links but with multiple VLANs, several ports would be in a blocking state, unused under stable conditions. The redundant links would essentially be used for backup purposes. The Cisco Per VLAN Spanning Tree Plus (PVST+) feature creates an STP instance for each VLAN. By tuning STP configuration per VLAN, each STP instance can use a different root switch and have different interfaces block. As a result, the traffic load...

Policy Routing

All the options for IP forwarding (routing) in this chapter had one thing in common The destination IP address in the packet header was the only thing in the packet that was used to determine how the packet was forwarded. Policy routing allows a router to make routing decisions based on information besides the destination IP address. Policy routing's logic begins with the ip policy command on an interface. This command tells IOS to process incoming packets with different logic before the normal...

Port Channels

When a network design includes multiple parallel segments between the same pair of switches, one switch ends up in a forwarding state on all the links, but the other switch blocks all but one of the ports of those parallel segments. As a result, only one of the links can be used at any point in time. Using Fast EtherChannel (FEC) (using FastE segments) and Gigabit EtherChannel (GEC) (using GigE segments) allows the combined links to be treated as one link from an STP perspective, so that all...

Port Fast Uplink Fast and Backbone Fast

The Cisco-proprietary PortFast, UplinkFast, and BackboneFast features each solve specific STP problems. Table 3-5 summarizes when each is most useful, and the short version of how they improve convergence time. Table 3-5 PortFast, UplinkFast, and BackboneFast_ Table 3-5 PortFast, UplinkFast, and BackboneFast_ Used on access ports that are not connected to other switches or hubs Immediately puts the port into forwarding state once the port is physically working Used on access layer switches that...

Queuing Tools CBWFQ and LLQ

This section hits the highlights of the modern queuing tools in Cisco IOS and covers detailed configuration for the more popular tools specifically class-based weighted fair queuing (CBWFQ) and low-latency queuing (LLQ). Because the CCIE Routing and Switching exam blueprint no longer includes the priority queuing (PQ) and custom queuing (CQ) legacy queuing methods, they are not covered in this book. Furthermore, WFQ is covered only in the context of CBWFQ and not as a standalone feature. Cisco...

Rapid Spanning Tree Protocol

IEEE 802.1w Rapid Spanning Tree Protocol (RSTP) enhances the 802.1d standard with one goal in mind improving STP convergence. To do so, RSTP defines new variations on BPDUs between switches, new port states, and new port roles, all with the capability to operate backwardly compatible with 802.1d switches. The key components of speeding convergence with 802.1w are as follows Waiting for only three missed Hellos on an RP before reacting (versus ten missed Hellos via the Maxage timer with 802.1d)...

References in This Chapter

Beau Williamson, Developing IP Multicast Networks, Volume I, Cisco Press, 2000 (Chapter 3) IGMPv1-IGMPv2 Interoperability, pages 73-76 Multicast in a Campus Network CGMP and IGMP Snooping (Document ID 10559), Router-Port Group Management Protocol, http www.cisco.com univercd cc td

RIP Configuration

This chapter does not go into detail on configuring RIPv2. However, make sure to review the list of RIPv2 configuration commands, and command syntax, listed in Table 7-6 of the Foundation Summary section for this chapter. Figure 7-2 shows the internetwork that will be used to illustrate RIP configuration concepts in Example 7-4. Note that most of the subnets are part of network 172.31.0.0, except where noted. Figure 7-2 Sample Internetwork Used for RIP Configuration Examples Network 172.31.0.0,...

RIP Convergence and Loop Prevention

The most interesting and complicated part of RIP relates to loop-prevention methods used during convergence after a route has failed. Some protocols, like OSPF, IS-IS, and EIGRP, include loop prevention as a side effect of their underlying route computations. However, RIP, like other distance vector protocols, uses several loop-prevention tools. Unfortunately, these loop-prevention tools also significantly increase convergence time a fact that is certainly the biggest negative feature of RIP,...

RIP Version

Chapters 7 through 9 and 11 each focus on a single routing protocol. This chapter covers Routing Information Protocol (RIP) Version 2, including most of the features, concepts, and commands. Chapter 10, IGP Route Redistribution, Route Summarization, and Default Routing, covers some RIP details, in particular, route redistribution between RIP and other routing protocols, and route summarization. Although RIPv2 has been removed from the latest version of the CCIE Routing and Switching...

Route Summarization Concepts

The ability to recognize and define how to most efficiently summarize existing address ranges is an important skill on both the written and lab exams. For the written exam, the question may not be as straightforward as, What is the most efficient summarization of the following subnets Rather, the math required for such a question might simply be part of a larger question. Certainly, such math is required for the lab exam. This section looks at the math behind finding the best summarization...

Router Port Group Management Protocol

RGMP is a Layer 2 protocol that enables a router to communicate to a switch which multicast group traffic the router does and does not want to receive from the switch. By being able to restrict the multicast destinations that a switch forwards to a router, a router can reduce its overhead. In fact, RGMP was designed to help routers reduce overhead when they are attached to high-speed LAN backbones. Although RGMP is Cisco proprietary, oddly enough it cannot work concurrently with...

SNMP Protocol Messages

The SNMPv1 and SNMPv2 protocol messages (RFC 3416) define how a manager and agent, or even two managers, can communicate information. For instance, a manager can use three different messages to get MIB variable data from agents, with an SNMP Response message returned by the agent to the manager supplying the MIB data. SNMP uses UDP exclusively for transport, using the SNMP Response message to both acknowledge receipt of other protocol messages and supply SNMP information. Table 5-4 summarizes...

SPF Calculation

So far, this chapter has covered a lot of ground related to the exchange of LSAs. Regardless of the OSPF network type and whether DRs are used, once a router has new or different information in its LSDB, it uses the Dijkstra SPF algorithm to examine the LSAs in the LSDB and derive the math-equivalent of a figure of a network. This mathematical model has routers, links, costs for each link, and the current (up down) status of each link. Figure 9-4 represents the SPF model of a sample network....

STP Configuration and Analysis

Example 3-1, based on Figure 3-6, shows some of the basic STP configuration and show commands. Take care to note that many of the upcoming commands allow the parameters to be set for all VLANs by omitting the VLAN parameter, or set per VLAN by including a VLAN parameter. Example 3-1 begins with SW1 coincidentally becoming the root switch. After that, SW2 is configured to become root, and SW3 changes its Root Port as a result of a configured port cost in VLAN 1. Figure 3-6 Network Used with...

Stubby Areas

OSPF can further reduce overhead by treating each area with one of several variations of rules, based on a concept called a stubby area. Stubby areas take advantage of the fact that to reach subnets in other areas, routers in an area must forward the packets to some ABR. Without stubby areas, ABRs must advertise all the subnets into the area, so that the routers know about the subnets. With stubby areas, ABRs quit advertising type 5 (external) LSAs into the stubby area, but instead ABRs create...

Switching and Bridging Logic

In this chapter so far, you have been reminded about the cabling details for Ethernet along with the formats and meanings of the fields inside Ethernet frames. A switch's ultimate goal is to deliver those frames to the appropriate destination(s) based on the destination MAC address in the frame header. Table 1-7 summarizes the logic used by switches when forwarding frames, which differs based on the type of destination Ethernet address and on whether the destination address has been added to...

The Mpls Vpn Control Plane

The MPLS VPN control plane defines protocols and mechanisms to overcome the problems created by overlapping customer IP address spaces, while adding mechanisms to add more functionality to an MPLS VPN, particularly as compared to traditional Layer 2 WAN services. To understand the mechanics, you need a good understanding of BGP, IGPs, and several new concepts created by both MP-BGP RFCs and MPLS RFCs. In particular, this section introduces and explains the concepts behind three new concepts...

The Solution Mpls Vpns

The protocols and standards defined by MPLS VPNs solve the problems shown in Figure 19-11 and provide a much larger set of features. In particular, the MPLS VPN RFCs define the concept of using multiple routing tables, called Virtual Routing and Forwarding (VRF) tables, which separate customer routes to avoid the duplicate address range issue. This section defines some key terminology and introduces the basics of MPLS VPN mechanics. MPLS uses three terms to describe the role of a router when...

Tuning Shaping for Voice Using LLQ and a Small Tc

Example 14-1 in the previous section shows default settings for queuing for the interface software queues (WFQ) and for the shaping queue (FIFO). Example 14-2 shows an alternative configuration that works better for voice traffic by using LLQ for the shaped traffic. Also, the configuration forces the Tc down to 10 ms, which means that each packet will experience only a short delay waiting for the beginning of the next Tc. By keeping Tc to a small value, the LLQ logic applied to the shaped...

Underlying Mechanics of Shaping

Shapers apply a simple formula to the Tc, Bc, and shaping rate parameters Tc Bc shaping rate Key Topic For example, in Figures 14-1 and 14-2, if the shaping rate (64 kbps) and the Bc (8000 bits) were both configured, the shaper would then calculate the Tc as 8000 64,000 0.125 seconds. Alternatively, if the rate and Tc had been configured, the shaper would have calculated Bc as Bc rate * Tc (a simple derivation of the formula listed earlier), or 64 kbps * 0.125 ms 8000 bits. (Both CB Shaping and...