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

This chapter covers what might be the single most important topic on both the CCIE Routing and Switching written and lab exams Border Gateway Protocol (BGP) Version 4. This chapter focuses on how BGP accomplishes its fundamental tasks 1. Forming neighbor relationships 2. Injecting routes into BGP from some other source 3. Exchanging those routes with other routers 4. Placing routes into IP routing tables All of these BGP topics have close analogies with those of BGP's IGP cousins, but of course...

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)

Building the BGP Table

The BGP topology table, also called the BGP Routing Information Base (RIB), holds the network layer reachability information (NLRI) learned by BGP, as well as the associated PAs. An NLRI is simply an IP prefix and prefix length. This section focuses on the process of how BGP injects NLRI into a router's BGP table, followed by how routers advertise their associated PAs and NLRI to neighbors. NOTE Technically, BGP does not advertise routes rather, it advertises PAs plus a set of NLRI that shares...

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....

Chapter

BGP neighbors must reach the established state, a steady state in which Update messages can be sent and received as needed. Although eBGP neighbors often share a common link, there is no requirement that neighbors must be connected to the same subnet. BGP sets TTL to 1 only for messages sent over eBGP connections, so the ebgp-multihop option is required only in that case. (The ibgp-multihop command does not exist.) The BGP router ID can be set to any syntactically valid number, in the format of...

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 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...

Comparison of PIMDM and PIMSM

One of the most confusing parts of the PIM-DM and PIM-SM designs is that it appears that if sources keep sending, and receivers keep listening, there is no difference between the end results of the end-user multicast packet flow using these two options. Once PIM-SM completes its more complicated processes, the routers near the receivers have all joined the SPT to the source, and the most efficient forwarding paths are used for each (S,G) tree. Although its underlying operation is a bit more...

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...

Configuring Trunking on Routers

VLAN trunking can be used on routers and hosts as well as on switches. However, routers do not support DTP, so you must manually configure them to support trunking. Additionally, you must manually configure a switch on the other end of the segment to trunk, because the router does not participate in DTP. The majority of router trunking configurations use subinterfaces, with each subinterface being associated with one VLAN. The subinterface number does not have to match the VLAN ID rather, the...

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...

Defining and Limiting LLQ Bandwidth

The LLQ priority command provides two syntax options for defining the bandwidth of an LLQ a simple explicit amount or bandwidth as a percentage of interface bandwidth. (There is no remaining bandwidth equivalent for the priority command.) However, unlike the bandwidth command, both the explicit and percentage versions of the priority command can be used inside the same policy map. IOS still limits the amount of bandwidth in an LLQ policy map, with the actual bandwidth from both LLQ classes...

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...

Dhcp

DHCP represents the next step in the evolution of dynamic IP address assignment. Building on the format of BOOTP protocols, DHCP focuses on dynamically assigning a variety of information and provides flexible messaging to allow for future changes, without requiring predefinition of MAC addresses for each client. DHCP also includes temporary leasing of IP addresses, enabling address reclamation, pooling of IP addresses, and, recently, dynamic registration of client DNS Hey Everybody My MAC...

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 Authentication

EIGRP authentication, much like OSPF authentication, requires the creation of keys and requires authentication to be enabled on a per-interface basis. The keys are used as the secret (private) key used in an MD5 calculation. (EIGRP does not support clear-text authentication.) Multiple keys are allowed and are grouped together using a construct called a key chain. A key chain is simply a set of related keys, each of which has a different number and may be restricted to a time period. By allowing...

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 Load Balancing

EIGRP allows for up to six equal-metric routes to be installed into the IP routing table at the same time. However, because of the complex EIGRP metric calculation, metrics may often be close to each other, but not exactly equal. To allow for metrics that are somewhat close in value to be considered equal, and added to the IP routing table, you can use the variance multiplier command. The multiplier defines a value that is multiplied by the lowest metric (in other words, the FD, which is the...

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...

EIGRP Updates

RTP allows the Updates to be sent as multicasts. If any neighbors fail to acknowledge receipt of the multicasted update, RTP resends Updates as unicasts just to those neighbors. The steps run as follows, using Figure 8-2 as an example 1. The EIGRP sender (R1 in Figure 8-2) starts a Retransmission Timeout (RTO) timer for each neighbor when sending a reliable message like an Update. (Cisco IOS actually calculates a Smoothed Round-Trip Time, or SRTT, to each neighbor, and derives RTO from the SRTT...

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

Link-state routing protocols define the content and structure of data that describes network topology, and define the processes by which routers exchange that detailed topology information. The name link state refers to the fact that the topology information includes information about each data link, along with each link's current operational state. All the topological data together comprises the link-state database (LSDB). Each link-state router applies the Dijkstra algorithm to the database...

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 Data Link Connection Identifiers

To connect two DTEs, an FR service uses a virtual circuit (VC) between pairs of routers. A router can then send an FR frame with the appropriate (typically) 10-bit Data Link Connection Identifier (DLCI) header field that identifies each VC. The intermediary FR switches forward the frame based on its DLCI, until the frame eventually exits the FR service out the access link to the router on the other end of the VC. FR DLCIs are locally significant, meaning that a particular DLCI value only...

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 Addressing and Subnetting

You need a postal address to receive letters similarly, computers must use an IP address to be able to send and receive data using the TCP IP protocols. Just as the postal service dictates the format and meaning of a postal address to aid the efficient delivery of mail, the TCP IP protocol suite imposes some rules about IP address assignment so that routers can efficiently forward packets between IP hosts. This chapter begins with coverage of the format and meaning of IP addresses, with...

IP Forwarding

IP forwarding, or IP routing, is simply the process of receiving an IP packet, making a decision of where to send the packet next, and then forwarding the packet. The forwarding process needs to be relatively simple, or at least streamlined, for a router to forward large volumes of packets. Ignoring the details of several Cisco optimizations to the forwarding process for a moment, the internal forwarding logic in a router works basically as shown in Figure 6-1. Figure 6-1 Forwarding Process at...

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...

Pv6 Multicast Static Routes

Just as in IPv4, multicast routing fundamentally builds its routing table based on the unicast routing table. Before any multicast traffic can be routed, that traffic must pass the router's RPF check. That is, it must have arrived on the interface that the router's unicast routing table indicates is the correct path back toward the traffic source. For tunnels, in particular, the RPF check can cause problems. If multicast traffic arrives over a tunnel instead of the physical interface over which...

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...

Layer 2 Security

The Cisco SAFE Blueprint document (available at http www.cisco.com go safe) suggests a wide variety of best practices for switch security. In most cases, the recommendations depend on one of three general characterizations of the switch ports, as follows Unused ports Switch ports that are not yet connected to any device for example, switch ports that are pre-cabled to a faceplate in an empty cubicle User ports Ports cabled to end-user devices, or any cabling drop that sits in some physically...

LLQ with More Than One Priority Queue

LLQ allows multiple queues classes to be configured as priority queues. This begs the question, Which queue gets scheduled first As it turns out, LLQ actually places the packets from multiple LLQs into a single internal LLQ. So, packets in the different configured priority queues still get scheduled ahead of non-priority queues, but they are serviced based on their arrival time for all packets in any of the priority queues. So why use multiple priority queues The answer is policing. By policing...

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...

Mechanics of MQC

MQC separates the classification function of a QoS tool from the action (PHB) that the QoS tool wants to perform. To do so, there are three major commands with MQC, with several subordinate commands The class-map command defines the matching parameters for classifying packets into service classes. The PHB actions (marking, queuing, and so on) are configured under a policy-map command. The policy map is enabled on an interface by using a service-policy command. Figure 12-4 shows the general flow...

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...

Miscellaneous Cbwfqllq Topics

CBWFQ and LLQ allow a policy map to either allocate bandwidth to the class-default class, or not. When a bandwidth command is configured under class class-default, the class is indeed reserved that minimum bandwidth. (IOS will not allow the priority command in class-default.) When class class-default does not have a bandwidth command, IOS internally allocates any unassigned bandwidth among all classes. As a result, class class-default might not get much bandwidth unless the class is configured...

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...

MPLS Unicast IP Forwarding

MPLS can be used for simple unicast IP forwarding. With MPLS unicast IP forwarding, the MPLS forwarding logic forwards packets based on labels. However, when choosing the interfaces out which to forward the packets, MPLS considers only the routes in the unicast IP routing table, so the end result of using MPLS is that the packet flows over the same path as it would have if MPLS were not used, but all other factors were unchanged. MPLS unicast IP forwarding does not provide any significant...

Mpls Vpns

One of the most popular of the MPLS applications is called MPLS virtual private networks (VPNs). MPLS VPNs allow a service provider, or even a large enterprise, to offer Layer 3 VPN services. In particular, SPs oftentimes replace older Layer 2 WAN services such as Frame Relay and ATM with an MPLS VPN service. MPLS VPN services enable the possibility for the SP to provide a wide variety of additional services to its customers because MPLS VPNs are aware of the Layer 3 addresses at the customer...

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...

Multicast Routing Basics

The main function of any routing protocol is to help routers forward a packet in the right direction, causing the packet to keep moving closer to its desired destination, ultimately reaching its destination. To forward a unicast packet, a router examines the packet's destination address, finds the next-hop address from the unicast routing table, and forwards the packet through the appropriate interface. A unicast packet is forwarded along a single path from the source to the destination. The...

Multicast Scoping

Multicast scoping confines the forwarding of multicast traffic to a group of routers, for administrative, security, or policy reasons. In other words, multicast scoping is the practice of defining boundaries that determine how far multicast traffic will travel in your network. The following sections discuss two methods of multicast scoping With TTL scoping, routers compare the TTL value on a multicast packet with a configured TTL value on each outgoing interface. A router forwards the multicast...

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...

Network Time Protocol

NTP Version 3 (RFC 1305) allows IP hosts to synchronize their time-of-day clocks with a common source clock. For instance, routers and switches can synchronize their clocks to make event correlation from an SNMP management station more meaningful, by ensuring that any events and traps have accurate time stamps. By design, most routers and switches use NTP client mode, adjusting their clocks based on the time as known by an NTP server. NTP defines the messages that flow between client and...

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 and LSAs

This section covers two major topics Although these might seem to be separate concepts, most OSPF design choices directly impact the LSA types in a network and impose restrictions on which neighbors may exchange those LSAs. This section starts with an OSPF design and terminology review, and then moves on to LSA types. Toward the end of the section, OSPF area types are covered, including how each variation changes how LSAs flow through the different types of OSPF stubby areas.

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...

OSPF Filtering

Intra-routing-protocol filtering presents some special challenges with link-state routing protocols like OSPF. Link-state protocols do not advertise routes they advertise topology information. Also, SPF loop prevention relies on each router in the same area having an identical copy of the LSDB for that area. Filtering could conceivably make the LSDBs differ on different routers, causing routing irregularities. IOS supports three variations of what could loosely be categorized as OSPF route...

OSPF Stub Router Configuration

Defined in RFC 3137, and first supported in Cisco IOS Software Release 12.2(4)T, the OSPF stub router feature not to be confused with stubby areas allows a router to either temporarily or permanently be prevented from becoming a transit router. In this context, a transit router is simply one to which packets are forwarded, with the expectation that the transit router will forward the packet to yet another router. Conversely, non-transit routers only forward packets to and from locally attached...