Additional ISIS Packet Information

The following sections provide additional information on the following packet types IS-IS packets Hello packets Link-state packets Sequence number packets IS-IS Packet Fields (Alphabetical Order) ATT Specifies the attachment bits (flag attachments to other areas). Checksum Gives the checksum of the contents of the LSP from the source ID to the end. Circuit Type Defines whether the link is Level 1 and or Level 2. End LSP Is the LSP ID of the last LSP in CSNP. Holding Time Defines how long to...

Aggregator

Typically, ATOMIC_AGGREGATE and AGGREGATOR are not used in defining and configuring BGP policies in routers and therefore will not be discussed in detail in this chapter. The remaining attributes will be illustrated and explained in detail in this chapter. The routing table of a router dictates how traffic destined to a certain destination exits that router. If the focus of traffic flow is shifted to a region where many routers are present, the routing policy depicted in the routing tables of...

Bad Checksum

OSPF-4-ERRRCV Received invalid packet Bad Checksum from 144.100.21.141, TokenRing0 0 This means that OSPF encountered an error in a packet that was received. This is because the OSPF checksum does not match the OSPF packet that was received by this router. A device between the neighbors, such as a switch, is corrupting the packet. The sending router's packet is invalid. In this case, either the sending router's interface is bad or a software bug is causing the error. The receiving router is...

Best Path Calculation

Material in this section is based on the Cisco document BGP Best Path Selection Algorithm, available at By design, a BGP speaker receiving updates picks only a single best update from a set of multiple updates and installs it in the routing table. BGP best-path calculation goes through a series of comparisons between multiple updates. The comparison is done over the BGP attributes, and a series of tests is performed until one update wins over the other and the best path update is placed in the...

BGP Route Not Getting Originated Cause BGP Is Autosummarizing to Classful Network Boundary

Sometimes, classful networks are advertised in BGP when other routing protocols are redistributed in BGP. For example, BGP might be trying to redistribute 100.100.100.0 24, but only 100.0.0.0 8 gets advertised. Another example could be that 131.108.0.0 16 is advertised where 131.108.5.0 24 was redistributed. BGP autosummarizes subnetted routes to their network boundaries when redistributed into BGP from any other routing protocol. For example, subnetted Class A routes automatically are...

BGP Route Not Getting Originated Cause IP Routing Table Does Not Have a Matching Route

BGP requires the IP routing table to have an exact matching entry for the prefix that BGP is trying to advertise using network and redistribute command. The prefix and mask of the network that BGP is trying to advertise must be identical in the IP routing table and in the BGP configuration. BGP will fail to originate any prefix related to this network if this discrepancy exists. Figure 15-9 shows the flowchart to follow to fix this problem. Figure 15-9. Problem-Resolution Flowchart Figure 15-9....

BGP4 Protocol Specification and Functionality

RFC 1771 defines the current Border Gateway Protocol 4 (BGP-4) implementation. BGP relies on a reliable transport mechanism to establish its connection and for exchanging information between BGP peers. BGP uses TCP port 179 for this purpose and benefits from the TCP protocol to offer reliable communication between BGP speakers. RFC 1771 describes in detail the requirements of BGP neighbor relationships, BGP update format, error notifications, and handling of special cases. Proper BGP...

Case Study ISDN Configuration Problem

This case study explores a problem that involves setting up IS-IS routing over an ISDN link. The objective is to put the troubleshooting knowledge acquired in this chapter to immediate use by trying to figure out any potential problems in the setup. RTA and RTB are connected over an ISDN link, as shown in Figure 11-12. Standard configuration is employed, as demonstrated in Example 11-39. Figure 11-12. Network Topology for ISDN Configuration Problem Figure 11-12. Network Topology for ISDN...

Understanding IP Routing

The primary objective of this book is to provide elaborate guidance for troubleshooting Internet Protocol (IP) routing problems on Cisco routers. In this regard, the subsequent text covers well-known routing protocols such as the following Open Shortest Path First Protocol (OSPF) Integrated Intermediate System-to-Intermediate System Protocol (IS-IS) Border Gateway Protocol (BGP) Protocol Independent Multicast (PIM) for multicast routing This chapter presents an introduction to IP routing and...

Understanding Intermediate Systemto Intermediate System ISIS

This chapter covers the following key topics Configuring IS-IS for IP routing This chapter presents the fundamental concepts behind the Intermediate System-to-Intermediate System (IS-IS) routing protocol. Specifically, the material covered is slanted toward Integrated IS-IS and its usability for routing in IP environments. The IS-IS protocol is one of the popular Interior Gateway Protocols (IGP) used on the Internet. OSPF, which is also covered in this book, is another popular IGP. The IS-IS...

Troubleshooting ISIS

This chapter covers the following key topics Troubleshooting of IS-IS adjacency problems Troubleshooting of IS-IS routing update problems Case study ISDN configuration problem Chapter 10, Understanding Intermediate System-to-Intermediate System (IS-IS), provides an overview of the IS-IS routing protocol, covering IS-IS protocol concepts and basic con-figuration on Cisco routers. In line with the overall theme of this book, this chapter covers troubleshooting of IS-IS routing problems. Cisco...

Understanding Protocol Independent Multicast PIM

This chapter covers the following key topics about Protocol Independent Multicast (PIM) Fundamentals of IGMP version 1, IGMP version 2, and reverse path forwarding (RPF) Host-to-host transmission has been the issue of many discussions in the technical world. As technologies advance, new methodologies for facilitating that transmission emerge. A transmission from one specific host to another specific host is known as a unicast. One-to-one transmission is easy. The big push, currently, is...

Understanding Border Gateway Protocol Version 4 BGP4

This chapter covers the following key topics about Border Gateway Protocol version 4 (BGP-4) BGP-4 protocol specification and functionality Scaling IBGP networks (route reflectors and confederations) An autonomous system (AS) is a set of devices under common administration. Between two or more autonomous systems, the Border Gateway Protocol advertises network reachability information. The Internet backbone relies solely on BGP to announce and receive IP prefixes, and the only routing protocol...

Understanding Routing Information Protocol RIP

This chapter covers the following key topics about Routing Information Protocol (RIP) Split horizon with poison reverse Why RIP doesn't support discontiguous networks Why RIP doesn't support variable-length subnet masking (VLSM) RIP is a distance vector protocol that uses hop count as its metric. This protocol is very simple and was intended for small networks. RIP is similar to gated, which was distributed by the FreeBSD version of UNIX. Before the RFC for RIP Version 1 (RIP-1) was written,...

Troubleshooting RIP

This chapter covers the following key topics Troubleshooting RIP routes installation Troubleshooting RIP routes advertisement Troubleshooting routes summarization in RIP Troubleshooting RIP redistribution problems Troubleshooting dial-on-demand (DDR) routing issues in RIP Troubleshooting route flapping problem in RIP This chapter discusses some of the common problems in RIP and tells how to resolve those problems. At this time, no RIP error messages will help troubleshooting RIP problems. As a...

Understanding Interior Gateway Routing Protocol IGRP

This chapter covers the following key topics about Interior Gateway Routing Protocol (IGRP) Metrics Split horizon with poison reverse Unequal-cost load balancing in IGRP In the mid-1980s, Cisco developed its own proprietary routing protocol, Interior Gateway Routing Protocol (IGRP), as a solution to some of the shortcomings of RIP, such as the hop-count limitation of 15. Like RIP, IGRP is a distance vector protocol. However, IGRP calculates its composite metric from a variety of variables, such...

Troubleshooting IGRP

This chapter covers the following key topics Troubleshooting IGRP route installation Troubleshooting IGRP route advertisement Troubleshooting IGRP redistribution problems Troubleshooting dial-on-demand (DDR) routing issues in IGRP Troubleshooting route flapping in IGRP Troubleshooting variance problem This chapter discusses common problems in IGRP networks and how to solve those problems. IGRP is a Cisco proprietary protocol. IGRP fixes some of the problems with RIP, but still it has similar...

Troubleshooting OSPF

This chapter covers the following OSPF troubleshooting topics Troubleshooting OSPF neighbor relationships Troubleshooting OSPF route advertisement Troubleshooting OSPF route installation Troubleshooting redistribution problems in OSPF Troubleshooting route summarization in OSPF Troubleshooting CPUHOG problems Troubleshooting dial-on-demand routing (DDR) issues in OSPF Troubleshooting SPF calculation and route flapping This chapter discusses common problems of OSPF and tells how to troubleshoot...

CLNS ping and traceroute

Cisco IOS Software provides ping and traceroute tools for ISO CLNP, which are analogous to the all-too-familiar IP version. ping clns and traceroute clns apparently were designed for use in ISO CLNP environments, yet they can be useful for troubleshooting IS-IS operation problems in IP environments. Contrary to popular belief, the clns router isis command is not required to enable the ping clns and traceroute clns commands to work. You might recall that, in addition to the IS-IS process, only...

Command Syntax Conventions

The conventions used to present command syntax in this book are the same conventions used in the IOS Command Reference. The Command Reference describes these conventions as follows Vertical bars ( ) separate alternative, mutually exclusive elements. Square brackets indicate optional elements. Braces indicate a required choice. Braces within brackets indicate a required choice within an optional element. Boldface indicates commands and keywords that are entered literally as shown. In actual...

Configuring ISIS for IP Routing

This section reviews the basic tasks involved in enabling IS-IS on Cisco routers. In addition to the basic configuration, numerous Cisco IOS Software commands exist for enabling various optimization and management capabilities, such as modifying hello timers, logging IS-IS adjacency changes, performing authentication, and so on. Chapter 11 covers some of these options in greater detail. For completeness, however, you should consult the IOS Network Protocols Configuration Guide, available at...

Debugs and Verification

Example 9-238 shows the summary-address configuration on R1. Note that R1 is not an ASBR. Also note that the range is using the format 255.255.255.0 instead of 0.0.0.255, as explained in the previous problem. In addition, in the previous example, the area range command was used to summarize the area routes, but that command cannot be used here When OSPF forms an adjacency, it floods all the link-state update packets to its neighbors. Sometimes, the flooding process takes a lot of time,...

Default Route and IGRP

In Cisco routers, IGRP does not recognize the 0.0.0.0 0 route as the default route. It uses its own method of propagating default route with the ip default-network command. The ip default-network command specifies a major network address and flags it as a default network. This major network could be directly connected, defined by a static route, or discovered by a dynamic routing protocol. The network specified by the ip default-network command must be in the routing table before the command...

Default Routes and EIGRP

Unlike IGRP, EIGRP recognizes the 0.0.0.0 0 route as the default route and allows it to be redistributed into EIGRP domain as the default route. EIGRP also uses its own method of propagating the default route with the ip default-network command, just as in IGRP. The ip default-network command works exactly the same as it does in IGRP. The ip default-network command specifies a major network address and flags it as a default network. This major network could be directly connected, defined by a...

Demand Circuit Keeps Bringing Up the Link Cause A Link Flap in the Network

The most common reason for a demand circuit to bring up the link is the existence of a link flap. A link flap occurs when a link in any part of the network goes up or down. This causes changes in the database information, and OSPF must bring up the link and refresh its database with the neighbor over the demand circuit. This is shown in the network setup in Figure 9-91. A link is flapping in area 0 and causes SPF in area 0. Because R1 is also a part of area 0, R1 will run SPF and then bring up...

Directly Connected External BGP Neighbors Not Coming Up Cause Layer 2 Is Down Preventing Communication with Directly

IP connectivity cannot occur until Layer 2 in the OSI reference model is up. Whether this Layer 2 information is learned dynamically or is configured statically, each router must have a correct Layer 2 rewrite information of adjacent routers. Ethernet, Frame Relay, ATM, and so on are most commonly used Layer 2 technologies. Most network administrators configure Layer 2 parameters in router configurations correctly sometimes, basic cabling issues also can cause Layer 2 issues. Among cabling...

Dynamic Routing

The last section discusses the essence of IP routing and indicates that dynamic automatic routing is very necessary for large network deployments. This section discusses the characteristics and classification of various IP routing protocols. Although all routing protocols have a common goal of gathering routing information to support packet-forwarding decisions, they can be classified into two broad categories, unicast and multicast, based on the type of data traffic they are designed to...

EBGPLearned Route Not Getting Installed in IP Routing Table Cause BGP Routes Are Dampened

Dampening is the way to minimize instability in a local BGP network caused by unstable BGP routes from EBGP neighbors. RFC 2439, BGP Route Flap Damping, describes in detail how dampening works. In short, dampening is the way to assign a penalty for a flapping BGP route. A withdrawal of a prefix is considered a flap. A penalty of 1000 is assigned for each flap if the flap penalty reaches the suppress limit because of continued flaps (default 2000), the BGP path is suppressed and is taken out of...

EIGRP Behavior

Unlike IGRP, EIGRP is an advanced distance vector protocol that carries the subnet mask information when an update is sent out. Therefore, EIGRP supports discontiguous network and variable-length subnet masking (VLSM). For more explanation about discontiguous networks and VLSM, refer to Chapter 2, Understanding Routing Information Protocol (RIP). Figure 6-8 shows the network diagram that illustrates EIGRP's support for discontiguous networks. Figure 6-8 shows two routers connected through a...

EIGRP Error Messages

Some EIGRP error messages that occur in the log have mystified many network admin-istrators. This section discusses some of the most common EIGRP errors that appear and the meanings behind these EIGRP error messages DUAL-3-SIA This message means that the primary route is gone and no feasible successor is available. The router has sent out the queries to its neighbor and has not heard the reply from a particular neighbor for more than three minutes. The route state is now stuck in active state....

EIGRP Is Not Installing Routes Cause Auto or Manual Summarization

When EIGRP fails to install routes in the routing table, the first thing to check is the topology table. Figure 7-25 shows the network setup for this case study. Figure 7-25. EIGRP Network Susceptible to Route-Installation Problem EiGFlP Ie not Inslabig roulis into Ih muling EiGFlP Ie not Inslabig roulis into Ih muling Figure 7-25. EIGRP Network Susceptible to Route-Installation Problem This section discusses how to troubleshoot consistent EIGRP route flapping. The most important tool for...

EIGRP Packet Format

Eigrp Header

Figure 6-4 shows the EIGRP packet header. Notice that following the autonomous systems number are the Type Length Value (TLV) triplets. The TLV triplets carry route entries, as well as provide the fields for DUAL process management. Some common TLVs are the EIGRP parameter TLV, the IP internal route TLV, and the IP external route TLV. The EIGRP packet parameters are described as follows Version Specifies different versions of EIGRP. Version 2 of EIGRP was imple-mented beginning with Cisco IOS...

EIGRP Reliable Transport Protocol

Five types of EIGRP packets exist, further categorized as reliable packets and unreliable packets. The reliable EIGRP packets are as follows Update Update packets contain EIGRP routing updates sent to an EIGRP neighbor. Query Queries are sent to neighbors when a route is not available and the router needs to ask the status of the route for fast convergence. Reply Reply packets to the queries contain the status of the route being queried for. The unreliable EIGRP packets are as follows Hello...

EIGRP Summarization Route Problem Cause Subnetworks of Summary Route Dont Exist in Routing Table

Consider the case shown in Figure 7-30, in which Router A is configured to send out a summary route of 172.16.80.0 255.255.240.0 on its Ethernet 0 interface to Router B. Example 7-52 shows the configuration of Router A. However, the next-hop router is not seeing the route, and the 172.16.80.0 255.255.240.0 route is not in the router's topology table. Example 7-53 shows a snapshot of the router's routing table. Figure 7-30. Network Diagram for Case Study on EIGRP Summarization Figure 7-30....

Equation 41 IGRP Metric Equation

Default values K1 K3 1, K2 K4 K5 0 BW 10 7 (min bandwidth along paths in kilobits per second) Delay (Sum of delays along paths in milliseconds) 10 From the equation, the load variable is a value from 1 to 255, in which 255 indicates 100 percent saturation of the link and 1 indicates virtually no traffic. The reli variable is also a value from 1 to 255, in which 1 indicates an unreliable link and 255 indicates a 100 percent reliable link. Referring to Equation 4-1, the term K5 (Reli + K4) is...

Equation 61 IGRP Metric

IGRP Metric KL XBW + iK - BWj- + K3 Delay L (256 - Load _ K1, K2, K3, K4, K5 Constants Default values K1 K3 1,K2 K4 K5 0 BW 107 (min bandwidth along paths in kilobits per second) Delay (Sum of delays along paths in milliseconds) 10 Load Load of interface Reli Reliability of the interface EIGRP is different than IGRP metric by a factor of 256 because of the Metric field IGRP uses only 24 bits in its update packet for the Metric field, whereas EIGRP uses 32 bits in its update packet for the...

Example 111 show clns neighbors Command Output

System Id Interface SNPA State Holdtime Type Protocol IS-IS RT5 Et0 0 00d0.58eb.ff01 Up 25 L1 IS-IS System Id Interface SNPA State Holdtime Type Protocol IS-IS Area Address(es) 49.0002 IP Address(es) 192.168.1.2* Uptime 02 15 11 RT5 Et0 0 00d0.58eb.ff01 Up 23 L1 IS-IS Area Address(es) 49.0001 IP Address(es) 10.1.1.5* Uptime 02 15 11 The show clns neighbors command provides a summary of known neighbors, the connecting interface, and the state of the adjacency. The show clns neighbors detail...

Example 1125 Displaying the Routing Information in an LSP

RT1 show isis database level-1 RT2.00-00 detail LSP Seq Num LSP Checksum LSP Holdtime 0x00001C9C 0x5F3E 1015 IS-Extended RT1.00 IP 10.1.2.0 24 IP 11.1.1.2 32 IP 11.1.1.6 32 IP 192.168.1.0 30 RT2.00-00 is the LSP ID of RT2. Detail output of the LSP, with ID RT2.00-00, shows the IP subnets for directly connected links, together with their metric information. Another interesting command is show isis topology, which displays a list of all known routers. For example, Example 11-26 shows the IS-IS...

Example 1139 Configurations for RTA and RTB in Figure 1112

Interface BRI1 0 ip address 192.168.31.1 255.255.255.0 isdn spid1 91947209980101 4720998 isdn spid2 91947209990101 4720999 dialer idle-timeout 1200 dialer map clns 49.0040.0000.0000.3200.00 name RTB broadcast 4723074 dialer map ip 192.168.31.3 name RTB broadcast 4723074 passive-interface LoopbackO net 49.0040.0000.0000.3100.00 is-type level-1 clns route 4 9.0040.0000.0000.3200.00 BRI1 0 ip address 192.168.31.3 255.255.255.0 isdn spid1 91947230740101 4723074 isdn spid2 91947230750101 4723075...

Example 31 Routing Table for R2 Shows No RIP Routes for Subnet 13110820

The possible causes for this problem are as follows Missing or incorrect network statement Layer 2 down Distribute list blocking the route Access list blocking RIP source address Access list blocking RIP broadcast multicast Incompatible version type Mismatch authentication key (RIP-2) Discontiguous network Invalid source Layer 2 problem (switch, Frame Relay, other Layer 2 media) Offset list with a large metric defined Routes that reached RIP hop-count limit Sender problem (discussed in the next...

Example 3125 Configuring R1 When No Routing Updates Will Go on the ISDN Link

Ip address 192.168.254.13 255.255.255.252 encapsulation ppp dialer map ip 192.168.254.14 name R2 57654 dialer-group 1 isdn switch-type basic-net3 ppp authentication chap Example 3-126 shows that RIP is sending the broadcast update toward R2. You can see that it's failing because of the encapsulation failed message. Also in Example 3-126, R1 is running a debug ip packet command with access-list 100 to display only the UDP port 520 output. RIP-1 and RIP-2 use UDP port 520 to exchange updates with...

Example 398 R2s Routing Table Reflects That the Subnetted Route Is Missing

R2 show ip route 155.155.155.0 255.255.255.0 R2 show ip route 155.155.0.0 Routing entry for 155.155.0.0 16 Known via rip, distance 120, metric 1 Last update from 131.108.1.1 on Ethernet0, 00 00 01 ago Routing Descriptor Blocks * 131.108.1.1, from 131.108.1.1, 00 00 01 ago, via Ethernet0 Route metric is 1, traffic share count is 1 Figure 3-37 shows the flowchart to fix this problem based on the autosummarization feature being enabled. Figure 3-37. Flowchart to Solve Why the Sender Is Not...

Example 51 R2 Routing Table Shows No IGRP Route for 13110820

The most common possible causes of this problem are as follows network statement is missing or incorrect. The distribute list is blocking the route. The access list is blocking the IGRP source address. The access list is blocking the IGRP broadcast. This is a discontiguous network. A Layer 2 problem (switch, Frame Relay, or other Layer 2 medium) has occurred. A sender AS mismatch has occurred. A sender's problem has occurred (discussed in the Troubleshooting IGRP Routes Advertisement section)....

Example 575 Configuring a Default Route to Set the Gateway of Last Resort

R1(config-term) ip route 0.0.0.0 0 0.0.0.0 131.108.1.1 Figure 5-31 shows the flowchart to follow to fix this problem. Figure 5-31. Problem-Resolution Flowchart Figure 5-31. Problem-Resolution Flowchart Example 5-76 shows the configuration of R1. No default-network statement is configured. Example 5-76 R1's Configuration Reveals That a Candidate Default I__ -J. II__ HI_J. __ _____ .

Example 61 show ip eigrp neighbor Command Output

The explanations of the heading of the output are as follows The explanations of the heading of the output are as follows H The list of the neighbors in the order in which they are learned. Address The IP address of the neighbors. Interface The interface from which the neighbors are learned. Hold The hold timer for the neighbor. If this timer reaches 0, the neighbor relationship is torn down. Uptime The timer that tracks how long this neighbor has been established. SRTT (Smooth Round Trip Time)...

Example 62 Configuring Eigrp Manual Summarization

Ip address 192.168.11.1 255.255.255.252 ip summary-address eigrp 1 192.168.8.0 255.255.252.0 Example 6-2 demonstrates how R1 in Figure 6-10 is summarizing addresses of 192.168.8.0 24, 192.168.9.0 24, and 192.168.10.0 24 into one update of 192.168.8.0 22. Summarization in EIGRP reduces the size of the routing table and the number of updates. It also limits the query range, which is crucial in terms of making a large EIGRP network more stable and more scalable.

Example 63 show ip route Output Shows Router 1 Choosing a Suboptimal Route Without Unequal Cost Load Balancing

Known via eigrp 1, distance 90, metric 2195456 Redistributing via eigrp 1 Advertised by eigrp 1 (self originated) Last update from 192.168.6.2 on SerialO, 00 00 20 ago Routing Descriptor Blocks * 192.168.6.2, from 192.168.6.2, 00 00 20 ago, via Serial0 Route metric is2195456, traffic share count is 1 Total delay is 21000 microseconds, minimum bandwidth is 1544 Kbit Reliability 255 255, minimum MTU 1500 bytes Loading 1 255, Hops 0 To use the unequal-cost load-balancing feature of EIGRP, you use...

Example 714 Configurations for Routers A and B in Figure 719

The previous section discusses the problems that EIGRP routers have when advertising routes to its neighbors. This section discusses troubleshooting problems when EIGRP doesn't install the routes in the routing table. The most common causes of this problem are as follows Auto or manual summarization configured Higher administrative distance The following sections detail the causes of this problem and how to resolve them. For overall troubleshooting methods, Figure 7-24 shows the flowchart for...

Example 747 debug ip routing Command Output Verifies Whether a Route Is Being Installed

RT add 150.150.0.0 16 via 10.1.1.2, eigrp metric 90 304128 RT delete route to 150.150.0.0 via 10.1.1.2, eigrp metric 90 304128 This debug shows all the routes that the routing table takes out and installs, although the output of the debug might be overwhelming to the routers. You can also use an access list to the debug so that the output shows only the routes in question. For example, if you want to Summarization is extremely important in a well-designed EIGRP network. Summarization is one of...

Example 87 Configuring the Virtual Link Between Routers E and B

Router ospf area 2 virtual-link 141.108.1.21 A virtual link itself is not a bad thing. The bad design would include an area that is not connected to Area 0, as shown in Figure 8-18, and then patching it up with a virtual link. Virtual links can be very useful in several scenarios. Figure 8-19 shows an example in which a virtual link can be used as a backup and for redundancy in case the link between routers A and B goes down, the Area 3 connectivity will not be broken. Also, if the link between...

Example 9110 show ip ospf neighbor Command Output Indicates Neighbor State Loading in This Case

OSPF Neighbor Stuck in LOADING Cause Mismatched MTU Size This is a unique problem that happens when an MTU mismatch occurs. If the MTUs are not the same across the link, this problem occurs. Specifically, if a neighbor's MTU is greater than the local router's, the neighbor sends a large MTU packet as a link-state update. This packet never reaches the local router as a result, the neighbor gets stuck in the LOADING state. Figure 9-42 shows the flowchart to follow to solve this problem. Figure...

Fast Forwarding in Routers

Even though this book is about routing protocols and how to troubleshoot routing-related problems, we would like to briefly mention in this introductory chapter that the high-speed forwarding requirements in today's networks have led to ingenious ways of packet processing on routers that extend beyond basic decision-making based on the IP routing table. The routing table remains critical for routing guidance, but instead of using the contents of the routing table directly, routers transform the...

Feedback Information

At Cisco Press, our goal is to create in-depth technical books of the highest quality and value. Each book is crafted with care and precision, undergoing rigorous development that involves the unique expertise of members from the professional technical community. Readers' feedback is a natural continuation of this process. If you have any comments regarding how we could improve the quality of this book or otherwise alter it to better suit your needs, you can contact us through e-mail at...

Figure 1211 IGMP Packet Format

The Type field indicates different types of IGMP packets Type 11 is the IGMP membership query. Type 12 is the IGMP version 1 membership report. Type 16 is the IGMP version 2 membership report. Type 17 indicates the IGMP version 2 leave group. The types listed are the most important ones. You can find other Type field information in RFC 2236. The Maximum Response Time field is used only in membership query messages. It spec-ifies the maximum time in units of 1 10 of a second that a host might...

Figure 946 Problem Resolution Flowchart

OSPF neighbor is not advertising roules. Not sure interface inl ils dalabase II OSPF is not enabled on the interface thai is supposed lo be advertised, OSPF will not put that Not sure interface inl ils dalabase and hence will not advertise to lis neighbor. Go to Ihe Debugs and Verification secton -

Flowcharts to Solve Common OSPF Problems

Troubleshooting OSPF Neighbor Relationships Is the interface defined as passive Is the access list blocking OSPF Helios Co the Hello and dead intervals match on l th sides Does the authentication type match on both sides Are both sides configured wilh the correct Are t> oth sides configured with matcfting area IDs Is th& re any stuMransit NSSA mismatch Is the router attempting to form an OSPF neighbor relationship tlirouqh a secondary IP address Is OSPF running on an asynchronous interface...

Flowcharts to Solve Common RIP Problems

Troubleshooting RIP Routes Installation if. flip un biua on the inl& rlfro& Is rlw lute rL ei> al Mio reeeivtrt noutei up'LipJJ h lliH nucx.L li 1 blDck icj thn PIP olik < fldJn SS- l-i he a ii lici blocking 1he HIJ3 broadiati > 6 fH HIP u Jor> cQ< TiJva(iN yv.lt (fig 1 K Ihttr un nulKKsnlinnlinri rnir.innlcti beHwemn Is Hie HIH updam jr fKi lionn a viihJ & ouic 7 Lay i 2 madia prupjy-jdr i p HIP hlUKleaiC multfoMt Is fln oflsel list conliguroti on 1tio snrvW or (5 tte...

Fundamentals of IGMP Version 1 IGMP Version 2 and Reverse Path Forwarding

Before diving into the intricacies of the PIM protocol, you need to understand the concept behind the Internet Group Management Protocol (IGMP) and reverse path forwarding (RPF). IGMP is the protocol that functions between the host, also called the receiver, and the multicast-enabled router. In short, IGMP allows the router to know that a host is interested in receiving multicast traffic for a specific group. IGMP is enabled on the router whenever PIM is enabled. IGMP messages are sent with a...

Further Reading

Refer to the following RFCs for more information about RIP. You can access all RFCs online at www.isi.edu in-notes rfcxxxx.txt, where xxxx is the number of the RFC that you want to read. Routing Information Protocol Extensions to RIP to Support Demand Circuits Triggered Extensions to RIP to Support Demand Circuits RIP-2 MD5 Authentication

IBGPLearned Route Not Getting Installed in IP Routing Table Cause IBGP Routes Are Not Synchronized

IBGP will not install or propagate a route to other BGP speakers unless IBGP-learned routes are synchronized. Synchronization means that for an IBGP-learned route, there must exist an identical route in the IP routing table provided by an IGP (OSPF, IS-IS, and so on). This means that the IGP must hold all external BGP routing information. This can be accomplished by redistributing EBGP into an IGP at the border routers of an AS. In Figure 15-18, R1 is originating 100.100.100.0 24 to its IBGP...

IBGPLearned Route Not Getting Installed in IP Routing Table Cause IBGP Next Hop Not Reachable

The cause of this problem is most common in IBGP-learned routes where BGP next-hop address should have been learned through an Interior Gateway Protocol (IGP). Failure to reach the next hop is an IGP problem, and BGP is merely a victim. With BGP, when IP prefixes are advertised to an IBGP neighbor, the NEXT-HOP attribute of the prefix does not change. The IBGP receiver must have an IP route to reach this next hop. Figure 15-20 shows the flowchart to follow to resolve this problem. Figure 15-20....

IGRP Behavior

Distance vector protocols are protocols that solely depend on neighbor routing advertisements to determine the best path to a destination. The advantage of the distance vector protocols is their simplicity to implement. However, because of the long convergence time, IGRP is not suitable for large networks. IGRP and RIP are both classical distance vector protocols. Although IGRP and RIP differ in metric calculation update timers, they exhibit the same behavior when it comes to sending and...

IGRP Packet Format

Figure 4-3 shows the IGRP packet format. In this figure, you can see that IGRP updates provide more information than RIP and, at the same time, are more efficient. None of the fields in an IGRP packet is left unused after the 12-octet header, each routing entry is filled one after another. Therefore, IGRP does not pad the update packet to force a 32-bit word boundary. With this efficient design, IGRP can carry a maximum of 104 fourteen-octet entries. Therefore, with its MTU size of 1500 bytes,...

IP Addressing Concepts

IP addressing is central to the operation of the IP protocol. The TCP IP stack shown in Figure 1-1 features a network interface to the underlying physical and data-link layers, which allow the IP protocol to be media independent. Media independence is probably one of the critical advantages of the IP protocol that has promoted its wide acceptance and ubiquity. IP uses a native addressing scheme, in line with its media-independent architecture, that has no bearing on the underlying local-area...

ISIS Link State Database

As a link-state protocol, IS-IS works by gathering reliable and complete information about the routing environment through the use of special packets known as Link State Protocol Data Units (LSPs). A protocol data unit (PDU) also means a packet. Each router generates an LSP, which captures local link-state information describing connected links, neighbor routers, IP subnets, related metric information, and so forth. Copies of the LSP are distributed to all routers in a specific area through a...

ISIS Nodes Links and Areas

IS-IS inherits the following ISO classification and definition of the two basic types of net-work nodes End systems are hosts in a network that typically do not have extensive routing capabilities. Intermediate systems refer to routers whose primary function is to route packets. Network nodes are interconnected by links. Again, in IS-IS, only two basic links types are of practical relevance Point-to-point links interconnect pairs of nodes, while broadcast type links are multipoint and can...

ISIS Troubleshooting Command Summary

IS-IS Troubleshooting Commands show clns route show clns cache show clns traffic clear clns cache clear clns es-neighbors clear clns is-neighbors clear clns neighbors clear clns route debug clns events debug clns packets debug clns routing show ip protocol show ip route summary show ip traffic debug isis adj-packets debug isis snp-packets debug isis spf-events debug isis spf-triggers debug isis spf-statistics debug isis update-packets

Metric

The RIP metric is based on hop count and can be between 1 and 15. The metric 16 is used for infinity, which means that if the route is unreachable, a metric of 16 is displayed. The question is, why was the metric chosen as 16 Why not 17 or 18 The metric filed in RIP-1 packet format clearly shows that it is 32 bits long. This means that, theoretically, RIP can support 232 hops. Although this is a large number, the metric of 15 was chosen to avoid a count to infinity problem. (This is also...

Mismatched Area ID

OSPF-4-ERRRCV Received invalid packet mismatch area ID, from backbone area must be virtual-link but not found from 170.170.3.3, EthernetO This means that the neighbor's interface connecting to this interface is in area 0 but that this interface is not in area 0. In this situation, the router will not form an OSPF adjacency with the neighbor that this packet comes from. This also happens if one side's virtual link is misconfigured. To avoid these messages, make sure that both sides have the...

Neighbor Relationships

BGP requires a neighbor relationship to be established before any information is exchanged between BGP speakers. BGP does not dynamically discover routers interested in running BGP instead, BGP is configured with a specific neighbor IP address. Like most other dynamic protocols, BGP uses periodic keepalive messages to ensure availability of BGP neighbors. The keepalive timer is one third of the holdtime. If three consecutive keepalive messages are missed from a particular BGP neighbor, the...

OSPF Neighbor ABR Not Advertising the Summary Route Cause Area Is Configured as Totally Stubby Area

When an area is configured as a stubby area, no external LSA can be leaked into that area. Similarly, an area can be configured as a totally stubby area, which means that no external or summary LSAs can be leaked into this area. Figure 9-50 shows an OSPF network setup used to produce this problem. R1 is an ABR, and area 2 is defined as a totally stubby area. Figure 9-50. Network Setup Used to Produce This Problem Figure 9-50. Network Setup Used to Produce This Problem Figure 9-51 shows the...

OSPF Neighbor Is Not Advertising External Routes Cause Area Is Configured as a Stub Area or NSSA

Ospf Flowchart

In OSPF, Type 5 LSAs are not allowed in a stub or NSSA area. When entering the redistribute command on a router that is completely in a stub or NSSA area, a warning message is displayed. This redistribute command in the configuration is incapable of importing any external LSAs into a stub or NSSA area. Figure 9-56 shows the flowchart to follow to solve this problem. Figure 9-56. Problem-Resolution Flowchart Figure 9-56. Problem-Resolution Flowchart Example 9-147 shows the configuration error...

OSPF Neighbor Is Not Advertising External Routes Cause Subnets Keyword Missing from the ASBR Configuration

When any protocol is redistributed into OSPF, if the networks that are being redistributed are subnets, you must define the subnets keyword under OSPF configuration. If the subnets keyword is not added, OSPF will ignore all the subnetted routes when generating the external LSA. The situation could arise when connected or static routes are being redistributed into or out of OSPF. In that case, the same rule applies The subnets keyword must be entered to redistribute subnetted routes. Figure 9-81...

OSPF Neighbor Is Not Advertising Routes Cause OSPF Not Enabled on the Interface That Is Supposed to Be Advertised

OSPF includes the interface subnet address in its database only if the OSPF is enabled on that interface. OSPF might not be enabled on an interface because of an incorrect network state-ment that doesn't cover the IP address assigned on an interface or a missing network statement for that interface address. In both cases, OSPF will exclude the interface address from its data-base and will not advertise to its neighbor. Figure 9-46 shows the flowchart to follow to solve this problem. Figure...

OSPF Neighbor List Is Empty Cause OSPF Not Enabled on the Interface

OSPF can be enabled on a per-interface basis. To enable OSPF on any interface, put a network command under router ospf and include the network address with the wildcard mask. When defining the network statement in OSPF, you should carefully examine the pa e 229 wildcard mask to see the range of addresses it covers. Figure 9-2 shows the flowchart to Page When a router receives an OSPF Hello from a neighbor, it sends the Hello packet by including that neighbor's router ID in the Hello packet. If...

OSPF Neighbor Not Advertising Default Routes Cause Missing defaultinformation originate Commands

OSPF does not originate the default route unless the OSPF default-information originate command is present in the OSPF configuration. This command originates the default route on the router on which it is configured. There is no other way in OSPF to generate the default route. Figure 9-59 shows a network setup that produces this problem. Figure 9-59. Network Setup That Produces This Problem Figure 9-60 shows the flowchart to follow to solve this problem. Figure 9-60. Problem-Resolution...

OSPF Neighbor Stuck in INITCause Access List on One Side Is Blocking OSPF Hellos

Ospf Flowchart

OSPF uses a multicast address of 224.0.0.5 for sending and receiving Hello packets. If an access list is defined on the interface and OSPF is enabled on that interface, this multicast address must be explicitly permitted in the access list otherwise, it can produce problems such as stuck in INIT. The stuck in INIT problem occurs only if one side is blocking OSPF Hellos. If both sides are blocking OSPF Hellos, the output of show ip ospf neighbor returns an empty list. Figure 9-23 shows the...

OSPF Not Installing Any Routes in the Routing Table Cause Network Type Mismatch

A mismatched network type produces a discrepancy in the database, and OSPF will not install those routes in the routing table. This situation is common in NBMA networks in which one side has a point-to-point network type and the other side has a broadcast network type. This problem also occurs if one side is defined as a point-to-multipoint network and the other side is left as nonbroadcast. In this example, one side is defined as broadcast and the other side is defined as point-to-point. When...

OSPF Not Installing External Routes in the Routing Table Cause Forwarding Address Is Not Known Through the Intra Area

When OSPF learns an external LSA, it makes sure that the forwarding address is known through an OSPF intra-area or interarea route before it installs it into the routing table. If the forwarding address is not know through an intra-area or interarea route, OSPF will not install the route in the routing table. This is in accordance with the RFC 2328 standard. Figure 9-76 shows a network with the following specifications R3 is an ASBR that is redistributing RIP routes into OSPF. R4 is running RIP...

PIM Dense Mode

PIM has two modes of operation dense mode and sparse mode. Dense mode uses a flood-and-prune mechanism to forward multicast packets. The router assumes that every multicast interface is interested in multicast packets, unless it is told otherwise. The router first forwards multicast packets to all the interfaces. Segments that don't want multicast packets receive prune messages from the neighboring routers, and the branch is pruned. When the router is first configured for multicast, the router...

PIM Sparse Mode

PIM sparse mode works the opposite way of dense mode. PIM dense mode assumes that all the multicast interfaces are interested in multicast packets, unless being told otherwise. In PIM sparse mode, the router assumes that none of the multicast inter-faces is interested in receiving multicast packets, unless a PIM join message is received on the interface. PIM sparse mode is more scalable than PIM dense mode, but the concept is more complex. PIM sparse mode uses the concept of a rendezvous point...

Pointto Point Media

Point-to-point media includes HDLC and PPP encapsulation links, Frame Relay ATM point-to-point subinterfaces, and similar point-to-point interfaces. The OSPF network type of point-to-point is on by default on these media. No DR or BDR election takes place on this medium type. All the OSPF packets are multicast-based. The OSPF creates adjacencies between neighboring routers for the purpose of exchanging routing information. Not every neighbor becomes adjacent in a broadcast environment. The...

Preface

Sitting in my office at Cisco on the third floor of building K, I read an e-mail from Kathy Trace from Cisco Press asking if I was interested in writing a book. She had read my technical tips that I had written for Cisco Connection Online and said that she wanted me as an author for Cisco Press. I was very enthusiastic about it and said to myself, Yeah It's a great idea Let's write a book But on what subject One of the topics that I had in mind was OSPF. Johnson used to sit right in front of my...

Problem Aspath Filtering Using Regular Expressions

All BGP updates that contain an announcement of IP prefixes have an AS_PATH field that lists all the autonomous systems that this update has traversed. BGP operators use filtering against this AS_PATH field to allow or deny IP prefixes and also to apply BGP policy based on AS_PATH filtering. This method offers greater flexibility in applying just a single line of filtering and not listing all IP prefixes, as in the case of distribute lists or prefix lists. Commonly seen problems are mostly the...

Problem BGP Route Not Getting Originated

BGP originates IP prefixes and announces them to neighboring BGP speakers (IBGP and EBGP) so that the Internet can reach those prefixes. For example, if an IP address associated with www.cisco.com fails to originate because of a BGP configuration mistake or a lack of protocol requirements, the Internet will never know about the IP address of www.cisco.com, resulting in no connectivity to this web site. Therefore, it is essential to look at BGP route-origination issues in detail. Several causes...

Problem Candidate Default Is Not Being Advertised Cause ip defaultnetwork Command Is Missing

In a classless environment, when a router needs to send a packet to a particular destination, it performs the following check in this order 1. Is the destination address one of my connected interface addresses If yes, use ARP for the address and then encapsulate the packet in an Ethernet frame and send it to the destination. 2. If no, do I have a route in my routing table for this destination address If yes, use that route from the routing table and send the packet. 3. If no, check whether the...

Problem Configuration Mistakes Cause Failed to Configure IBGP Neighbor as a Route Reflector Client

Configuring route reflectors is fairly simple. In route-reflector BGP configuration, IBGP neighbors' peering addresses are listed as route-reflector clients however, a BGP operator inadvertently might configure an incorrect IBGP peering address as a route-reflector client. Figure 15-27 shows that R1 is an RR. R8 and R2 are RRCs of R1. Figure 15-27. Simple Route-Reflection Environment Figure 15-27. Simple Route-Reflection Environment Example 15-59 shows the required configuration needed to make...

Problem Cpuhog Messages During Adjacency Formation Cause Router Is Not Running Packet Pacing Code

When OSPF forms an adjacency, it floods all its link-state packets to its neighbor. This flooding sometimes takes a lot of CPU. Also, releases of Cisco IOS Software before 12.0T did not support packet pacing, which means that a router will try to send data as fast as it can over a link. If a link is slow or the router on the other side is slow in responding, this results in retransmission of the LSA and eventually leads to CPUHOG messages. Packet pacing adds a pacing interval between the LS...

Problem Cpuhog Messages During LSA Refresh Period Cause Router Is Not Running LSA Group Pacing Code

This problem occurs when the Cisco IOS Software code is not Release 12.0 or later. In Cisco IOS Software Release 12.0, the LSA group pacing feature was introduced to eliminate this CPU problem that can occur every 30 minutes. In previous versions of Cisco IOS Software, all LSAs refresh every 30 minutes to synchronize the age of all LSAs. Therefore, there is a significant flood every 30 minutes to refresh all LSAs at the same time. This flooding causes the CPUHOG messages every 30 minutes....

Problem Directly Connected External BGP Neighbors Not Initializing

This section discusses issues when a directly connected EBGP neighbor relationship is unsuccessful. The autonomous system (AS) will not send or receive any IP prefix updates to or from a neighboring AS unless the neighbor relationship reaches the Established state, which is the final stage of BGP neighbor establishment, as described in Chapter 14, Understanding Border Gateway Protocol Version 4 (BGP-4). When an AS has a single EBGP connection, no IP connectivity can occur until BGP finalizes...

Problem EBGPLearned Route Not Getting Installed in IP Routing Table

Just as with IBGP, EBGP routes might not get installed in the IP routing table, resulting in a lack of IP traffic reachability to those routes. Multiple causes of this problem might exist, depending on which EBGP scenario is being looked at. The most common causes of EBGP routes not getting installed are as follows The BGP next hop is not reachable in case of multihop EBGP. The multiexit discriminator (MED) value is infinite. The sections that follow discuss these causes and how to resolve the...

Problem Extended Access Lists Fails to Capture the Correct Masked Route

To reduce the size of Internet BGP routing tables, BGP operators are forced to advertise aggregated prefixes and suppress subnetted IP blocks. To achieve this, almost all ISPs expect their peering ISPs and customers to advertise aggregated blocks of, say, 21 (255.255.248.0) of IP blocks and will refuse to accept any prefix with a mask greater than 21. Proper BGP filtering must be in place at peering points so that prefixes with masks greater than 21 can be filtered out and only prefixes with...

Problem IGRP Broadcast Is Keeping the ISDN Link Up Cause IGRP Broadcasts Have Not Been Denied in the Interesting

ISDN links typically are used as backup links when primary links go down. Cisco IOS Software requires that routers are instructed on the kind of traffic that can bring up the ISDN link and keep it up. Such traffic is called interesting traffic. Network operators typically want data traffic to be considered as interesting traffic, to bring up and keep up the ISDN link. IGRP or other routing protocol updates should not be defined as interesting traffic. If this is not done, the ISDN link comes up...

Problem IGRP Not Using Unequal Cost Path for Load Balancing Cause variance Command Is Missing or Misconfigured

To use the variance feature (unequal-cost-path load balancing), it must be configured under the router igrp command. By default, IGRP does not do unequal-cost-path load balancing. Also, when the variance factor is multiplied by the current best metric, the resulting number is compared with other available path metrics. Any available path metric that is under this resulting number will be used for unequal-path load balancing. Figure 5-39 shows the network setup susceptible to this problem. The...

Problem IGRP Routes Are Flapping Cause Packet Drops on Senders or Receivers Interface

When IGRP is used in a large Frame Relay environment where there are several neigh-bors on one Frame Relay interface, there is a possibility of a packet loss. The packet loss in IGRP means that the whole update is lost. If a sender or receiver drops an IGRP update, it has to wait for another update because the IGRP updates are not retransmitted after it is lost. The most common reason for packet drops on Frame Relay interfaces is a result of broad-cast drops in the broadcast queue of Frame...

Problem IGRP Updates Are Not Going Across the Dialer Interface Cause Missing Broadcast Keyword in a dialer map Statement

When a dialer interface say, ISDN comes up, it could be desirable to run a routing protocol over this link. Static routes might do the job, but in networks with a large number of routes, static routes might not scale well. Therefore, running a dynamic routing protocol is necessary. In some situations, the ISDN link is up but no routing information is going across. Without a routing protocol, no destination addresses can be learned and no traffic can be sent to those destinations. This problem...

Problem in Propagating IBGP Route to Ibgpebgp Neighbor Cause IBGP Route Was Not Synchronized

A scenario might arise in which an IBGP learned route is not propagated to any BGP neighbor, whether IBGP or EBGP. One case could be that when an IBGP-learned route is not synchronized, that route is not considered as a candidate to advertise to other BGP neighbors. As you remember from previous discussions in Chapter 14, a BGP route is synchronized only if it has been learned through an IGP or a static route first. In Cisco IOS Software, BGP advertises only what it considers the best path to...

Problem in Propagating Originating BGP Route to Ibgpebgp Neighbors Cause Misconfigured Filters

A scenario might arise in which the BGP configuration to originate and propagate routes looks good, but BGP neighbors are not receiving the routes. The originator's BGP table shows all the routes. There is a possibility that configured filters are the cause of the problem. When implementing BGP in Cisco IOS Software, operators have many options to configure filters to control which routes to propagate to which neighbors. These filters could be fairly straightforward or could get very complex....

Problem Internal BGP Neighbors Not Coming Up

IBGP can experience issues similar to EBGP in neighbor relationship. IBGP is an important piece of overall BGP-running networks. Chapter 14 discusses the importance and usage of IBGP. This section addresses some commonly seen issues exclusive to IBGP neighbor relationship problems. The causes of this problem are identical to the previous problem of nondirectly connected external BGP neighbors not coming up The route to the nondirectly connected IBGP neighbor address is missing. The...

Problem Lowest MED Not Selected as Best Path

In some scenarios, the router does not select the lowest MED advertised by neighbors as the best path. Figure 15-51 shows a network setup that has AS 109 (R1) connected to AS 110 at two BGP peering points (R3 and R5) AS 109 has one connection with AS 111 (R4). R1 is receiving 100.100.100.0 24 from all three EBGP connections. All neighbors are advertising MEDs to influence return traffic from AS 109. R3 and R5 are advertising MEDs of 50 and 30, respectively, whereas R4 is sending a MED of 40....