Neighbor Relationship

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...

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...

Summary

This chapter elaborated on the architecture of the IS-IS routing protocol, discussing basic concepts as well as advanced protocol mechanisms involving the link-state database. The chapter also provided insight into configuration procedures required for enabling IS-IS routing on Cisco routers. Even though this book is focused on use of IS-IS for IP routing, some time was dedicated to exploring the origins of the IS-IS protocol as a dynamic routing application for ISO CLNP. IS-IS is specified in...

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...

Troubleshooting ISIS Routing Update Problems

Configuration in IS-IS is fairly simple and straightforward. In the two-stage process discussed in Chapter 10, the routing process is enabled globally, and IS-IS adjacency formation and LSP flooding is enabled on an interface by applying the command ip router isis. This command also puts the IP subnet information of the interface into the router's LSP that is generated and flooded to adjacent neighbors. This section covers IS-IS routing update problems on the premise that there are no adjacency...

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...

Understanding Open Shortest Path First OSPF

This chapter covers the following key topics about the Open Shortest Path First (OSPF) protocol OSPF is a link-state interior gateway protocol designed for a large complex network. An IETF standard, OSPF is widely deployed in many large networks. Development began in 1987, and OSPF Version 2 was established in 1991 with RFC 1247. The goal was to have a link-state protocol that is more efficient and scalable than RIP. RFC 2328 (April 1998) is the latest revision to OSPF Version 2. OSPF runs on...

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...

Problem Traffic Takes a Different Interface from What Shows in Routing Table Cause Next Hop of the Route Is Reachable

In some scenarios, BGP and the routing table path to a certain destination prefix show Exit A, but actual traffic leaves through Exit B. Packet forwarding to a destination takes place from the routing table, and network operators do expect to see this behavior. However, in most cases, the next hops of prefixes in the routing table are not directly connected and packet forwarding eventually takes place based on the next-hop path. Figure 15-36 tries to explain one such simple case. Figure 15-36....

The Diffusing Update Algorithm

The Diffusing Update Algorithm (DUAL) is the brain behind the operation of EIGRP. It is an algorithm that tracks all the routes advertised from a neighbor and then selects a loop-free path to the destination. Before discussing the details of DUAL, you must understand several terms and concepts Feasible distance (FD) Feasible distance is the minimum metric along the path to a destination. Figure 6-1 shows the feasible distance calculation to reach Network 7 for each of Router A's neighbors, from...

Troubleshooting Dialon Demand Routing Issues in RIP

Dial-on-demand routing (DDR) is common in scenarios in which the ISDN or similar dialup links are used as a backup link. When the primary link goes down, this backup link comes up. RIP begins sending and receiving updates on this link as long as the primary link is down. The dialup links can be used as a backup for the primary link in two ways Use the backup interface command. Use a floating static route with a dialer list that defines interesting traffic. The first method is very simple The...

Troubleshooting Dialon Demand Routing DDR Issues in IGRP

Dial-on-demand routing is very common when the ISDN or similar dialup links are used as a backup link. When the primary link goes down, this backup link comes up. IGRP starts sending and receiving updates on this link as long as the primary link is down. Two ways exist for using the dialup links as a backup for the primary link Using the backup interface command Using floating static routes with a dialer list that defines interesting traffic The first method is simple The command is typed under...

How This Book Is Organized

Although this book could be read cover to cover, it is designed to be flexible and to allow you to easily move between chapters and sections of chapters to cover just the material that you need more work with. Chapter 1, Understanding IP Routing This chapter provides an overview of IP routing protocols with focus on the following topics - Routing protocol administrative distance The remaining chapters alternate between chapters that provides coverage of key aspects of a specific routing...

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...

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,...

Why RIP Doesnt Support Variable Length Subnet Masking

The capability to specify a different subnet mask for the same network number is called variable-length subnet masking (VLSM). RIP and IGRP are classful protocols and are incapable of carrying subnet mask information in their updates. Before RIP or IGRP sends an update, it performs a check against the subnet mask of the network that is about to be advertised, with the subnet mask of the interface sourcing the update. If the two subnet masks don't match, the update gets dropped. The following...

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...

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...

Troubleshooting BGP

This chapter covers the following troubleshooting topics Troubleshooting BGP neighbor relationships Troubleshooting BGP route advertisement origination and receiving Troubleshooting a BGP route not installing in routing table Troubleshooting BGP when route reflectors are used Troubleshooting outbound traffic flow issues because of BGP policies Troubleshooting load-balancing scenarios in small BGP networks Troubleshooting inbound traffic flow issues because of BGP policies Troubleshooting BGP...

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...

Default Routes and RIP

Cisco's RIP implementation supports the propagation of a default route, also known as 0.0.0.0 0. When RIP finds a default route in its routing table, it automatically advertises this in the RIP update. One important thing to remember here is that the default route must have a valid metric. For example, if the default route is learned through OSPF and the metric is 20, RIP will advertise this router with a metric of infinity (16). So, for this situation, the default-metric command must be used...

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....

Troubleshooting Redistribution Problems in OSPF

This section describes problems related to redistribution in OSPF. When a router in OSPF does the redistribution, it becomes an ASBR. The routes that are redistributed into OSPF could be directly connected routes, static routes, or dynamically learned routes from another routing protocol or another OSPF process. The following are problems that can happen during redistribution ASBR is not advertising redistributed routes. OSPF is not installing external routes in the routing table. The second...

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)...

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....

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...

DUAL Finite State Machine

When EIGRP loses its successor or primary route, EIGRP immediately tries to reconverge by looking at its topology table to see if any feasible successors are available. If a feasible successor is available, EIGRP immediately promotes the feasible successor to a successor and informs its neighbors about the change. The feasible successor then becomes the next hop for EIGRP to forward the packets to the destination. The process by which EIGRP converges locally and does not involve other routers...

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

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...

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...

Troubleshooting SPF Calculation and Route Flapping

This section explains the most common reasons behind route flapping in OSPF and SPF calculation. Whenever there is a change in topology, OSPF runs the SPF algorithm to compute the shortest path first tree again. Unstable links existing within the OSPF network could cause constant SPF calculation. This section discusses the problem of SPF running constantly in the network for the following reasons Interface flap within the network Neighbor flap within the network This is a common problem in...

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...

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...

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...

Troubleshooting Routes Flapping Problem in RIP

Running RIP in a complex environment can sometimes cause flapping of routes. Route flapping refers to routes coming into and going out of the routing table. To check whether the routes are indeed flapping, check the routing table and look at the age of the routes. If the ages are constantly getting reset to 00 00 00, this means that the routes are flapping. Several reasons exist for this condition. This section discusses one of the common reasons packet loss because the packet is dropping on...

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...

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...

EIGRP Summarization

Two types of summarization take place in EIGRP autosummarization and manual summarization. Autosummarization is the default behavior for EIGRP, just as it is for RIP and IGRP. Basically, when the router sends out a routing update, it automatically summarizes the route to its natural major network when the route is advertised across a major network boundary. Figure 6-9 shows an example of autosummarization. In Figure 6-9, Router R1 needs to send an update about the network 132.168.1.0 to R2...

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...

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...

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...

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 Enhanced Interior Gateway Routing Protocol EIGRP

This chapter covers the following key topics about Enhanced IGRP (EIGRP) EIGRP neighbor relationships The Diffusing Update Algorithm (DUAL) EIGRP reliable transport protocol EIGRP query process Default routes and EIGRP Unequal-cost load balancing in EIGRP As the size of network grows larger, you can see that the classical distance vector routing protocols such as IGRP and RIP won't scale to the needs of the network. Some of the biggest scalability problems of IGRP and RIP are as follows Full...

Problem OSPF Neighbor Stuck in 2WAYCause Priority 0 Is Configured on All Routers

It is normal in broadcast media to have a 2-WAY state because not every router becomes adjacent on broadcast media. Every router enters into FULL state with the DR and the BDR. In this example, there are only two routers on Ethernet both are configured with priority 0. Priority 0 means that this router will not take part in DR BDR election process. This configuration is useful when there are low-end routers on the segment and the desire is not to make those low-end routers DRs. For this...

Routing Protocol Administrative Distance

The previous sections in this chapter provide a high-level overview of IP routing protocols from the perspectives of design, architecture, and operation. The section discusses briefly generic implementation-related issues that impact operation of these protocols on Cisco routers. Details of operation and configuration of each protocol are covered in the protocol-specific chapters. Cisco IOS Software provides common command resources for configuring and enabling the capabilities of IP routing...

OSPF4Badlsatype Invalid lsa Bad LSA type Type 6 Error Message

This is normal if the neighboring router is sending the multicast OSPF (MOSPF) packet. For more information on MOSPF, refer to RFC 1584. Cisco routers do not support MOSPF, so they simply ignore it. To get rid of these messages, simply type the following If the type is something other than 6, it's probably a bug or a memory corruption error. Refer to the section OSPF Neighbor Stuck in LOADING to learn more about how to fix the BAD LSA problem. This message means that OSPF received an invalid...

EIGRP Neighbor Relationships

Unlike IGRP, EIGRP must establish neighbor relationships before updates are sent out. When an EIGRP process is configured on the router, the router begins to exchange EIGRP hello packets over the multicast address of 224.0.0.10. Neighbor relationships form between routers when they receive each other's hello packet. Over LAN broadcast media such as Ethernet, Token Ring, or FDDI, the hello packets are sent every 5 seconds. Over WAN multipoint interfaces with a bandwidth of T1 or greater, and...

EIGRP Packet Format

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...

Problem in Propagating BGP Route to IBGP Neighbor but Not to EBGP Neighbor Cause BGP Route Was from Another IBGP Speaker

In some cases, certain routes are not propagated to IBGP neighbors but are propagated only to EBGP neighbors. When IBGP speakers in an AS are not fully meshed and have no route reflector or confederation configuration, any route that is learned from an IBGP neighbor will not be given to any other IBGP neighbor. Such routes are advertised only to EBGP neighbors, as illustrated in Figure 15-13. Chapter 14 explains using route reflectors and confederations. You also can find information on this...

EIGRP Query Process

Although EIGRP is an advanced distance vector routing protocol and convergence time is low, an EIGRP router still relies on its neighbor to advertise routing information. To achieve fast convergence, EIGRP can't rely on a flush timer like IGRP. EIGRP needs to actively search for the lost routes for fast convergence. This process is called the query process, and it was briefly discussed in the previous few sections. In the query process, queries are sent when the primary route is lost and no...

Troubleshooting Eigrp Neighbor Relationships

This section discusses methods of troubleshooting issues regarding EIGRP neighbor relationships. The following are the most common causes of problems with EIGRP neighbor relationships Uncommon subnet, primary, and secondary address mismatch Access list denying multicast packets Manual change (summary router, metric change, route filter) Figure 7-1 illustrates a general troubleshooting flowchart on EIGRP neighbor relationships. Figure 7-1. General Flowchart on Troubleshooting EIGRP Neighbor...

Troubleshooting Eigrp Route Advertisement

Sometimes, EIGRP has issues with route advertisement. This section discusses methods for troubleshooting EIGRP route advertisement problems, which can be categorized as follows EIGRP is not advertising routes to neighbors when the network administrators think that it should. EIGRP is advertising routes to neighbors when the network administrators think that it shouldn't. EIGRP is advertising routes with a metric that is not understood by the network administrators. EIGRP Is Not Advertising...

Appendix Answers to Review Questions

This appendix provides answers to the review questions that appear at the end of Chapter 1 and the even-numbered chapters that cover the key aspects of RIP, IGRP, EIGRP, OSPF, IS-IS, PIM, and BGP. What is connectionless data networking - Connectionless networking refers to transferring data in independent units referred to as packets, without the need to predefine the path of data flow. Instead, the packets are forwarded using a hop-by-hop routing paradigm between the source and destination....

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...

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...

Unequal Cost Load Balancing in IGRP

IGRP and RIP provide the capability to install up to six parallel equal-cost paths for load balancing. IGRP has an added feature that RIP does not have the capability to do unequal-cost load balancing, the capability to load-balance traffic over multiple paths that do not have the same metric to the destination. The advantage of this feature is that it offers the flexibility of load balancing, thus making more efficient use of the link. IGRP uses the variance command to perform unequal-cost...

Unequal Cost Load Balancing in EIGRP

EIGRP and IGRP use the same equation to calculate their metrics, and they share the same behavior when it comes to unequal-cost load balancing. EIGRP also can install up to six parallel equal-cost paths for load balancing, like IGRP can, and EIGRP also uses the same variance command as IGRP to do unequal-cost path load balancing. Consider the network in Figure 6-12. Figure 6-12. Unequal-Cost Load Balancing Example Figure 6-12. Unequal-Cost Load Balancing Example Remember the rules for multipath...

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...

Problem Router Is Not Summarizing Interarea Routes Cause area range Command Is Not Configured on ABR

You must ensure that the area range command is configured on the correct router. Area range summarization can be done only on the ABR. In summarization, instead of originating separate LSAs for each network, the ABR originates summary LSAs to cover those ranges of addresses. Sometimes, the network mask is configured wrong and summarization doesn't work because of the misconfiguration. When configuring the area range command, make sure that the summarization mask is in the form of a prefix mask...

Protocol Extension to RIP

RIP Version 2 (RIP-2) made some improvements and enhancements to RIP-1. RIP-2 supports VLSM and discontiguous networks, and it offers the following enhancements Figure 2-10 shows the RIP-2 packet format. The sections that follow discuss each of the enhancements and new packet fields in greater detail. The Route Tag field is a 2-byte field that allows RIP routes to be assigned with a unique integer value. The routing table display shows the route tag for each RIP route, if assigned. This route...

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...

Problem Nondirectly Connected External BGP Neighbors Not Coming Up

As discussed in Chapter 14, in some cases, EBGP neighbors are not directly connected. BGP neighbor relationships can be established in the following situations as well Between loopback interfaces of two routers. Between routers trying to make EBGP neighbor relationship that are separated by one or more routers. Such a neighbor relationship is termed EBGP multihop in Cisco iOs Software. EBGP multihop can be used for several reasons. Peering between loopbacks between EBGP typically is done when...

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...

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...

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...

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...

RIP Behavior

RIP follows certain rules when it sends and receives updates. This section covers the rules for sending and receiving updates. When RIP sends an update, it performs several checks. In Figure 2-3, two routers are running RIP together. Router 1 is connected to two majornets, 131.108.0.0 16 and 137.99.0.0 16. The majornet 131.108.0.0 is further divided into two subnets 131.108.5.0 24 and 131.108.2.0 24, which is actually connected to Router 2. Before Router 1 sends a RIP update to Router 2, it...

Troubleshooting OSPF Neighbor Relationships

This section discusses the problems related to establishing OSPF neighbor relationships. OSPF neighbor relationship problems can be of any type. Sometimes, the neighbor list is empty (that is, an OSPF neighbor might not even see the Hellos from each other). Sometimes, the problem is that the neighbor is stuck in a specific state. Recall from Chapter 8, Understanding Open Shortest Path First (OSPF), that the normal state of an OSPF neighbor is FULL. If the state is something other than FULL for...

Problem OSPF Neighbor Stuck in Exstartexchange

This is an important state during the OSPF adjacency process. In this state, the router elects a master and a slave and the initial sequence number. The whole database also is exchanged during this state. If a neighbor is stuck in EXSTART EXCHANGE for a long time, it is an indication of a problem. For more information on the EXSTART EXCHANGE state, refer to Chapter 8. The most common possible causes of this problem are as follows Mismatched interface MTU Duplicate router IDs on neighbors...

Debugs and Verification

Chapter 14 discusses using filters in BGP. Discussing every single filter is outside the scope of this book however, some of most commonly seen real-world filtering mistakes and misconceptions are discussed. Using a distribute list allows for standard access lists (1 to 99) and extended access lists (100 to 199). Example 15-32 gives a sample configuration of both. Example 15-32 Sample Distribute List Configuration Using Standard and Extended Access Lists R1 access-list 1 permit 100.100.100.0...

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...

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...

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...

Multiaccess Media

Multiaccess media includes Ethernet, Fast Ethernet, Gigabit Ethernet, FDDI, Token Ring, and similar multiaccess media. OSPF runs as a broadcast network type over these media. The OSPF network type of broadcast is on by default over these media. In this network type, the DR and the BDR are elected to reduce the flooding on the segment. The multicast capabilities of OSPF are used to form adjacencies and to efficiently distribute the information to other routers on the segment. In broadcast...

Problem Multiple Exit Points Exist but Traffic Goes Out Through One or Few Exit Routers Cause BGP Policy Definition

This problem commonly is seen when an AS receives the same prefix announcements from mul-tiple EBGP connections but traffic destined to those prefixes prefers only one or two exit points. As illustrated by Figure 15-34, AS 109 has multiple connections to other autonomous systems. AS 109 has three EBGP connections with AS 110, two with AS 111, and one with AS 112. AS 111 is peering with AS 110 and AS 112. Figure 15-34. Autonomous System with Multiple Connections to Other Autonomous Systems with...

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...

Problem Convergence Time Improvement for RR and Clients Cause Use of Peer Groups

When an RR is serving many clients, any update that it receives from IBGP EBGP peers must be generated and propagated as separate updates for each RRC. If the number of BGP updates and RRCs is large, this process could become CPU-intensive for the RR. This results in slower propagation of BGP updates and hence results in slower convergence in the network overall. Peer-group clubs configure BGP neighbors in one group. Any common update that needs to go to all members of the peer group are...

Troubleshooting Inbound IP Traffic Flow Issues Because of BGP Policies

Just as in managing outbound IP traffic from an AS, Cisco IOS Software offers BGP operators configuration options to manage inbound traffic in an AS. It is important that inbound traffic from other autonomous systems be managed well. If this does not happen, capacity of the network will not be fully utilized. This causes congestion in one part of the network while the other parts are underutilized. The end result of this mismanagement of inbound traffic flow is sluggish throughput, slow...

Problem Path with Lowest RID Is Not Chosen as Best

This is the scenario in which two or more paths from EBGP neighbors have identical BGP attributes and BGP best-path selection is done based on the RID. The BGP best-path selection rule states that, in case all other attributes are identical, the path with the lowest RID should be selected as best. In this case, the path with the highest RID is selected as best. In Cisco IOS Software, if BGP selects a best path based on the RID and a new path comes in with a lower RID, with all other attributes...

Scaling IBGP in Large Networks Route Reflectors and Confederations

It is a common understanding that there exists a rule stating that IBGP neighbors must be fully meshed with each other. This section addresses why this is a requirement and how to avoid fully meshed IBGP. It is important to understand two rules of prefix advertisement 1. When a prefix is received from an EBGP neighbor, the router must advertise that prefix to all other EBGP and IBGP neighbors. 2. When a prefix is received from an IBGP neighbor, it can be advertised ONLY to EBGP neighbors, NOT...

Troubleshooting OSPF Route Installation

This section discusses the problems related to route installation. This means that OSPF routers have fully synchronized their databases with those of their neighbors but are not installing routes in the routing table. After the route is in the database, there can be several reasons that the route is not installed in the database. This section discusses those reasons in detail and also tells how to solve these kinds of problems. The most common reasons for OSPF failing to install routes in 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...

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...

Problem RIP Is Not Installing All Possible Equal Cost Paths Cause maximumpath Command Restricts RIP from Installing

By default, Cisco routers support only four equal paths for the purpose of load balancing. The maximum-path command can be used for up to six equal-cost paths. If the command is not configured properly, it can cause a problem, as discussed in this section. When con-figured improperly, the maximum-path command allows only one path to the destination, even though more than one path exists. Configuring the command as maximum-path 1 should be done only when load balancing is not desired. Figure...

Problem Sender Is Not Advertising RIP Routes

Typically, an IP network running RIP has routers that have a consistent view of the routing table. In other words, all routers have routing tables that contain reachability information for all the IP subnets of the network. This might differ in cases when filtering of certain subnets is done at some routers and not at others. Ideally, all RIP routers have routes of the complete network. When the routing information differs from one router to the other, one of two possibilities could exist Some...

Problem Sender Is Not Advertising IGRP Routes

Typically, an IP network running IGRP has routers that have a consistent view of the routing table. In other words, all routers have routing tables that contain reachability information for all the IP subnets of the network. This might differ when filtering of certain subnets is done at some routers and not at others. Ideally, all IGRP routers are aware of all routes of the complete network. When the routing information differs from one router to the other, one of two possibilities could exist...

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...

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...

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...

Troubleshooting RIP Routes Installation

This section discusses several possible scenarios that can prevent RIP routes from getting installed in the routing table. This section is selected first in the troubleshooting list because the most common problem in RIP is that routes are not installed in the routing table. If the routes are not installed in the routing table, the router will not forward the packets to destinations that are not in the routing table. When this happens, it creates reachability problems. Users start complaining...

Troubleshooting IGRP Route Installation

This section discusses the most common scenarios that can prevent IGRP routes from getting installed in the routing table. This is the most useful section in this chapter because the most common problem in IGRP is that routes are not in the routing table. If a specific destination is not in the routing table, the packet destined for that address will be dropped. The easiest way to find out whether a specific route is in the routing table is with the show ip route x.x.x.x command, where x.x.x.x...

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

When a dialer interface (ISDN, for example) comes up, you might want 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. Therefore, running a dynamic routing protocol such as RIP is necessary. In some situations, the ISDN link might be up, but no routing informa-tion is going across. Without a routing protocol, no destination addresses can be learned and no traffic can be sent to those...

Problem Route Reflector Client Stores an Extra BGP Update Cause Clientto Client Reflection

The problem here stems from RRCs receiving extra BGP updates, which consume extra memory and take up CPU to process them. In Figure 15-28, RRC R8 peers IBGP with RR R1 R8 is peering IBGP with RRC R2 as well. Because of this peering relationship, R2 receives an extra BGP update for all the routes originated propagated by R8. Such a setup typically is done when a physical circuit exists between RRCs and the BGP operator wants to run BGP directly over them. In standard network design, such BGP...

Static and Dynamic Routes

Static path information can be manually programmed into the router and simply force the router to utilize a particular interface or next-hop IP address for forwarding packets with matching destination addresses. Static routes potentially could match a broad range of network addresses. Yet another way to obtain routing information is to use distributed applications enabled on routers that allow automatic collection and sharing of routing infor-mation. These routing applications frequently are...

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 RIP Broadcast Is Keeping the ISDN Link Up Cause RIP Broadcasts Have Not Been Denied in the Interesting Traffic

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

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 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 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....

Troubleshooting OSPF Route Advertisement

This section discusses the problems related with OSPF route advertisement. OSPF is a link-state protocol. When it forms neighbor relationships, it exchanges the entire link-state database with its neighbor(s). If any database information is not shared with the neighbor, the link-state characteristics of OSPF will break. The most common reasons for OSPF to not share the database information about a specific link are as follows The OSPF neighbor is not advertising routes. The OSPF neighbor (ABR)...