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 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....

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 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...

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...

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 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...

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 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 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 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 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...

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...

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...

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...

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...

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 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

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 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 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

Propagate Out Default Route Thru Ospf

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 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 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 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

Bgp Route Filter

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 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...

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...

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...

Problem Redistributed Routes Are Not Getting Installed in the Routing Table Cause Metric Is Not Defined During

Ospf Flowchart

IGRP has a composite metric made up of bandwidth, delay, reliability, load, and MTU however, by default, it utilizes only bandwidth and delay. OSPF's metric is based on interface cost. Cost is derived from the bandwidth of the link. Cisco uses 100,000,000 bandwidth to get the cost. IGRP does not understand the metrics of other protocols (except EIGRP) so it is necessary to input a default metric when doing redistribution. Figure 5-32 shows the network setup susceptible to the problem in which...

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 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 RIP2 Routing Table Is Huge Cause Autosummarization Is

When a RIP update crosses a major network, it summarizes to the classful boundary. For example, 131.108.1.0, 131.108.2.0, and 131.108.3.0 will be autosummarized to 131.108.0.0 16 when advertised to a router with no 131.108.X.X addresses on its inter-faces. Disabling the autosummarization feature increases the size of the routing table. In some situations, this feature must be turned off (for example, if discontiguous networks exist, as discussed earlier). Figure 3-39 shows the flowchart to...

Problem RIP2 Routing Table Is Huge Cause ip summaryaddress Is Not Used

Figure 3-40 shows the network setup that could produce a large routing table. Figure 3-40. Network Setup That Could Generate a Large Routing Table Figure 3-40. Network Setup That Could Generate a Large Routing Table Figure 3-40 shows that R2 is announcing several subnets of 131.108.0.0 network. Notice that the link between R1 and R2 is also part of the 131.108.0.0 network, so autosummar-ization cannot play any role to solve the problem of receiving a subnet route that could be summarized. The...

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

Ospf Flowchart

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...

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

Bgp Route 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...

Problem Subnetted Routes Missing from the Routing Table of R2Cause Autosummarization Feature Is Enabled

In some situations, subnetted routes are not advertised in RIP. Whenever RIP sends an update across a major network boundary, the update will be autosummarized. This is not really a problem this is done to reduce the size of the routing table. Figure 3-36 shows a network setup in which R1 has subnets of 155.155.0.0, but R2 shows none of these subnets in its routing table. Either R1 is not advertising them to R2, or R2 is not receiving them. The chances of R1 not advertising more specific...

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...

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...

RIP1 Packet Format

The maximum datagram size in RIP is 512 octets. The first byte is used for commands such as rip update request and rip update response. The next byte is used for the Version field, which is set to 1 for RIP-1. The next 2 bytes must be 0. The 2-byte field after this is used for the address family identifier the next 14 bytes are allocated for the network address, as shown in Figure 2-2. In the case of IP, only 4 bytes of those 14 are used for the IP address. The remaining 10 bytes are unused in...

Routes Toward R2

Figure 3-21 shows the network setup in which Router R1 is not sending RIP routes toward R2. Figure 3-21 shows the network setup in which Router R1 is not sending RIP routes toward R2. The sections that follow carefully dissect how to troubleshoot this problem based on specific causes. Sender Is Not Advertising RIP Routes Cause Missing or Incorrect network Statement One of the requirements for enabling RIP on a router's interface is to add the network statement under the router rip command. The...

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...

Split Horizon with Poison Reverse

Another technique to avoid routing loop is split horizon with poison reverse. Using this technique, routes learned on an interface are advertised back on the same interface with an IGRP metric of infinity. This is called poison update. When the router receives the poison update, it considers the route as invalid. For example, in Figure 4-2, Router 1 receives an update for Network X from Router 2. With poison reverse, this specific route is advertised back to Router 2, but with an IGRP metric of...

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 BGP Neighbor Relationships

Directly Connected External BGP Neighbors Wot Initializing Can you ping thedireclly connected BGP neighbor Is neighbor IP address configured correctly Nondirectly Connected External BGP Neighbors Not Coming Up Is ebgp-mullihop command configured 1 update-source command configured Is this router exhibiting the same behavior with all neighbors This router may be malfunction ng. Is the interface access lis blocking TCPvBGP

Troubleshooting BGP Route Not Installing in Routing Table

This section discusses issues related to BGP routes not getting installed in the IP routing table. If a router must forward an IP packet by looking at the IP destination address in IP packet, the router must have an IP routing table entry for the subnet of the IP destination address. If the BGP process fails to create an IP routing table entry, all traffic destined for missing IP subnets in the routing table will be dropped. This is a generic behavior of hop-by-hop IP packet forwarding done by...

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 IGMP Joins

Neighbor Joins

As discussed in Chapter 12, Understanding Protocol Independent Multicast (PIM), IGMP joins are a line of communication between the multicast receiver (host) and the router. IGMP joins are used by the multicast receiver to inform the multicast router that hosts on the local segment are interested in certain multicast groups this allows the router to forward multicast packet to the segment. This section discusses issues with IGMP joins. Refer to Figure 13-1 for the troubleshooting flowchart on...

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...

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...

Troubleshooting ISIS Adjacency Problems

IS-IS adjacency-related problems normally are caused by link failures and configuration errors. On Cisco routers, link failures easily can be identified by inspection of a show interface command output. Also, because IS-IS routing is not required to establish IP connectivity to directly attached routers, it is easy to discern whether the problem is media-related or specific to the IS-IS configuration. The show clns neighbors command is usually the starting point for troubleshooting IS-IS...

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...

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)...

Troubleshooting Outbound IP Traffic Flow Issues Because of BGP Policies

BGP's real power is in managing IP traffic flows coming in and going out of the AS. BGP in general and Cisco IOS Software in particular offer a great deal of flexibility in manipulating BGP attributes LOCAL_PREFERENCE, MED, and so forth to control BGP best-path calculation. This best-path decision determines how traffic exits the AS. With the large size of BGP networks today, it is crucial that BGP operators understand how BGP attributes should be managed. This section discusses what problems...

Troubleshooting RIP Redistribution Problems

This section talks about problems that can happen during redistribution in RIP. Redistribution refers to the case when another routing protocol or a static route or connected route is being injected into RIP. Special care is required during this process to avoid any routing loops. In addition, metric (hop count) should be defined during this process, to avoid problems. The most prevalent problem encountered with RIP redistribution is that redistributed routes are not being installed in the...

Troubleshooting RIP Routes Advertisement

All the problems discussed so far deal with the problem on the receiving end or the problem in the middle (Layer 2). A third possible cause exists when routes are not being installed in the routing table. The sender could be having a problem sending RIP updates for some reason. As a result, the receiver cannot install the RIP routes in the routing table. This section talks about the things that can go wrong on the sender's side. This section discusses some of the possible scenarios that can...

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 Route Flapping Problem in IGRP

Running IGRP in a complex environment sometimes can cause flapping of routes. Route flapping means that the routes keep coming and going from the routing table. To see 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, the routes are flapping. There could be several reasons for this. This section discusses one of the most common reasons packet loss. Packet loss prevents an IGRP update from...

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...

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...

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...

Warning and Disclaimer

This book is designed to provide information about troubleshooting IP routing protocols, including RIP, IGRP, EIGRP, OSPF, IS-IS, PIM, and BGP. Every effort has been made to make this book as complete and as accurate as possible, but no warranty or fitness is implied. The information is provided on an as is basis. The authors, Cisco Press, and Cisco Systems, Inc. shall have neither liability nor responsibility to any person or entity with respect to any loss or damages arising from the...

Who Should Read This Book

This is an intermediate-level book that assumes that you have a general understanding of IP routing technologies and other related protocols and technologies used in building IP networks The primary audience for this book consists of network administrators and network operation engineers responsible for the high availability of their networks, or those who plan to become Cisco Certified Internetwork Experts.

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...

Acknowledgments

Alhamdulillah I thank God for giving me the opportunity to write this book, which I hope will help many people in resolving their routing issues. I would like to thank my manager, Srinivas Vegesna, and my previous manager and mentor, Andrew Maximov, for supporting me in this book project. Special thanks goes to Bob Vigil, who let me use some of his presentation material in the RIP and IGRP chapter. I would also like to thank Alex Zinin for clearing some of my OSPF concepts that I used in this...

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....

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 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 SPF Calculation and Route Flapping

Ospf Flowchart

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...

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

When Use Ibgp

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...

Troubleshooting PIM Dense Mode

Pim Sparse Dense Mode Diagram

Multicast dense mode operation is very simple it uses the flood and prune mechanism to form a multicast forwarding tree. Because of the simplicity in operation, troubleshooting PIM dense mode is also very simple. Most of the PIM dense mode problem is related to Reverse-Path Forwarding (RPF) check failure and Time to Live (TTL) value problems. Figure 13-3 shows the troubleshooting flowchart for multicast dense mode. Figure 13-3. Flowchart for Troubleshooting Multicast Dense Mode Figure 13-3....

About the Technical Reviewers

Brian Morgan, CCIE 4865, CCSI, is the Director of Data Network Engineering at Allegiance Telecom, Inc. He has been in the networking industry for more than 12 years. Before going to Allegiance, Morgan was an instructor consultant teaching ICND, BSCN, BSCI, CATM, CVOICE, and BCRAN. He is a co-author of the Cisco CCNP Remote Access Exam Certification Guide and a technical editor of numerous Cisco Press titles. Harold Ritter, CCIE 4168, is a network consulting engineer for Cisco Advanced Network...

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...

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...

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....