Appendix K. Topics from Previous Editions
Cisco changes the exams, renaming the exams on occasion, and changing the exam numbers every time it changes the exam with a new blueprint. We then make new editions of the books to match the new exams. (Once we even made a new edition of the book without a new version of the exam, just because it was a long time between new versions of the exam.) As a result, the current CCNA Routing and Switching exam (200-125) is the seventh version of that exam, and the two-book CCNA R&S Cert Guides are basically the eighth editions of the content in these books.
As with every new edition, the book content is based on Cisco’s exam topics; that is, the book attempts to cover the topics Cisco lists as exam topics. However, Cisco not only adds topics but also removes topics for each new edition. In some cases, I feel the need to keep some of the content covering exam topics that Cisco chose to remove. There are a few reasons why. Sometimes I just feel the need to keep that content around for that one reader in one thousand who might care. Also, more than a few schools use these books as textbooks. So, I decided to copy some of the old material as DVD appendixes.
In some cases, an old topic that exists as a complete chapter is an extra appendix available as softcopy only. For other, smaller topics, I have collected them into this DVD appendix. These topics were in some past editions, or even in drafts that did not get published in one or two cases. Regardless, the material is here in case you find it useful. But certainly do not feel like you have to read this appendix for the current exam.
The topics in this appendix are as follows:
Dial access with modems and ISDN
GLBP concepts and configuration
HSRP tracking
OSPFv2 link-state advertisements
OSPFv3 link-state advertisements
Note
The content under the heading “Dial Access with Modems and ISDN” was most recently published for the 200-101 Exam in 2013, in Chapter 15 of the Cisco CCNA Routing and Switching ICND2 200-101 Official Cert Guide.
Dial Access with Modems and ISDN
The two Internet access technologies discussed in this section require us to think back to the early days of the Internet for some perspective. The Internet had many booming growth periods over time, but one such period took off in the very early 1990s, when commercial traffic was beginning to drive huge growth in the Internet.
Back in those early days of the Internet, for consumers, most people accessed the Internet using dial-up. That is, they used their analog phone line and an analog modem and basically placed a phone call to an ISP.
As a brief bit of background, when using a home telephone line, a phone call creates an electrical circuit that uses analog signals. Computers use digital signals; so to use an analog circuit, something had to convert from digital to analog. The solution: an analog modem.
Analog modems would sit at each end of the call—one at the customer site, and one at the ISP. To send the digital data from the customer’s PC or router, the modem would modulate, or convert, the digital signal to an analog signal. The sending modem would then transmit the analog signals to the receiving modem, which would then demodulate the analog back into the original digits. (The term modem comes from the squashing of those two terms together: modulate and demodulate.)
Figure K-1 shows the general idea, with two examples. One shows a PC with an external modem, meaning that the PC connects to the modem with a cable. The other shows an internal modem. The ISP would then have a matching set of modems, called a modem bank. A phone call to the ISP’s phone number would ring to any available modem, allowing a customer to connect to any one of the ISP modems and be connected to the Internet.
Today, most ISPs refer to this option as dial access or simply dial. And even though ISPs have used it for decades, most ISPs still offer dial services. Dial can be inexpensive in some markets and a workable service for people in remote areas where faster Internet access options are not available.
Note
Telcos refer to the telephone cable that runs into a customer’s home or business the local loop.
Dial access happens to have several cost advantages compared to other consumer Internet access options. The ISP purposefully puts a point of presence (PoP) in most local calling areas, so the phone call to connect to the Internet is free, rather than having a long-distance charge. Also the equipment cost fell pretty quickly over time, so the price to get started is relatively low. And in many markets, almost every home has a home phone line already, so there is no need to spend more for the physical access link. As a result, the only added cost is the fee to the ISP to allow access into the Internet.
Of course, there are negatives, too. You can either surf the Internet or make a voice phone call, but not both. To use the Internet, you had to make a phone call first, so the Internet was not “on” all the time. But the speed is the biggest issue, with a fast modem having a bit rate over the line of only 56 Kbps, an incredibly slow speed by today’s standards.
Over time, the telcos of the world set out to improve over the analog modem option. One early improvement used an entirely new technology called Integrated Services Digital Network (ISDN). ISDN allowed some of the same cost advantages as analog modems, but with faster speeds. For instance:
ISDN used the same local loop (local phone line), which most people already had.
ISDN required the equivalent of a phone call to the ISP, just like with analog modems.
ISPs already had a PoP in each local calling areas to support analog modems, so these ISDN calls would not require any long-distance charges.
The big advantage of ISDN was speed. ISDN uses digital signals over the local loop, instead of analog. In addition, it supports two calls at the same time, each at 64 Kbps, over that one local loop phone line. Both calls (channels) could be dialed to the ISP, for a 128-Kbps Internet service. Or, the user could make one voice phone call and have one 64-Kbps Internet connection at the same time. ISDN did cost a little more—you had to pay the telco for the upgraded ISDN service—but you got concurrent Internet and voice, plus better speed than analog modems.
Figure K-2 shows some particulars of ISDN. The consumer side of an ISDN used a line called a Basic Rate Interface (BRI), which has the two 64-Kbps channels for user traffic. Physically, the connection used some type of ISDN-aware device, often referred to as an ISDN modem, taking the place of an analog modem.
The ISP side of the connection could use many different technologies, as well, including an ISDN technology called a Primary Rate Interface (PRI). This technology turned a T1 physical line into 23 ISDN channels ready to accept those ISDN calls, as shown on the right.
Both analog modems and ISDN filled big needs for Internet access in the early days of the Internet. Using existing phone lines that people already paid for anyway was a great business model. However, their relatively slow speeds led to innovation to faster Internet access—both from the telcos of the world and their emerging competitors of the time, the cable TV companies. Table K-1 summarizes a few of the key comparison points so far.
The content under the heading “Gateway Load Balancing Protocol (GLBP)” was most recently published for the 200-101 Exam in 2013, in Chapter 6 of the Cisco CCNA Routing and Switching ICND2 200-101 Official Cert Guide. The content was in two separate sections, one on GLBP concepts, one on GLBP configuration.
Gateway Load Balancing Protocol (GLBP)
This section first discusses GLBP concepts, followed by GLBP configuration.
GLBP Concepts
Hot Standby Router Protocol (HSRP) and Virtual Router Redundancy Protocol (VRRP), which were introduced before Gateway Load Balancing Protocol (GLBP), balanced the packet load per subnet. However, because traffic loads vary unpredictably from subnet to subnet, Cisco wanted a First Hop Redundancy Protocol (FHRP) option with better load-balancing options than just the per-subnet load balancing of HSRP and VRRP. To meet that need, Cisco introduced GLBP.
GLBP balances the packet load per host by using an active/active model in each subnet. Each GLBP router in a subnet receives off-subnet packets from some of the hosts in the subnet. Each host still remains unaware of the FHRP, allowing the hosts to configure the same default gateway/router setting and for the hosts to make no changes when a router fails.
GLBP creates a world that at first glance looks like HSRP, but with a few twists that let GLBP balance the traffic. Like HSRP, all the routers configure a virtual IP address, which is the IP address used by hosts as their default router. Like with HSRP, hosts use a default router setting that points to the virtual IP address, and that setting does not need to change. GLBP differs from HSRP with regard to the MAC addresses it uses and the Address Resolution Protocol (ARP) process, because GLBP actually uses ARP Reply messages to balance traffic from different hosts through different routers.
With GLBP, one router acts in a special role called the active virtual gateway (AVG). The AVG replies to all ARP requests for the virtual IP address. Each router has a unique virtual MAC address, so that the AVG can reply to some ARP Requests with one virtual MAC, and some with the other. As a result, some hosts in the subnet send frames to the Ethernet MAC address of one of the routers, with other hosts sending their frames to the MAC address of the second router.
As an example, Figure K-3 shows the process by which a GLBP balances traffic for host A based on the ARP Reply sent by the AVG (R1). The two routers support virtual IP address 10.1.1.1, with the hosts using that address as their default router setting.
The figure shows three messages, top to bottom, with the following action:
1. Host A has no ARP table entry for its default router, 10.1.1.1, so host A sends an ARP Request to learn 10.1.1.1’s MAC address.
2. The GLBP AVG, R1 in this case, sends back an ARP Reply. The AVG chooses to include its own virtual MAC address in the ARP Reply, VMAC1.
3. Future IP packets sent by host A are encapsulated in Ethernet frames, destined to VMAC1, so that they arrive at R1.
From now on, host A sends off-subnet packets to R1 due to host A’s ARP table entry for its default gateway (10.1.1.1). Host A’s ARP table entry for 10.1.1.1 now refers to a MAC address on R1 (VMAC1), so packets host A sends off-subnet flow through R1.
To balance the load, the AVG answers each new ARP Request with the MAC addresses of alternating routers. Figure K-4 continues the load-balancing effect with the ARP Request for 10.1.1.1 coming from host B. The router acting as AVG (R1) still sends the ARP Reply, but this time with R2’s virtual MAC (VMAC2).
Here are the steps in the figure:
1. Host B sends an ARP Request to learn 10.1.1.1’s MAC address.
2. The GLBP AVG (R1) sends back an ARP Reply, listing VMAC2, R2’s virtual MAC address.
3. For future packets sent off-subnet, host B encapsulates the packets in Ethernet frames, destined to VMAC2, so that they arrive at R2.
The process shown in Figures K-3 and K-4 balances the traffic, per host, but the routers must also be ready to take over for the other router if it fails. GLBP refers to each router as a forwarder. When all is well, each router acts as forwarder for its own virtual MAC address, but it listens to GLBP messages to make sure the other forwarders are still working. If another forwarder fails, the still-working forwarder takes over the failed forwarder’s virtual MAC address role and continues to forward traffic.
Configuring and Verifying GLBP
GLBP configuration mimics HSRP configuration to a great degree.
Example K-1 shows a GLBP configuration with both routers using GLBP group 1, with virtual IP address 10.1.1.1, with the glbp 1 ip 10.1.1.1 interface subcommand.
Example K-1 GLBP Configuration on R1 and R2, Sharing IP Address 10.1.1.1
! First, the configuration on R1
R1# show running-config
! Lines omitted for brevity
interface GigabitEthernet0/0
ip address 10.1.1.9 255.255.255.0
glbp 1 ip 10.1.1.1
glbp 1 priority 110
glbp 1 name GLBP-group-for-book
! The following configuration, on R2, is identical except for
! the interface IP address, and the GLBP priority
R2# show running-config
! Lines omitted for brevity
interface GigabitEthernet0/0
ip address 10.1.1.129 255.255.255.0
glbp 1 ip 10.1.1.1
glbp 1 name GLBP-group-for-book
Once configured, the two routers negotiate as to which will be the AVG. As with HSRP, if both come up at the same time, R1 will win, with a priority set to 110 with the glbp 1 priority 110 command versus R2’s default priority of 100. However, if either router comes up before the other, that router goes ahead and takes on the AVG role.
Sifting through the GLBP show command output takes a little more work than with HSRP, in particular because of the added detail in how GLBP works. First, consider the show glbp brief command on Router R1, as shown in Example K-2. (Note that many show glbp commands have the same options as equivalent HSRP show standby commands.)
Example K-2 GLBP Status on R1 with show glbp brief
R1# show glbp brief
Interface Grp Fwd Pri State Address Active router Standby router
Gi0/0 1 - 110 Active 10.1.1.1 local 10.1.1.129
Gi0/0 1 1 - Listen 0007.b400.0101 10.1.1.129 -
Gi0/0 1 2 - Active 0007.b400.0102 local
Before looking at the right side of the output, first consider the context for a moment. This example lists a heading line and three rows of data. These data rows are identified by the Grp and Fwd headings, short for Group and Forwarder. With only one GLBP group configured, R1 lists lines only for group 1. More important, each row defines details about a different part of what GLBP does, as follows:
Fwd is -: This line refers to none of the forwarders, and instead describes the AVG.
Fwd is 1: This line describes GLBP forwarder (router) 1.
Fwd is 2: This line describes GLBP forwarder (router) 2.
The output usually lists the line about the AVG first, as noted with a dash in the Forwarder column. Now look at the highlighted portions on the right of Example K-2. This line will list the virtual IP address and identify the active AVG and the standby AVG. This particular command, from Router R1, lists R1 itself (“local”) as the active router. So, R1 is the current AVG.
Each of the next two lines lists status information about one of the forwarder roles; that is, a router that uses a virtual MAC address, receives frames sent to that address, and routes the packets encapsulated in those frames. To that end, the Address column lists MAC addresses, specifically the virtual MAC addresses used by GLBP, and not the interface MAC addresses.
Each forwarder row also identifies the router that currently uses the listed virtual MAC in the Active Router column. In Example K-2, 0007.b400.0101 is used by the router with interface IP address 10.1.1.129 (which happens to be R2). 0007.b400.0102 is supported by the local router (the router on which the show command was issued), which is R1.
The brief output of the show glbp brief command lists many details, but it takes some effort to learn how to sift through it all. For more perspective on the output, Example K-3 lists this same show glbp brief command, this time on R2. Note that the Fwd column again identifies the first line of output as being about the AVG, with the next two lines about the two forwarders.
Example K-3 GLBP Status on R2 with show glbp brief
R2# show glbp brief
Interface Grp Fwd Pri State Address Active router Standby router
Gi0/0 1 - 100 Standby 10.1.1.1 10.1.1.9 local
Gi0/0 1 1 - Active 0007.b400.0101 local -
Gi0/0 1 2 - Listen 0007.b400.0102 10.1.1.9 -
The State column in the output in Examples K-2 and K-3 can pull the GLBP concepts together. First, to define the meaning of the state values, the following short list defines the states expected for the first line of output, about the AVG, and then about each GLBP forwarder:
AVG: One router should be the active AVG, with the other acting as standby, ready to take over the AVG role if the AVG fails.
Each forwarder: One router should be active, while the other should be listening, ready to take over that virtual MAC address if that forwarder fails.
Table K-2 collects the values of the State column from Examples K-2 and K-3 for easier reference side by side. Note that, indeed, each line has either an active/standby pair (for the AVG) or an active/listen pair (for the forwarder function).
Finally, the show glbp command lists a more detailed view of the current GLBP status. Example K-4 shows a sample from Router R1. Note that the first half of the output has similar information compared to HSRP’s show standby command, plus it lists the IP and MAC addresses of the routers in the GLBP group. Then, the end of the output lists a group of messages per GLBP forwarder.
Example K-4 GLBP Status on R1 with show glbp
R1# show glbp
GigabitEthernet0/0 - Group 1
State is Active
2 state changes, last state change 00:20:59
Virtual IP address is 10.1.1.1
Hello time 3 sec, hold time 10 sec
Next hello sent in 2.112 secs
Redirect time 600 sec, forwarder timeout 14400 sec
Preemption disabled
Active is local
Standby is 10.1.1.129, priority 100 (expires in 8.256 sec)
Priority 110 (configured)
Weighting 100 (default 100), thresholds: lower 1, upper 100
Load balancing: round-robin
IP redundancy name is "GLBP-group-for-book"
Group members:
0200.0101.0101 (10.1.1.9) local
0200.0202.0202 (10.1.1.129)
There are 2 forwarders (1 active)
Forwarder 1
State is Listen
2 state changes, last state change 00:20:34
MAC address is 0007.b400.0101 (learnt)
Owner ID is 0200.0202.0202
Redirection enabled, 598.272 sec remaining (maximum 600 sec)
Time to live: 14398.272 sec (maximum 14400 sec)
Preemption enabled, min delay 30 sec
Active is 10.1.1.129 (primary), weighting 100 (expires in 8.352 sec)
Client selection count: 1
Forwarder 2
State is Active
1 state change, last state change 00:24:25
MAC address is 0007.b400.0102 (default)
Owner ID is 0200.0101.0101
Redirection enabled
Preemption enabled, min delay 30 sec
Active is local, weighting 100
Client selection count: 1
Note
The content under the heading “HSRP Tracking” was most recently published for the 200-101 Exam in 2013, in Appendix B of the Cisco CCNA Routing and Switching ICND2 200-101 Official Cert Guide.
HSRP Tracking
Chapter 20 shows how to configure the HSRP priority so that one router will be preferred as the active router in an HSRP group. For example, Example 20-1 in Chapter 20 shows two routers in the same HSRP group, with R1 using a slightly better (numerically higher) priority of 110 and R2 using default priority 100, so that R1 becomes the active router when both routers are up and working. Figure K-5 shows a similar design, with Router R1 as active (priority 110) and R2 as standby (priority 100), using HSRP virtual IP address 10.1.1.1.
The network design benefits from another HSRP feature: interface tracking. IOS can track the state of an interface, with a variable for the interface as being either up or down. Then, you can change the HSRP priority value based on tracking variables, changing HSRP’s choice of which router is primary based on other events and status inside the router.
For example, notice the big X over the upper WAN link in Figure K-5. What happens when R1’s WAN link is down? Clearly, the WAN path through R2 and R4 should probably be used. However, all the hosts on the left still use R1 as their default gateway, when using R2 would clearly be more efficient. Instead, the HSRP configuration could be changed as follows:
Set the priority values as noted earlier, so that under normal operation, R1 is active: R1 = 110, R2 = 100.
R1 tracks its S0/0/0 interface, such that when S0/0/0 fails, R1 lowers its HSRP priority by 20.
R2 tracks its S0/0/1 interface, such that when S0/0/1 fails, R2 lowers its HSRP priority by 20.
When the standby router priority becomes better (higher) than the currently active router, take over the role of HSRP active (a feature called preemption).
Example K-5 completes the picture of HSRP interface tracking. Example K-5 shows R1’s basic HSRP configuration, with tracking of the WAN interface as described here, and enables preemption. (Similar configuration would need to be added to R2 as well.) As a result, when all links work, R1 remains the active HSRP router, with priority 110. If R1’s WAN link then fails, R1’s priority falls to 90 and R2’s remains at 100, so R2 preempts R1’s active role so that R2 takes over as the active HSRP router.
Example K-5 HSRP Configuration on R1
interface GigabitEthernet0/0
ip address 10.1.1.9
standby version 2
standby ip 10.1.1.1
standby 1 priority 110
standby 1 track serial0/0/0 20
standby 1 preempt
FHRPs all provide some level of tracking. For example, HSRP can track interfaces and use more complex object tracking that considers multiple factors to reach a decision.
Note
The content under the heading “(OSPFv2) Link-State Advertisements” was most recently published for the 200-101 Exam in 2013, in Chapter 8 of the Cisco CCNA Routing and Switching ICND2 200-101 Official Cert Guide, under the heading “Link-State Advertisements.”
(OSPFv2) Link-State Advertisements
Many people tend to get a little intimidated by OSPF LSAs when first learning about them. The output of the show ip ospf database command—a command that lists a summary of the output—is pretty long. Commands that look at specific LSAs list a lot more information. The details appear to be in some kind of code, using lots of numbers. It can seem like a bit of a mess.
However, if you examine LSAs while thinking about OSPF areas, and area design, some of the most common LSA types will make a lot more sense. For instance, think about the LSDB in one area. The topology details includes routers and the links between the routers. As it turns out, OSPF defines the first two types of LSAs to define those exact details, as follows:
One router LSA for each router in the area
One network LSA for each network that has a DR plus one neighbor of the DR
Next, think about the subnets in the other areas. The ABR creates summary information about each subnet in other areas—basically just the subnet IDs and masks—as a third type of LSA:
One summary LSA for each subnet ID that exists in a different area
The next few pages discuss these three LSA types in a little more detail; Table K-3 lists some information about all three for easier reference and study.
Note
In some networks, both OSPF and other routing protocols are used. In that case, one or more routers run both OSPF and the other routing protocol, with those routers acting as an OSPF Autonomous System Border Router, or ASBR, redistributing routing information between OSPF and the other protocol. In such a case, the ASBR creates a Type 4 LSA, which describes the ASBR itself, and Type 5 LSAs for each external route learned from the other routing protocol and then advertised into OSPF.
Router LSAs Build Most of the Intra-Area Topology
OSPF needs very detailed topology information inside each area. The routers inside area X need to know all the details about the topology inside area X. And the mechanism to give routers all these details is for the routers to create and flood router (Type 1) and network (Type 2) LSAs about the routers and links in the area.
Router LSAs, also known as Type 1 LSAs, describe the router in detail. Each lists a router’s RID, its interfaces, its IPv4 addresses and masks, its interface state, and notes about what neighbors the router knows out its interfaces.
To see a specific instance, first review Figure K-6. It lists internetwork topology, with subnets listed. As a small internetwork, the engineer chose a single-area design, with all interfaces in backbone area 0.
With the single-area design planned for this small internetwork, the LSDB will contain four router LSAs. Each router creates a router LSA for itself, with its own RID as the LSA identifier. The LSA lists that router’s own interfaces, IP address/mask, with pointers to neighbors.
Once all four routers have copies of all four router LSAs, SPF can mathematically analyze the LSAs to create a model. The model looks a lot like the concept drawing in Figure K-7. Note that the drawing shows each router with an obvious RID value. Each router has pointers that represent each of its interfaces, and because the LSAs identify neighbors, SPF can figure out which interfaces connect to which other routers.
Network LSAs Complete the Intra-Area Topology
Whereas router LSAs define most of the intra-area topology, network LSAs define the rest. As it turns out, when OSPF elects a DR on some subnet and that DR has at least one neighbor, OSPF treats that subnet as another node in its mathematical model of the network. To represent that network, the DR creates and floods a network (Type 2) LSA for that network (subnet).
For instance, back in Figure K-6, one Ethernet LAN and one Ethernet WAN exist. The Ethernet LAN between R2 and R3 will elect a DR, and the two routers will become neighbors; so, whichever router is the DR will create a network LSA. Similarly, R1 and R4 connect with an Ethernet WAN, so the DR on that link will create a network LSA.
Figure K-8 shows the completed version of the intra-area LSAs in area 0 with this design. Note that the router LSAs actually point to the network LSAs when they exist, which lets the SPF processes connect the pieces together.
Note
The drawings in the last two figures work a little like a jigsaw puzzle. The SPF algorithm basically solves the jigsaw puzzle, but by looking at all the numbers inside the different LSAs, to see which LSAs fit next to which other LSAs.
Finally, note that in this single-area design example no summary (Type 3) LSAs exist at all. These LSAs represent subnets in other areas, and there are no other areas. The next example shows some summary LSAs.
LSAs in a Multiarea Design
Migrating from a single-area design to a multiarea design has a couple of effects on LSAs:
Each area has a smaller number of router and network LSAs.
The ABRs have a copy of the LSDB for each area to which they connect.
The ABRs each have a router LSA in each area’s LSDB.
Each area has a need for some summary (Type 3) LSAs to describe subnets in other areas.
Before focusing on these summary LSAs, first work through a new example for a moment. Figure K-9 begins this new example using the same internetwork topology as Figure K-6, but now with a multiarea design, with Router R1 as the only ABR.
Figure K-9 Multiarea Design for the Same Internetwork as Figure K-6
Next, consider what router and network LSAs should be in the area 4 LSDB. Remember, inside an area, the LSDB should have router LSAs for routers inside the area, and network LSAs for certain networks inside the area (those with a DR that has at least one neighbor). So, the area 4 LSDB will include two router LSAs (for R1 and R4), plus one network LSA, for the network between R1 and R4, as shown in Figure K-10.
Figure K-10 Router and Network LSAs in Area 4 Only, Assuming the Multiarea Design in Figure K-9
Now focus on the subnets in the entire internetwork for a moment. Breaking it down by area, we have the following:
Three subnets in area 23
Two subnets in area 4
Two subnets in area 0
The routers inside area 4 need to know about the five subnets outside area 4, and to do that, the ABR (R1) advertises summary LSAs into area 4.
A summary (Type 3) LSA describes a subnet that sits in another area. First, it has to list the subnet ID and mask to identify the specific subnet. The LSA also lists the RID of the ABR that creates and advertises the summary LSA into the area. By identifying the ABR, from a topology perspective, these subnets appear to be connected to the ABR. In this new example, ABR R1 creates and floods the five summary LSAs shown in the upper left of Figure K-11.
Note
The OSPF summary LSA does not mean that the router is performing route summarization, which is the process of taking multiple routes, for multiple subnets, and advertising them as one route for a larger subnet.
Note
The content under the heading “Mismatched OSPF Network Types” was most recently published for the 200-101 Exam in 2013, in Chapter 11 of the Cisco CCNA ICND2 Routing and Switching 200-101 Official Cert Guide. (For this appendix, the content has been edited for clarity in the context of this book.)
Mismatched OSPF Network Types
OSPF defines a concept for each interface called a network type. The OSPF network type tells OSPF some ideas about the data link to which the interface connects. In particular, the network type tells a router:
Whether the router can dynamically discover neighbors on the attached link (or not)
Whether to elect a DR and BDR (or not)
This book happens to cover configuration details that require only two OSPF network types. The OSPF network type called point-to-point is used by default in obvious point-to-point topologies. Those include serial interfaces that use some point-to-point data link protocol, like HDLC or PPP, as well as the point-to-point GRE tunnels included in Chapter 15, “Private WANs with Internet VPN.” The other OSPF network type, the broadcast network type, is used on broadcast media, in which all devices can communicate directly with all other devices. Ethernet interfaces default to use an OSPF network type of broadcast.
The OSPF network type changes OSPF’s behavior on an interface, in particular, in regard to whether the router dynamically discovers other routers using Hello messages, and whether the routers on the link attempt to elect a DR and BDR or not. Of the two OSPF network types used by default in this book, both types allow the routers to dynamically discover the neighboring OSPF routers. However, only the broadcast network type causes the router to use a DR/BDR; the point-to-point network type does not, because a DR/BDR would serve no useful purpose in a point-to-point topology.
The show ip ospf interface command lists an interface’s current OSPF network type. Example K-6 shows Router R1, from the earlier examples, with a network type of “broadcast” on its G0/0 interface.
Example K-6 Displaying the OSPF Network Type on an Interface
R1# show ip ospf interface g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 10.1.1.1/24, Area 0, Attached via Network Statement
Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
! Lines omitted for brevity
It is possible to change the OSPF network type on an interface and, by making poor choices about the settings on neighboring routers, to prevent the routers from becoming OSPF neighbors. Normally, engineers either leave this setting at its default value or change the setting for all routers on the same link. However, by choosing poorly, and using different network types on different neighboring routers, problems can occur.
For instance, if Routers R1 and R2 from the sample internetwork used in this appendix still connect to the same VLAN, both using their G0/0 interfaces, they both by default use OSPF network type broadcast. These routers work best on their Ethernet interfaces with an OSPF network type of broadcast. As a result, both dynamically learn about each other as an OSPF router, and they both try to use a DR/BDR. However, if R1 was changed to use network type point-to-point on its G0/0 interface instead, problems occur. The result? The routers actually still become neighbors, because both network type broadcast and network type point-to-point allow for the dynamic discovery of OSPF neighbors. However, the two routers fail to exchange their LSDBs, as shown by R1, because one router is attempting to use the process that relies on a DR, while the other router is not. Example K-7 shows an example matching this paragraph’s description.
Example K-7 Mismatched OSPF Network Types Causing a Failure to Exchange LSDBs
R1# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)# interface gigabitethernet0/0
R1(config-if)# ip ospf network point-to-point
R1(config-if)# ^Z
R1#
R1# show ip route ospf
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
+ - replicated route, % - next hop override
Gateway of last resort is not set
R1#
! Lines omitted for brevity
Note that to the depth discussed in this book, using the default OSPF network types makes perfect sense. However, more complex topologies can drive the need to use different OSPF network types, even with different settings on some routers in the same subnet. The CCNP and CCIE R&S exams include more details about how to use the OSPF network type. Note that the ICND2 200-105 Cert Guide’s Appendix I, “Implementing Frame Relay,” discusses one use of OSPF network types with Frame Relay.
Note
The content under the heading “(OSPFv3) Link-State Advertisements” was most recently published for the 200-101 Exam in 2013, in Chapter 17 of the Cisco CCNA Routing and Switching ICND2 200-101 Official Cert Guide.
(OSPFv3) Link-State Advertisements
The next section examines OSPF LSAs as defined by OSPFv3 for use in advertising IPv6 routes.
OSPFv3 LSDB and LSAs
Once OSPFv3 routers become neighbors, they proceed to exchange their LSDBs over that subnet. In most cases, the two routers exchange their LSDBs directly, and when finished, each router lists its neighbor as having reached a full state. Once in a full state, the two routers should have the same link-state advertisements (LSA) for that area.
This section takes a brief look at the LSDB and the LSAs in an area, which once again look similar to the LSDB and LSAs used for OSPFv2. Then this section looks at one rare configuration issue that allows two routers to become OSPFv3 neighbors for a short time, while causing the topology exchange process to fail.
Verifying OSPFv3 LSAs
OSPFv3 uses similar concepts, with slightly different naming for the equivalent of OSPFv2’s Type 1, 2, and 3 LSAs. As explained back in the previous section, “(OSPFv2) Link-State Advertisements,” OSPFv2 uses the Type 1 router LSA and Type 2 network LSA to define the topology inside an area. The Type 3 summary LSA then describes for one area a subnet that exists in some other area—an interarea subnet, if you will.
For the configuration options shown for OSPFv2 in this book, only these three types of LSAs are needed in the OSPFv2 LSDB.
OSPFv3 keeps those same three LSA concepts, renaming the summary LSA. The following list summarizes these three key OSPFv3 LSA types and the reasons why OSPFv3 routers create each:
One router LSA (Type 1 LSA) for each router in the area (including ABRs attached to the area)
One network LSA (Type 2 LSA) for each network that has a DR plus one neighbor of the DR
One interarea prefix (Type 3 LSA) LSA for each IPv6 prefix (subnet) that exists in a different area
For example, in area 4 in the sample network used in this appendix, two routers exist: internal router R4 and ABR R1. So, the area 4 LSDB will have a router LSA for each router. One network exists in this area for which a DR will be used (the Ethernet WAN between R1 and R4). R1 and R4 will become neighbors, as well, so one network LSA will be created for that network. Finally, ABR R1 will know about five different IPv6 prefixes that exist outside area 4, so ABR R1 should create and flood five interarea prefix LSAs into area 4. Figure K-12 shows the conceptual model of these LSAs for area 4.
Beyond this basic LSA structure, OSPFv3 does make several changes to LSAs compared to OSPFv2. The details inside these LSAs change, and OSPFv3 adds several new LSA types not seen in OSPFv2. However, these details are beyond the scope of this book.
To see the LSAs of Figure K-12 in an actual router, Example K-8 lists the beginning of the area 4 LSDB as it exists in Router R4. The example highlights the headings and the IPv6 prefixes of the interarea prefix LSAs. Note that the output indeed shows two router LSAs, one line for the single network LSA and five lines with the interarea prefixes.
Example K-8 LSDB Content in Area 4, as Viewed from R4
R4# show ipv6 ospf database
OSPFv3 Router with ID (4.4.4.4) (Process ID 4)
Router Link States (Area 4)
ADV Router Age Seq# Fragment ID Link count Bits
1.1.1.1 258 0x80000072 0 1 B
4.4.4.4 257 0x80000003 0 1 None
Net Link States (Area 4)
ADV Router Age Seq# Link ID Rtr count
4.4.4.4 257 0x80000001 4 2
Inter Area Prefix Link States (Area 4)
ADV Router Age Seq# Prefix
1.1.1.1 878 0x80000069 2001:DB8:1:1::/64
1.1.1.1 878 0x80000068 2001:DB8:1:2::/64
1.1.1.1 364 0x8000000A 2001:DB8:1:13::/64
1.1.1.1 364 0x8000000A 2001:DB8:1:23::/64
1.1.1.1 364 0x8000000A 2001:DB8:1:12::/64
! Lines omitted for brevity
Note
The content under the heading “IPv6 Routing Protocols as Discussed in this Book” has not been previously published in this book, but was written in an early draft for the Cisco CCNA Routing and Switching ICND2 200-105 Official Cert Guide.
IPv6 Routing Protocols as Discussed in This Book
This book discusses the traditional approach to OSPF only; that is, other than this brief introduction, it ignores OSPFv3 address families. In the earlier chapters of this book, all the OSPF examples you have seen for IPv4 are OSPFv2 examples. This section shows examples of OSPFv3, for IPv6 routes only, and specifically OSPFv3 without using the address families feature.
EIGRP (discussed in Chapter 24) has gone through a similar transformation as OSPF through its history of IPv6 support. Cisco calls the traditional EIGRP style either EIGRP classic mode or EIGRP autonomous system mode. EIGRP supports a newer mode called EIGRP named mode; this mode, like OSPFv3’s address family mode, uses address families in the configuration. Chapter 24 discusses EIGRP for IPv6 classic mode.
To close the list, Routing Information Protocol (RIP) also supports IPv6, but its history does not track quite as closely. RIP has two versions that support IPv4, with the expected names of RIP version 1 (RIPv1) and RIP version 2 (RIPv2). To support IPv6, a working group created a new version of RIP, called RIP Next Generation (RIPng), with the name chosen in reference to the Star Trek TV series. (Yep.) (Cisco often refers to this protocol today as RIP for IPv6.) To date, RIPng support in IOS has not gone through a similar transformation to include address families.
Table K-4 summarizes the names of the protocols for easier review and study. This book discusses the more classic configuration options, and does not discuss address family configuration in any detail.
