802.1Q Tunnels

Tunneling is a way to send 802.1Q-tagged frames across a foreign network (such as a Service Provider’s network) and still preserve the original 802.1Q tag. The SP configures their end of the trunk link as a tunnel port and assigns a VLAN to carry your traffic within their network. The SP switch then adds a second 802.1Q tag to each frame that came in the tunnel port. Other switches in the SP network see only this second tag, and do not read the original tag. When the frame exits the SP network, the extra tag is removed, leaving the original 802.1Q tag to be read by the receiving switch in your network.


Layer 2 Protocol Tunneling (GBPT)

If a Service Provider separates sections of your network, you can use Layer 2 protocol tunneling to tunnel CDP, Spanning Tree Protocol (STP), and VLAN Trunking Protocol (VTP) frames across the SP’s cloud. This is called Generic Bridge PDU Tunneling (GBPT). Frames from the above control protocols are encapsulated as they enter the SP’s network on a tunnel port, and de-encapsulated when they exit that network.


Troubleshooting Trunking

Troubleshooting trunking links happens mostly at the physical and datalink layers. Start with the most basic assumptions and work your way “up” the OSI model. It is important to show that physical layer connectivity is present, before moving on to, for instance before trying to troubleshoot IP problems.

• Are both sides of the link in the correct trunking mode?

• Is the same trunk encapsulation on both sides?

• If 802.1Q, is the same native VLAN on both sides?

• Are the same VLANs permitted on both sides?

VLAN Trunking Protocol (VTP)

VTP is a protocol that runs over trunk links and synchronizes the VLAN databases of all switches in the VTP domain. A VTP domain is an administrative group—all switches within that group must have the same VTP domain name configured or they do not synchronize databases.
VTP works by using Configuration Revision numbers and VTP advertisements:

• All switches send out VTP advertisements every five minutes, or when there is a change to the VLAN database (when a VLAN is created, deleted, or renamed).

• VTP advertisements contain a Configuration Revision number. This number is increased by one for every VLAN change.

• When a switch receives a VTP advertisement, it compares the Configuration Revision number against the one in its VLAN database.

• If the new number is higher, the switch overwrites its database with the new VLAN information, and forwards the information to its neighbor switches.

• If the number is the same, the switch ignores the advertisement.

• If the new number is lower, the switch replies with the more up-to-date information contained in its own database.

VTP Switch Roles

A switch can be a VTP:

• Server—The default VTP role. Servers can create, delete, and rename VLANs. They originate both periodic and triggered VTP advertisements and synchronize their databases with other switches in the domain.
  

• Client—Clients cannot make VLAN changes. They originate periodic VTP advertisements and synchronize their databases with other switches in the domain.

• Transparent—It can create, delete, and rename VLANs, but its VLANs are only local. It does not originate advertisements or synchronize its database with any other switches. It forwards VTP advertisements out its trunk links, however.

VTP Pruning

By default, switches flood broadcasts, multicasts, and unknown unicasts across trunk links. Suppose a host in VLAN 10 on Switch B sends a broadcast. Hosts in VLAN 10 on Switch C need to see that broadcast, but Switch A has no ports in VLAN 10, so it doesn’t need to receive the broadcast traffic.

Enabling VTP pruning causes the switch to keep track of VLAN port assignments in its downstream switches. The switch then sends flooded traffic only on trunks toward switches that have ports assigned to the VLAN originating the traffic. It prunes flooded traffic from all other trunks. VTP pruning increases the available bandwidth by preventing unnecessary traffic on trunk links.

There are two versions of VTP: Version 1 and Version 2. To use Version 2, all switches in the domain must be capable of using it. Configure one server for Version 2, and the information is propagated through VTP.
Version 2 has the following added features:

• It supports Token Ring VLANs.

• Transparent switches pass along messages from both versions of VTP.

• Consistency checks are performed only when changes are configured through the CLI or SNMP.

Configuring VTP

VTP configuration is done at the global config mode. To configure the switch’s VTP mode:
(config)#vtp {server | client |transparent}

To configure the VTP domain name:
(config)#vtp domain name

To configure a VTP password (all switches in the domain must use the same password):
(config)#vtp password password

To configure the switch to use VTP Version 2:
(config)#vtp version 2

To enable pruning:
vtp pruning

To specify which VLANs are to be pruned:
(config-if)#switchport trunk pruning vlan {add | except | none | remove} vlan-list [,vlan[,vlan[,,,]]


Verifying and Monitoring VTP

To get basic information about the VTP configuration, use show vtp status. The example shows the default settings:

Troubleshooting VTP

The following are some common things to check when troubleshooting problems with VTP:

• Make sure you are trunking between the switches. VTP is sent only over trunk links.

• Make sure the domain name matches on both switches (name is case sensitive).

• If the switch is not updating its database, make sure it is not in transparent mode.

• If using passwords, make sure they all match. To remove a password, use no vtp password.

Adding a New Switch to a VTP Domain

Adding a new switch in client mode does not prevent it from propagating its incorrect VLAN information. A server synchronizes to a client if the client has the higher configuration revision number. You must reset the revision number back to 0 on the new switch. The easiest way to do this is to change the domain name. Then change it back to the correct one, and attach the switch to the network.

VLAN Trunking

A trunk is a link that carries traffic for more than one VLAN. Trunks multiplex traffic from multiple VLANs. Trunks connect switches and allow ports on multiple switches to be assigned to the same VLAN.

Two methods of identifying VLANs over trunk links are:

• Inter-Switch Link (ISL)—A Cisco proprietary method that encapsulates the original frame in a header, which contains VLAN information. It is protocol-independent and can identify Cisco Discovery Protocol (CDP) and bridge protocol data unit (BPDU) frames.
  

• 802.1Q—Standards-based, tags the frames (inserts a field into the original frame immediately after the source MAC address field), and supports Ethernet and Token Ring networks.

When a frame comes into a switch port, the frame is tagged internally within the switch with the VLAN number of the port. When it reaches the outgoing port, the internal tag is removed. If the exit port is a trunk port, then its VLAN is identified in either the ISL encapsulation or the 802.1Q tag. The switch on the other end of the trunk removes the ISL or 802.1Q information, checks the VLAN of the frame, and adds the internal tag. If the exit port is a user port, then the original frame is sent out unchanged, making the use of VLANs transparent to the user.

If a nontrunking port receives an ISL-encapsulated frame, the frame is dropped. If the ISL header and footer cause the MTU size to be exceeded, it might be counted as an error.

If a nontrunking port receives an 802.1Q frame, the source and destination MAC addresses are read, the tag field is ignored, and the frame is switched normally at Layer 2.


Configuring a Trunk Link

Ports can become trunk ports either by static configuration or dynamic negotiation using Dynamic Trunking Protocol (DTP). A switch port can be in one of five DTP modes:

• Access—The port is a user port in a single VLAN.
• Trunk—The port negotiates trunking with the port on the other end of the link.
• Non-negotiate—The port is a trunk and does not do DTP negotiation with the other side of the link.
• Dynamic Desirable—Actively negotiates trunking with the other side of the link. It becomes a trunk if the port on the other switch is set to trunk, dynamic desirable, or dynamic auto mode.

Configure a port for trunking at the interface configuration mode:

(config-if)#switchport mode {dynamic {auto | desirable} | trunk}

If dynamic mode is used, DTP negotiates the trunking state and encapsulation. If trunk mode is used, you must specify encapsulation:

(config-if)#switchport trunk encapsulation {isl | dot1q | negotiate}


Native VLAN with 802.1Q

If you are using 802.1Q, specify a native VLAN for the trunk link with the command:

(config-if)#switchport trunk native vlan vlan no

Frames from the native VLAN are sent over the trunk link untagged. Native VLAN is the VLAN the port would be in if it were not a trunk, and it must match on both sides of the trunk link. VLAN 1 is the default native VLAN for all ports.


VLAN Mapping

ISL trunking recognizes only VLANs numbered 1–1001, but 802.1Q can use VLANs 0–4094. If you are using both ISL and 802.1Q in your network and have VLANs numbered above 1001, you have to map the 802.1Q VLANS to ISL numbers. Some rules about mapping VLANs include:

• You can configure only eight mappings.
• Mappings are local to the switch; the same mappings must be configured on all switches in the network.
• You can map only to Ethernet ISL VLANs.
• The 802.1Q VLANs with the same number as mapped ISL VLANs are blocked. (For example, you map 802.1Q VLAN 1500 to ISL VLAN 150, then 802.1Q VLAN 150 is blocked on that switch.)
• You should not map the 802.1Q native VLAN.

VLANs Allowed on the Trunk

By default, a trunk carries traffic for all VLANs. You can change that behavior for a particular trunk link by giving the following command at the interface config mode:

switchport trunk allowed vlan vlans

Make sure that both sides of a trunk link allow the same VLANs.


Verifying a Trunk Link

Two commands you can use to verify your trunk configuration are:

#show running-config
#show interfaces [interface no.] switchport | trunk


Using the trunk keyword with the show interfaces command gives information about the trunk link:

VLAN Implementation

VLANs are used to break large campus networks into smaller pieces. The benefit of this is to minimize the amount of broadcast traffic on a logical segment.

What Is a VLAN?

A virtual LAN (VLAN) is a logical LAN, or a logical subnet. It defines a broadcast domain. A physical subnet is a group of devices that shares the same physical wire. A logical subnet is a group of switch ports assigned to the same VLAN, regardless of their physical location in a switched network.

Two types of VLANs are:

• End-to-end VLAN—VLAN members are assigned by function and can reside on different switches. They are used when hosts are assigned to VLANs based on functions or workgroups, rather than physical location. VLANs should not extend past the Building Distribution submodule. Figure 2-1 shows end-to-end VLANs.
  

• Local VLAN—Hosts are assigned to VLANs based on their location, such as a floor in a building. A router accomplishes sharing of resources between VLANs. This type is typically found in the Building Access submodule. Figure 2-2 shows an example of local VLANs.

End-To-End VlanS


Local VLANS


VLAN membership can be assigned either statically by port or dynamically by MAC address using a VLAN Membership Policy Server (VMPS).

Creating a VLAN in Global Config Mode:

VLANs must be created before they may be used. VLANs may be created in global configuration mode or in VLAN database mode. Creating VLANs in global configuration is easy—just identify the VLAN number and name it!

(config)#vlan 12
(config-vlan)#name MYVLAN


Creating a VLAN in Database Mode

Creating a VLAN in VLAN database mode is very similar to global configuration. There are no advantages to either method. Either method creates an entry in a VLAN.DAT file. Remember that copying the configuration, by itself, does not move the VLAN information! To do that you must move the VLAN.DAT file.

#vlan database
(vlan)#vlan 12 name MYVLAN

Delete a VLAN by using the same command with no in front of it. There is no need to include the name when deleting.


Assigning Ports to VLANs

When statically assigning ports to VLANs, first make it an access port, and then assign the port to a VLAN. At the interface configuration prompt:

(config-if)#switchport mode access
(config-if)#switchport access vlan 12
The commands are similar when using dynamic VLAN assignment. At interface configuration mode:

(config-if)#switchport mode access
(config-if)#switchport access vlan dynamic

If you use dynamic, you must also enter the IP address of the VMPS server at global configuration mode:

(config-if)#vmps server ip address


Verifying VLAN Configuration

To see a list of all the VLANs and the ports assigned to them, use the command show vlan. To narrow down the information displayed, you can use these keywords after the command: brief, id, vlan-number, or name vlan-name:

Other verification commands include:

show running-config interface interface no.—Use the following to verify the VLAN membership of the port:


show mac address-table interface interface no. vlan vlan no.— Use the following to view MAC addresses learned through that port for the specified VLAN:


show interfaces interface no. switchport—Use the following to see detailed information about the port configuration, such as entries in the Administrative Mode and Access Mode VLAN fields:



Troubleshooting VLAN Issues

The following are three steps in troubleshooting VLAN problems:

• Check the physical connectivity—Make sure the cable, the network adapter, and switch port are good. Check the port’s link LED.
  

• Check the switch configuration—If you see FCS errors or late collisions, suspect a duplex mismatch. Also check configured speed on both ends of the link. Increasing collisions can mean an overloaded link, such as with a broadcast storm.
  

• Check the VLAN configuration—If two hosts cannot communicate, make sure they are both in the same VLAN. If a host cannot connect to a switch, make sure the host and the switch are in the same VLAN.

IPv6 Routing

MP-BGP for IPv6

Multiprotocol BGP (RFC 2858) involves two new extensions to BGP4 that allow BGP to carry reachability information for other protocols, such as IPv6, multicast IPv4, and MPLS. The extensions allow NEXT_HOP to carry IPv6 addresses and NLRI (network layer reachability
information) to an IPv6 prefix.

Example 8-5 shows the BGP commands as they might be applied.


OSPFv3

OSPFv3 is one of the first routing protocols available for IPv6 and. Due to its open-standard heritage, it is widely supported in IPv6. OSPFv3 is the only routing protocol discussed on the BSCI test, so it is covered in more depth here.

OSPFv3, which supports IPv6, is documented in RFC 2740. Like OSPFv2, it is a link-state routing protocol that uses the Dijkstra algorithm to select paths. Routers are organized into areas, with all areas touching area 0.

OSPF speakers meet and greet their neighbors using Hellos, exchange LSAs (link-state advertisements) and DBDs (database descriptors), and run SPF against the accumulated link-state database.

OSPFv3 participants use the same packet types as OSPFv2, form neighbors in the same way, flood and age LSAs identically, and support the same NBMA topologies and rare techniques such as NSSA and ondemand circuits.

OSPFv3 differs from its predecessors principally in its new address format. OSPFv3 advertises using multicast addresses FF02::5 and FF02::6, but uses its link-local address as the source address of its advertisements. Authentication is no longer built in, but relies on the underlying capabilities of IPv6.


OSPFv3 LSAs

OSPFv3 and OSPFv2 use a similar set of LSAs, but version 3 has a few changes from OSPFv2. Types 3 and 4 have been slightly renamed, but still fulfill the same functionality as they did with OSPFv2. Type 8 is new and assists in discovering neighbors. Types 1 and 2 no longer
pass routes. Instead they pass router IDs. Prefixes are associated as leaf objects that hang off those nodes and are advertised using Type 9, which is also new.

LSAs are sourced from the link-local address of an interface and destined for a multicast address. FF02::5 is the “all OSPF routers” address and FF02::6 is the “all OSPF DRs” address.

The OSPFv3 LSA types are collected together in Table 8-1. Notice that types one through seven exactly match their OSPFv2 predecessor, while type 8 and type 9 are new to OSPFv3.


Configuration
OSPF configuration is similar to RIPng and EIGRP. The routing process is created and routing properties are assigned to it. Interfaces are then associated with the process under interface configuration mode. Assuming that ipv6 unicast-routing and interface IP addresses are already in place, the commands to implement OSPFv3 are shown in Example 8-6.


Cost may be overridden with the ipv6 ospf cost command as shown in Example 8-7.

The summary-range command is shown to demonstrate summarization.


Troubleshooting
Troubleshoot OSPFv3 just like OSPFv2. Start by looking at show ipv6 route to verify routes have been advertised. Assuming the route is in the routing table, test reachability using ping ipv6. You can also look at the ospf setup using show ipv6 ospf 1 interface, show ipv6
ospf, or show ipv6 ospf database.


Integrating IPv4 and IPv6

There are several strategies for migrating from IPv4 to IPv6. Each of these strategies should be considered when organizations decide to make the move to IPv6 because each has positive points to aiding a smooth migration. It should also be said that there does not have to be a global decision on strategy—your organization may choose to run dual-stack in the U.S., go completely to IPv6 in Japan, and use tunneling in Europe. The transition mechanisms include:

• Dual stack—Running IPv6 and IPv4 concurrently.
  

• IPv6 to IPv4 tunneling (6-to-4)—Routers that straddle the IPv4 and IPv6 worlds to encapsulate the IPv6 traffic inside IPv4 packets.
  

• Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)—This protocol is similar to 6-to-4, but it treats the IPv4 network as an NBMA network.
  

• Teredo/Shipworm—Encapsulates IPv6 packets in IPv4/UDP segments.

NAT-PT, ALG, and BIA/BIS
Instead of replacing IPv4, there are several ways to coordinate the functioning of IPv4 and v6 concurrently. NAT-protocol translation is an example of this coexistence strategy. NAT-PT maps IPv6 addresses to IPv4 addresses. If IPv6 is used on the inside of your network, a NATPT device will receive IPv6 traffic on its inside interface and replace the IPv6 header with an IPv4 header before sending it to an outside interface. Reply traffic will be able to follow the mapping backward to enable two-way communication. NAT-PT is able to interpret application traffic and understand when IP information is included in the application data. It is also possible to connect IPv4 and IPv6 routing domains using application-level gateways (ALG), proxies, or Bump-in-the-API (BIA) and Bump-in-the-Stack (BIS), which are NAT-PT implementations within a host.

IPv6 Routing

IPv6 is not enabled by default on Cisco routers. To enable IPv6 routing, the command is Router(config)#ipv6 unicast-routing.

After IPv6 is enabled, addresses are assigned to interfaces much like version 4:

Router(config-if)#ipv6 address prefix/prefix-length

To make this less abstract, a more complete example that shows an IPv6 implementation is shown in Example 8-1.


Static Routing

Static routing with IPv6 works exactly like it does with version 4. Aside from understanding the address format, there are no differences. Static routes are not currently on the BSCI test. The syntax for the IPv6 static route command is shown below, and Example 8-2 is supplied so that the command may be viewed in context as it might be applied.

Router(config)# ipv6 route ipv6-prefix/prefix-length {ipv6-address | interface-type interface-number [ipv6-address]} [administrativedistance] [administrative-multicast-distance | unicast | multicast] [tag tag]


RIPng for IPv6

RIPng is the IPv6 of RIP and is defined in RFC 2080. Like RIPv2 for IPv4, RIPng is a distance vector routing protocol that uses a hop count for its metric and has a maximum hop count of 15. RIPng also uses periodic multicast updates—every 30 seconds—to advertise routes. The multicast address is FF02::9.

RIPng is not on the BSCI exam at present, but it is presented here for completeness and to round out your appreciation for IPv6 routing and to prepare the reader for trial implementations of IPv6.

There are two important differences between the old RIP and the nextgeneration RIP. First, RIPng supports multiple concurrent processes, each identified by a process number (this is similar to OSPFv2). Second, RIPng is initialized in global configuration mode and then
enabled on specific interfaces.

Example 8-3 shows the syntax used to apply RIPng to a configuration. Notice that the syntax is very similar to traditional RIP.


Like RIP for IPv4, troubleshoot RIPng by looking at the routing table (show ipv6 route), by reviewing the routing protocols (show ipv6 protocols), and by watching routing updates propagated between routers (debug ipv6 rip).


EIGRP
EIGRP has been expanded to support IPv6, although you’ll need to verify that a specific version of IOS is capable of doing this. EIGRP for IPv6 is based on the IPv4 version. EIGRP is still an advanced distance vector routing protocol that uses a complex metric. EIGRP still has a reliable update mechanism and uses DUAL to retain fall-back paths. Like EIGRP in IPv4, it sends multicast hellos every five seconds (but the multicast address is now FF02::A). EIGRP is enabled as described in the following:

Router(config)#ipv6 router eigrp as
Router(config-rtr)#router-id ipv4-address|ipv6-address
Router(config-rtr)#interface type number
Router(config-if)#ipv6 eigrp as

Like EIGRP for IPv4, troubleshoot by looking at the routing table (show ipv6 route), by reviewing the routing protocols (show ipv6 protocols), and by monitoring neighbors (show ipv6 eigrp neighbors). Example 8-4 shows the configuration for IPv6 EIGRP. Notice that the
routing protocol must be enabled under each interface.

IPv6 Introduction

IPv6 is an extension of IP with several advanced features:

• Larger address space
• Simpler header
• Autoconfiguration
• Extension headers
• Flow labels
• Mobility
• “Baked in” security

Of these, many capabilities have been backported to IPv4. The primary adoption of IPv6 will be driven by the need for more addresses. Given the growth in Internet use and the emergence of large groups of Internet users in developing countries, this is a significant requirement.

IPv6 Routing Prefix

IPv4 addresses are 32 bits long, whereas IPv6 addresses are 128 bits. IPv6 addresses are composed of the following elements (see Figure 8-1):

• The first three bits (/3) of unicast always 001.
• The next 13 bits (/16) are Top-Level Aggregator (TLA) the upstream ISP.
• The next 24 bits (/40) are the next-level aggregator or regional ISP.
• Enterprises are assigned /48 and have 16 bits of subnetting.

IPv6 Interface ID


The host portion of the address is last 64 bits. It can be assigned manually, using DHCP v6, or using stateless autoconfiguration. An end-system uses stateless autoconfiguration by waiting for a router to advertise the local prefix. If the end system has a 64-bit MAC, it concatenates the prefix and its MAC to form an IPv6 address. If the end system has a 48-bit MAC, it flips the global/local bit and inserts 0xFFEE into the middle of the MAC. The resulting 64-bit number is called the EUI64. The prefix and EUI64 are concatenated to form the address. Figure 8-2 shows how a host uses its MAC address to create its IPv6 address.


Simplified Presentation of IPv6 Address
There are two ways to shorten the representation of an IPv6 address. Take the example address
4001:0000:0001:0002:0000:0000:0000:ABCD.

• Leading zeros may be omitted. This makes the example 4001:0:1:2:0:0:0:ABCD.
• Sequential zeros may be shown as double colons once per address. This makes the example 4001:0:1:2::ABCD.

IPv6 Header


The IPv6 header is similar to the IPv4 header. The largest changes have to do with the larger addresses, aligning fields to 64-bit boundaries and moving fragmentation to an extension header.

The fields are:

• Version—6.
• Priority—Similar to DSCP in version 4, this eight-bit field is used to describe relative priority.
• Flow—20-bit flow label allows tagging in a manner similar to MPLS.
• Length—The length of the data in the packet.
• Next Header—Indicates how the bits after the IP header should be interpreted. Could indicate TCP or UDP, or it could show an extension header.
• Hop Limit—Similar to TTL.
• Source and Destination—IPv6 addresses.

Zero or more extension headers could follow, including:

• Hop-by-hop options—Options for intermediate devices.
• Destination options—Options for the end node.
• Source routing—Specifies “way stations” that the route must include.
• Fragmentation—Used to divide packets.
• Authentication—Used to attest to source. Replaces the AH header from IPSec.
• Encryption—Replaces the IPSec ESP header.

Multicast MAC Address

Multicast MAC addresses all start with the first 25 bits 01005E. The last 23 bits are the left-most bits from the IP address. Example 7-4 shows how a MAC address of 0100.5E40.0305 maps to a multicast IP address of 227.64.3.5.


The first four bits of multicast IP addresses are always 1110, and the last 23 bits map to the MAC. That leaves five bits that are dropped. Remember that this is an issue, because every multicast MAC maps to many multicast IPs! Example 7-5 shows how a MAC address of 0100.5E40.0305 could map to several different multicast IP addresses. Notice that the first two octets can vary in the IP addresses.


Multicast IP Addresses

Multicasts use the IP address range 224.0.0.0 to 239.255.255.255. The first four bits of the first octet are always binary 1110, and the remaining 28 bits identify the multicast group. Some addresses are reserved:

• 224.0.0.1 is the all-hosts group.
• 224.0.0.2 is the all-routers group.
• The rest of the 224.0.0.0/24 range is reserved for link-local protocols.
• 224.0.1.0 to 238.255.255.255 are for use over the Internet, and they are called globally-scoped addresses.
• Source-specific multicast uses 232.0.0.0/8 addresses.
• 233.0.0.0/8 is used to assign static multicast addresses for use by an organization. The second and third octets of the address are the organization’s Autonomous System number. This is called GLOP, which is a combination of global and scope.
• 239.0.0.0/8 is for local use within an organization, and it is called a limited scope or an administratively scoped address.

Protocol Independent Multicast (PIM)

PIM is a protocol used between routers to keep track of where to forward traffic for each multicast group. It can use information gathered from any routing protocol. PIM can run in dense mode or sparse mode.

PIM Dense Mode
PIM dense mode uses source-based trees. When running in dense mode, PIM assumes that every router needs to receive multicasts. Any router that doesn’t want to receive it must send a prune message upstream to the server. PIM dense mode is most appropriate when:

• Multicast servers and receivers are near each other.
• There are just a few servers and many receivers.
• You have a high volume of multicast traffic.
• The multicast stream is fairly constant.

PIM Sparse Mode
PIM sparse mode uses shared distribution trees. It does not assume that any routers want to receive the multicast, but instead waits to hear an explicit message from them, joining the group. Then it adds branches to the tree to reach the hosts behind those routers. PIM sparse mode uses RPs to connect hosts and servers. After the connection is made, PIM switches over to a source tree. Sparse mode is used when:

• Pockets of users are widely dispersed around the network.
• Multicast traffic is intermittent.

PIM Sparse-Dense Mode
An interface can be configured in sparse-dense mode. Then, if the router knows of an RP for its group, it uses sparse mode. Otherwise, it uses dense mode. In addition, it makes the interface capable of receiving multicasts from both sparse and dense-mode groups.


Configuring Multicast Routing and PIM

Use the following command to enable multicast routing:

(config)# ip multicast-routing

PIM mode must be configured at each interface with the following command. Configuring PIM on an interface also enables Internet Group Management Protocol (IGMP) on that interface:

(config-if)# ip pim {sparse-mode | dense-mode | sparse-dense-mode}

When using sparse mode, an RP must be specified. A router knows that it is an RP when it sees its own address in the command:

(config)# ip pim rp-address ip-address


Auto-RP
Auto-RP automates the discovery of RPs in a sparse or sparse-dense PIM network. RPs advertise themselves to a router designated as an RP mapping agent. The mapping agent then decides on one RP per group and sends that information to the other routers.

To configure a router as an RP, type the following:

(config)# ip pim send-rp-announce type number scope ttl group-list access-list-number

To configure a router as a mapping agent, type the following:

(config)# ip pim send-rp-discovery scope ttl


PIM Version 2
Cisco routers with recent versions of the IOS use PIM Version 2 by default. Some differences between PIM Version 1 and PIM Version 2 include:

• PIM Version 1 is Cisco proprietary, whereas PIM Version 2 is standards-based.
• Both versions can dynamically map RPs to multicast groups. PIM Version 1 uses an auto-RP mapping agent, and PIM Version 2 uses a bootstrap router (BSR).
• PIM Version 1 uses a Time-to-Live value to bound its announcements, and PIM Version 2 uses a configured domain border.
• In PIM Version 2, sparse and dense mode are group properties, not interface properties.

To configure PIM Version 2, configure at least one router as a BSR, and selected routers as RPs. To configure a BSR, use the following:

(config)# ip pim bsr-candidate interface hash-mask-length [priority]

To configure a router as a candidate RP, use the following:

(config)# ip pim rp-candidate type number ttl group-list access-list-number