HP 5500 Ei 5500 Si Switch Series Configuration Guide
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345 node from the outgoing interface list and determines whether it has receivers for that IPv6 multicast group. If not, the router continues to forward the prune message to its upstream router. Multicast source registration The purpose of IPv6 multicast source registration wi ll inform the RP about the existence of the IPv6 multicast source. Figure 95 IPv6 multicast source registration As shown in Figure 95, the IPv6 mu lticast source registers with the RP as follows: 1. The IPv6 multicast source S sends the first IPv6 multicast packet to IPv6 multicast group G. After receiving the multicast packet, the DR that directly connects to the multicast source encapsulates the packet in a register message. Then it sends the message to the corresponding RP by unicast. 2. When the RP receives the register message, it extracts the multicast packet from the register message and forwards the multicast IPv6 multicast packet down the RPT, and sends an (S, G) join message hop by hop toward the IPv6 multicast source. The routers along the path from the RP to the IPv6 multicast source form an SPT branch. Each rout er on this branch generates an (S, G) entry in its forwarding table. The source-side DR is the root of the SPT, and the RP is the leaf of the SPT. 3. The subsequent IPv6 multicast data from the IPv6 multicast source travels along the established SPT to the RP, and then the RP forwards the data along the RPT to the receivers. When the IPv6 multicast traffic arrives at the RP along th e SPT, the RP sends a register-stop message to the source-side DR by unicast to stop the sour ce registration process. NOTE: The RP is configured to initiate an SPT switchover as de scribed in this section. Otherwise, the DR at the IPv6 multicast source side keeps encapsulatin g multicast data in register messages and the registration process will not stop unless no outgoing interfaces exist in the (S, G) entry on the RP. Switchover to SPT In an IPv6 PIM-SM domain, an IPv6 multicast group corresponds to one RP and one RPT. Before the SPT switchover occurs, the DR at the IPv6 multicast source side encapsulates all multicast data destined to the multicast group in register messages and sends these messages to the RP. After receiving these register messages, the RP extracts the multicast data and sends the multicast data down the RPT to the DRs at the
346
receiver side. The RP acts as a transfer station for all IPv6 multicast packets. The whole process involves
the following issues:
• The DR at the source side and the RP need to implement complicated encapsulation and
de-encapsulation of IPv6 multicast packets.
• IPv6 multicast packets are delivered along a path that might not be the shortest one.
• An increase in IPv6 multicast traffic heavily burdens the RP, increasing the risk of failure.
To solve the issues, IPv6 PIM-SM allows an RP or the DR at the receiver side to initiate an SPT switchover
process:
1. The RP initiates an SPT switchover process.
The RP can periodically check the passing-by IPv6 mu lticast packets. If it finds that the traffic rate
exceeds a configurable threshold, the RP sends an (S, G) join message hop by hop toward the IPv6
multicast source to establish an SPT between the DR at the source side and the RP. Subsequent IPv6
multicast data travels along the established SPT to the RP.
For more information about the SPT switchover initiated by the RP, see Multicast source
regi
stration .
2. The receiver-side DR initiates an SPT switchover process
After receiving the first IPv6 multicast packet, th e receiver-side DR initiates an SPT switchover
process, as follows:
{ The receiver-side DR sends an (S, G) join message hop by hop toward the IPv6 multicast source.
When the join message reaches the source-side DR, all the routers on the path have installed
the (S, G) entry in their forwarding table, and thus an SPT branch is established.
{ When the IPv6 multicast packets travel to the router where the RPT and the SPT deviate, the
router drops the multicast packets received from the RPT and sends an RP-bit prune message
hop by hop to the RP. After receiving this prune message, the RP sends a prune message toward
the IPv6 multicast source (suppose only one receiver exists) to implement SPT switchover.
{ IPv6 multicast data is directly sent from the source to the receivers along the SPT.
IPv6 PIM-SM builds SPTs through SPT switchover more economically than IPv6 PIM-DM does through the
flood-and-prune mechanism.
Assert
I P v6 PIM -SM uses a s i mil ar asser t me chanis m as I P v6 PIM - DM do es. For more i nformation, se e Assert.
IPv6 BIDIR-PIM overview
In some many-to-many applications, such as multi-side video conference, there might be multiple
re c eive rs i n t e re s t e d i n m u l t i p l e I P v 6 m u l t ic as t s o u rc e s s i mu l t a n e o us ly. Wi t h I P v 6 PI M - D M o r I P v 6 PI M - S M ,
each router along the SPT must create an (S, G) entry for each IPv6 multicast source, consuming a lot of
system resources. IPv6 BIDIR-PIM is introduced to address this problem. Derived from IPv6 PIM-SM, IPv6
BIDIR-PIM builds and maintains bidirectional RPTs, each of which is rooted at an RP and connects IPv6
multiple multicast sources with multiple receivers. Traffic from the IPv6 multicast sources is forwarded
through the RP to the receivers along the bidirectional RPT. In this case, each router needs to maintain
only a (*, G) multicast routing entry, saving system resources.
IPv6 BIDIR-PIM is suitable for networks with dense multicast sources and dense receivers.
The working mechanism of IPv6 BIDIR-PIM is summarized as follows:
• Neighbor discovery
347 • RP discovery • DF election • Bidirectional RPT building Neighbor discovery IPv6 BIDIR-PIM uses the same neighbor discovery mechanism as IPv6 PIM-SM does. For more information, see Neighbor discovery . RP discovery IPv6 BIDIR-PIM uses the same RP discovery mechanism as IPv6 PIM-SM does. For more information, see RP discovery . In IPv6 PIM-SM, an RP must be specified with a real IPv6 address. In IPv6 BIDIR-PIM, however, an RP can be specified with a virtual IPv6 address, which is called the rendezvous point address (RPA). The link corresponding to the RPA’s subnet is called the rendez vous point link (RPL). All interfaces connected to the RPL can act as RPs, which back up one another. In IPv6 BIDIR-PIM, an RPF interface is the interface pointing to an RP, and an RPF neighbor is the address of the next hop to the RP. DF election On a network segment with multiple multicast routers, the same multicast packets might be forwarded to the RP repeatedly. To address this issue, IPv6 BIDIR- PIM uses a DF election mechanism to elect a unique designated forwarder (DF) for each RP on every ne twork segment within the IPv6 BIDIR-PIM domain, and allows only the DF to forward multicast data to the RP. NOTE: DF election is not necessary for an RPL. Figure 96 DF election As shown in Figure 96, without the DF election mechanism, both Router B and Router C can receive multicast packets from Route A, and they might both forward the packets to downstream routers on the local subnet. As a result, the RP (Router E) receives duplicate multicast packets. With the DF election mechanism, once receiving the RP information, Router B and Router C initiate a DF election process for the RP: Ethernet Router B Router C Router A IPv6 Multicast packets DF election message RP Source Router D Router E
348 1. Router B and Router C multicast DF election messa ges to all PIM routers (224.0.0.13). The election messages carry the RP’s address, and the priority and metric of the unicast route, MBGP route, or multicast static route to the RP. 2. The router with a route of the highest priority becomes the DF. 3. In the case of a tie, the router with the route with the lowest metric wins the DF election. 4. In the case of a tie in the metric, the router with the highest link-local IPv6 address wins. Bidirectional RPT building A bidirectional RPT comprises a receiver-side RPT and a source-side RPT. The receiver-side RPT is rooted at the RP and takes the routers directly connected with the receivers as leaves. The source-side RPT is also rooted at the RP but takes the routers directly connected with the IPv6 multicast sources as leaves. The processes for building these two parts are different. Figure 97 RPT building at the receiver side As shown in Figure 97, the pr ocess for building a receiver-side RP T is similar to that for building an RPT in IPv6 PIM-SM: 1. When a receiver joins IPv6 multicast group G, it uses an MLD message to inform the directly connected router. 2. After getting the receiver information, the router sends a join message, which is forwarded hop by hop to the RP of the IPv6 multicast group. 3. The routers along the path from the receiver’s directly connected router to the RP form an RPT branch, and each router on this branch adds a (* , G) entry to its forwarding table. The * means any IPv6 multicast source. When a receiver is no longer interested in the mu lticast data addressed to IPv6 multicast group G, the directly connected router sends a prune message, wh ich goes hop by hop along the reverse direction of the RPT to the RP. After receiving the prune message, each upstream node deletes the interface connected with the downstream node from the outgoing interface li st and checks whether it has receivers in that IPv6 multicast group. If not, the router continues to forward the prune message to its upstream router.
349 Figure 98 RPT building at the multicast source side As shown in Figure 98, the process of building a source-side RPT is relatively simple: 1. When an IPv6 multicast source sends IPv6 multicas t packets to IPv6 multicast group G, the DF in each network segment unco nditionally forwards the packets to the RP. 2. The routers along the path from the source’s directly c o n n e c t e d r o u t e r t o t h e R P f o r m a n R P T b r a n c h . Each router on this branch adds a (*, G) entry to its forwarding table. The * means any IPv6 multicast source. After a bidirectional RPT is built, multicast traffic is forwarded along the source-side RPT and receiver-side RPT from IPv6 multicast sources to receivers. NOTE: If a receiver and an IPv6 multicast source are at the same side of the RP, the source-side RPT and the receiver-side RPT might meet at a node b e f o r e r e a c h i n g t h e R P . I n t h i s c ase, IPv6 multicast packets from the IPv6 multicast source to the receiver are directly forw arded by the node to the receiver, instead of by the RP. IPv6 administrative scoping overview Division of IPv6 PIM-SM domains Typically, an IPv6 PIM-SM/IPv6 BIDIR-PIM domain contains only one BSR, which is responsible for advertising RP-set information within the entire IPv6 PIM-SM/IPv6 BIDIR-PIM domain. The information for all multicast groups is forwarded within the network scope administered by the BSR. We call this IPv6 non-scoped BSR mechanism. To implement refined management, an IPv6 PIM-SM/IPv6 BIDIR-PIM domain can be divided into one IPv6 global scope zone and multiple IPv6 administr atively scoped zones (IPv6 admin-scope zones). We call this IPv6 administrative scoping mechanism. The IPv6 administrative scoping mechanism effectively releases stress on the management in a single-BSR domain and enables provision of zone -specific services using private group addresses.
350 IPv6 admin-scope zones correspond to IPv6 multicast groups with different scope values in their group addresses. The boundary of the IPv6 admin-scope zone is formed by zone border routers (ZBRs). Each IPv6 admin-scope zone maintains one BSR, which serves multicast groups within a specific scope. IPv6 multicast protocol packets, such as assert messages and bootstrap messages, for a specific group range cannot cross the IPv6 admin-scope zone boundary. IPv6 multicast group ranges served by different IPv6 admin-scope zones can overlap. An IPv6 multicast group is valid only within its local IPv6 admin-scope zone, functioning as a private group address. The IPv6 global scope zone maintains a BSR, which serves the IPv6 multicast groups with the Scope field in their group addresses being 14. Relationship between IPv6 admin-scope zones and the IPv6 global scope zone The IPv6 global scope zone and each IPv6 admin-scope zone have their own C-RPs and BSRs. These devices are effective only in their respective IPv6 admin-scope zones. That is, BSR election and RP election are implemented independently within each IPv6 admin-scope zone. Each IPv6 admin-scope zone has its own boundary. The multicast information cannot cross this border in either direction. A better understanding of the IPv6 global scope zone an d IPv6 admin-scope zones should be based on geographical space and group address range. • Geographical space IPv6 admin-scope zones are logical zones specific to particular multicast groups. The multicast packets of these multicast groups are confined wi thin the local IPv6 admin-scope zone and cannot cross the boundary of the zone. Figure 99 Relationship between admin-scope zones and the global scope zone in geographic space As shown in Figure 99, for multicast groups with the same Scope field in their group addresses, IPv6 admin-scope zones must be geographically separated from one another. Namely, a router must not serve different admin-scope zones. In other words, different admin-scope zones contain different routers, whereas the global scope zone covers all routers in the IPv6 PIM-SM/IPv6 BIDIR-PIM domain. Multicast packets that do not belong to any admin-scope zones can be transmitted in the entire IPv6 PIM-SM/IPv6 BIDIR-PIM domain. • Multicast group address Scope field As shown in Figure 100, the Sc ope field in each IPv6 multicast group address indicates the admin-scope zone the correspond ing multicast group belongs to.
351 Figure 100 IPv6 multicast address format The admin-scope zone range increases with the value of the Scope field. For example, value E indicates IPv6 global scope, which contains other admin-scope zones with the Scope field values smaller than E. Possible values of the Scope field are given in Table 2. Table 2 Values of the Scope field Value Meanin g Remarks 0, F Reserved N/A 1 Interface-local scope N/A 2 Link-local scope N/A 3 Subnet-local scope IPv6 admin-scope zone 4 Admin-local scope IPv6 admin-scope zone 5 Site-local scope IPv6 admin-scope zone 6, 7, 9 through D Unassigned IPv6 admin-scope zone 8 Organization-local scope IPv6 admin-scope zone E Global scope IPv6 global-scope zone IPv6 PIM-SSM overview The source-specific multicast (SSM) model and the an y-source multicast (ASM) model are opposites. The ASM model includes the IPv6 PIM-DM and IPv6 PIM-SM modes. You can implement the SSM model by leveraging part of the IPv6 PIM-SM technique. It is also called IPv6 PIM-SSM. The SSM model provides a solution for source-specific multicast. It maintains the relationships between hosts and routers through MLDv2. In actual application, MLDv2 and part of IPv6 PIM-SM technique is adopted to implement the SSM model. In the SSM model, receivers know exactly where an IPv6 multicast source is located by using advertisements, consultancy, and so on. This model do es not require RP or RPT, and it does not require a source registration process for the purpose of discovering IPv6 multicast sources in other IPv6 PIM domains. In IPv6 PIM-SSM, the term channel refers to an IPv6 multicast group, and the term channel subscription refers to a join message. The working mechanism of IPv6 PIM-SSM is summarized as follows: • Neighbor discovery • DR election • SPT building
352 Neighbor discovery IPv6 PIM-SSM uses the same neighbor discovery mechanism as in IPv6 PIM-SM. For more information, see Neighbor discovery . DR election IPv6 PIM-SSM uses the same DR election mechanism as in IPv6 PIM-SM. For more information, see DR ele ction . SPT building The decision to build an RPT for IPv6 PIM-SM or an SPT for IPv6 PIM-SSM depends on whether the IPv6 multicast group that the receiver will join falls into the IPv6 SSM group range. The IPv6 SSM group range that IANA has reserved is FF3x::/32, where x represents any legal address scope. Figure 101 Building an SPT in IPv6 PIM-SSM As shown in Figure 101, Hosts B and C are IPv6 multicast information receivers. They send an MLDv2 report message to the respective DRs to announce that they are interested in the information about the specific IPv6 multicast source S and that sent to the IPv6 multicast group G. The DR that has received the report first determines whether the IPv6 group address in this message falls into the IPv6 SSM group range and then does the following: • If the IPv6 group address in the message does fall into the IPv6 SSM group range, the IPv6 PIM-SSM model is built. The DR sends a channel subscription message hop by hop toward the IPv6 multicast s o u r c e S . A n ( S , G ) e n t r y i s c r e a t e d o n a l l r o u t e r s o n t h e p a t h f r o m t h e D R t o t h e s o u r c e . T h u s , a n S P T is built in the network, with the source S as its root and receivers as its leaves. This SPT is the transmission channel in IPv6 PIM-SSM. • If the IPv6 group address in the message does not fa ll into the IPv6 SSM group range, the DR follows the IPv6 PIM-SM process. The receiver-side DR sends a (*, G) join message to the RP, and the source-side DR registers the IPv6 multicast source. Source Server Host A Host B Host C ReceiverReceiver IPv6 multicast packets SPT Subscribe message DR DR RP
353 Relationships among IPv6 PIM protocols In an IPv6 PIM network, IPv6 PIM-DM cannot work with IPv6 PIM-SM, IPv6 BIDIR-PIM, or IPv6 PIM-SSM. However, IPv6 PIM-SM, IPv6 BIDIR-PIM, and IPv6 PIM-SSM can work together. When they work together, which one is chosen for a receiver trying to join a group depends, as shown in Figure 102. Figure 102 Relationships among IPv6 PIM protocols For more information about MLD SSM mapping, see Configuring MLD (available only on the HP 5500 EI) . Protocols and standards • R F C 3973 , Protocol Independent Multicast-Dense Mode(PIM-DM):Protocol Specification(Revised) • RFC 4601, Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification (Revised) • RFC 3956, Embedding the Rendezvous Point (RP) Address in an IPv6 Multicast Address • RFC 5015, Bidirectional Protocol Independent Multicast (BIDIR-PIM) • RFC 5059, Bootstrap Router (BSR) Mechanism for Protocol Independent Multicast (PIM) • RFC 4607, Source-Specific Multicast for IP • draft-ietf-ssm-overview-05, An Overview of Source-Specific Multicast (SSM) Configuring IPv6 PIM-DM IPv6 PIM-DM configuration task list
354 Task Remarks Enabling IPv6 PIM-DM Required Enabling state-refresh capability Optional Configuring state refresh parameters Optional Configuring IPv6 PIM-DM graft retry period Optional Configuring IPv6 PIM common features Optional Configuration prerequisites Before you configure IPv6 PIM-DM, complete the following tasks: • Enable IPv6 forwarding and configure an IPv6 unic ast routing protocol so that all devices in the domain are interoperable at the network layer. • Determine the interval betw een state refresh messages. • Determine the minimum time to wait before receiving a new refresh message. • Determine the hop limit value of state-refresh messages. • Determine the graft retry period. Enabling IPv6 PIM-DM With IPv6 PIM-DM enabled, a router sends hello mess ages periodically to discover IPv6 PIM neighbors and processes messages from the IPv6 PIM neighbors. When you deploy an IPv6 PIM-DM domain, enable IPv6 PIM-DM on all non-border interfaces of routers. IMPORTANT: • All the interfaces of the same device must operate in the same IPv6 PIM mode. • IPv6 PIM-DM cannot be used for IPv6 multicast groups in the IPv6 SSM group range. To enable IPv6 PIM-DM: Step Command Remarks 1. Enter system view. system-view N/A 2. Enable IPv6 multicast routing. multicast ipv6 routing-enable Disabled by default. 3. Enter interface view. interface interface-type interface-number N/A 4. Enable IPv6 PIM-DM. pim ipv6 dm Disabled by default. For more information about the multicast ipv6 routing-enable command, see IP Multicast Command Reference . Enabling state-refresh capability Pruned interfaces resume multicast forwarding when the pruned state times out. To prevent this, the router directly connected with the IPv6 multicast source periodically sends an (S, G) state-refresh message, which is forwarded hop by hop along the initial flooding path of the IPv6 PIM-DM domain, to refresh the