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HP 5500 Ei 5500 Si Switch Series Configuration Guide

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    							 115 
2.
 
Check that . Use the  display current-configuration  command to verify that multicast routing is 
enabled. If not, carry out the  multicast routing-enable command in system view to enable IP 
multicast routing. In addition, check that IGMP  is enabled on the corresponding interfaces.  
3. Use the  display igmp interface  command to verify that the IGMP  version on the interface is lower 
than that on the host.  
4.  Use the  display current-configuration interface  command to verify that no ACL rule has been 
configured to restrict the host from joining the mu lticast group G. If the host is restricted from 
joining the multicast group G, the ACL rule must be modified to allow receiving the reports for the 
multicast group G.  
Inconsistent memberships on routers on the same subnet 
Symptom 
Different memberships are maintained on different IGMP routers on the same subnet.  
Analysis  
•   A router running IGMP maintains multiple param eters for each interface, and these parameters 
influence one another, forming very complicated relationships. Inconsistent IGMP interface 
parameter configurations for routers on the same subnet will surely result in inconsistency of 
memberships.  
•   In addition, although an IGMP router is compatible  with a host that is running a different version of 
I G M P,  a l l  r o u t e r s  o n  t h e  s a m e  s u b n e t  m u s t  r u n  t h e  s a m e  v e r s i o n  o f  I G M P.  I n c o n s i s t e n t  I G M P  v e r s i o n s  
running on routers on the same subnet also leads to inconsistency of IGMP memberships.  
Solution  
1.  Use the  display current-configuration  command to verify the IGMP configuration information on 
the interfaces.  
2.  Use the  display igmp interface  command on all routers on the same subnet to verify the 
IGMP-related timer settings. The settings sh ould be consistent on all the routers.  
3. Use the  display igmp interface  command to verify that all the routers on the same subnet are 
running the same version of IGMP.
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    							 116 
Configuring PIM (available only on the HP 
5500 EI) 
PIM overview 
Protocol Independent Multicast (PIM) provides IP multicast forwarding by leveraging unicast static routes 
or unicast routing tables generated by any unicast ro uting protocol, such as routing information protocol 
(RIP), open shortest path first (OSPF), intermediate system to intermediate system (IS-IS), or border 
gateway protocol (BGP). Independent of the unicast routing protocols running on the device, multicast 
routing can be implemented as long as the correspo nding multicast routing entries are created through 
unicast routes. PIM uses the reverse path forwarding (RPF) mechanism to implement multicast forwarding. 
When a multicast packet arrives on an interface of the device, it undergoes an RPF check. If the RPF check 
succeeds, the device creates the corresponding routin g entry and forwards the packet. If the RPF check 
fails, the device discards the packet. For more information about RPF, see  Configuring multicast routing 
and f

orwarding (available only on the HP 5500 EI) .  
Based on the implementation mechanism,  PIM falls into the following categories: 
•   Protocol Independent Multicast–Dense Mode (PIM-DM)  
•   Protocol Independent Multicast–Sparse Mode (PIM-SM) 
•   Bidirectional Protocol Independent Multicast (BIDIR-PIM) 
•   Protocol Independent Multicast Source-Specific Multicast (PIM-SSM) 
The term router in this document refers to both routers and Layer 3 switches. 
The term interface in the PIM features refers to Layer 3 interfaces, including VLAN interfaces and 
route-mode (or Layer 3) Ethernet ports. You can set an  Ethernet port to operate in route mode by using the 
port  link-mode  route  command (see  Layer 2—LAN Switching Configuration Guide ). 
PIM-DM overview 
PIM-DM is a type of dense mode multicast protocol.  It uses the push mode for multicast forwarding, and 
is suitable for small-sized networks with densely distributed multicast members. 
The basic implementation of PIM-DM is as follows:  
•   PIM-DM assumes that at least one multicast group member exists on each subnet of a network. 
Therefore, multicast data is flooded to all nodes  on the network. Then, branches without multicast 
forwarding are pruned from the forwarding tree, le aving only those branches that contain receivers. 
This flood-and-prune process takes place periodically. Pruned branches resume multicast 
forwarding when the pruned state times out. Data is then flooded again down these branches, and 
then the branches are pruned again. 
•   When a new receiver on a previously pruned branch joins a multicast group, to reduce the join 
latency, PIM-DM uses a graft mechanism to  resume data forwarding to that branch. 
Generally speaking, the multicast forwarding path is a sour ce tree. That is, it is a forwarding tree with the 
multicast source as its root and multicast group me mbers as its leaves. Because the source tree is the 
shortest path from the multicast source to the receivers, it is also called a shortest path tree (SPT).
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    							 117 
The working mechanism of PIM-DM is summarized as follows:  
•  Neighbor discovery 
•   SPT building 
•   Graft 
•   Assert 
Neighbor discovery 
In a PIM domain, a PIM router discovers PIM neighbors, maintains PIM neighboring relationships with 
other routers, and builds and maintains SPTs by peri odically multicasting hello messages to all other PIM 
routers (224.0.0.13) on the local subnet. 
 
  NOTE: 
Every PIM-enabled interface on a router sends hello  messages periodically, and thus learns the PIM 
neighboring information pertinent to the interface.  
 
SPT building 
The process of building an SPT is the flood-and-prune process.  
1.  In a PIM-DM domain, when a multicast source S  sends multicast data to multicast group G, the 
multicast packet is first flooded throughout the do main. The router first performs RPF check on the 
multicast packet. If the packet passes the RPF check,  the router creates an (S, G) entry and forwards 
the data to all downstream nodes in the network. In  the flooding process, an (S, G) entry is created 
on all the routers in the PIM-DM domain.  
2.  Then, nodes without receivers downstream are pruned. A router having no receivers downstream 
sends a prune message to the upstream node to  tell the upstream node to delete the 
corresponding interface from the outgoing interface list in the (S, G) entry and stop forwarding 
subsequent packets addressed to that  multicast group down to this node. 
An (S, G) entry contains the multicast source  address S, multicast group address G, outgoing 
interface list, and incoming interface. 
For a given multicast stream, the interface that re ceives the multicast stream is referred to as 
upstream, and the interfaces that forward the mu lticast stream are referred to as downstream. 
A prune process is first initiated by a leaf router. As shown in  Figure 38, a r
 outer without any receiver 
a t t a c h e d  t o  i t  ( t h e  r o u t e r  c o n n e c t e d  w i t h  H o s t  A ,   for example) sends a prune message. This prune process 
goes on until only necessary branches are left in th e PIM-DM domain. These branches constitute the SPT.
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    							 118 
Figure 38 SPT building 
 
 
The flood-and-prune process takes place periodically. A pruned state timeout mechanism is provided. A 
pruned branch restarts multicast forwarding when th e pruned state times out and then is pruned again 
when it no longer has any multicast receiver. 
 
  NOTE: 
Pruning has a similar implementation in PIM-SM.  
 
Graft 
When a host attached to a pruned node joins a multicast group, to reduce the join latency, PIM-DM uses 
a graft mechanism to resume data forwarding to  that branch. The process is as follows:  
1. The node that needs to receive multicast data se nds a graft message toward its upstream node as 
a request to join the SPT again.  
2.  After receiving this graft message, the upstream  node puts the interface on which the graft was 
received into the forwarding state and responds  with a graft-ack message to the graft sender.  
3. If the node that sent a graft message does not re ceive a graft-ack message from its upstream node, 
it will keep sending graft messages at a configurabl e interval until it receives an acknowledgment 
from its upstream node. 
Assert 
Where more than one multicast router exists, the as sert mechanism shuts off duplicate multicast flows 
onto the same multi-access network. It does this  by electing a unique multicast forwarder on the 
multi-access network.
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Figure 39 Assert mechanism 
 
 
As shown in Figure 39, after Router A and Router B receive an (S, G) packet from the upstream node, they 
both forward the packet to the local subnet. As a result, the downstream node Router C receives two 
identical multicast packets, and both Router A and Ro uter B, on their own local interface, receive a 
duplicate packet forwarded by the other. After detect ing this condition, both routers send an assert 
message to all PIM routers (224.0.0.13) on the local subnet through the interface on which the packet 
was received. The assert message contains the multicast source address (S), the multicast group address 
(G), and the preference and metric of the unicast route/MBGP route/multicast static route to the source. 
By comparing these parameters, either Router A or  Router B becomes the unique forwarder of the 
subsequent (S, G) packets on the multi-access subnet. The comparison process is as follows: 
1.  The router with a higher preference to the source wins;  
2. If both routers have the same preference to the sour ce, the router with a smaller metric to the source 
wins;  
3.  If a tie exists in route metric to the source, the router with a higher IP address of the local interface 
wins. 
PIM-SM overview 
PIM-DM uses the flood-and-prune principle to build SPTs for multicast data distribution. Although an SPT 
has the shortest path, it is built with a low efficiency. Therefore, the PIM-DM mode is not suitable for large- 
and medium-sized networks. 
PIM-SM is a type of sparse mode multicast protocol.  It uses the pull mode for multicast forwarding and is 
suitable for large-sized and medium-sized networks wi th sparsely and widely distributed multicast group 
members. 
The basic implementation of PIM-SM is as follows: 
•   PIM-SM assumes that no hosts need to receive multicast data. In the PIM-SM mode, routers must 
specifically request a particular multicast stream before the data is forwarded to them. The core task 
for PIM-SM to implement multicast forwarding will  build and maintain rendezvous point trees (RPTs). 
An RPT is rooted at a router in the PIM domain as the common node, or rendezvous point (RP), 
through which the multicast data travels along the RPT and reaches the receivers. 
•   When a receiver is interested in the multicast data addressed to a specific multicast group, the 
router connected to this receiver sends a join mess age to the RP that corresponds to that multicast 
group. The path along which the message goes hop by hop to the RP forms a branch of the RPT.  
•   When a multicast source sends multicast streams to a multicast group, the source-side designated 
router (DR) first registers the multicast source with  the RP by sending register messages to the RP by
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    							 120 
unicast until it receives a register-stop message from the RP. The arrival of a register message at the 
RP triggers the establishment of an SPT. Then, the multicast source sends subsequent multicast 
packets along the SPT to the RP. After reaching the RP, the multicast packet is duplicated and 
delivered to the receivers along the RPT. 
 
  NOTE: 
Multicast traffic is duplicated only where the distribution tree branches, and this process automatically 
repeats until the multicast tr affic reaches the receivers. 
 
The working mechanism of PIM-SM is summarized as follows:  
•  Neighbor discovery 
•   DR election 
•   RP discovery 
•   RPT building  
•   Multicast source registration 
•   Switchover to SPT 
•   Assert 
Neighbor discovery 
PIM-SM uses a similar neighbor discovery mechanism as PIM-DM does. For more information, see 
Neighbor discovery .  
DR election 
PIM-SM also uses hello messages to elect a DR for a  multi-access network (such as Ethernet). The elected 
DR will be the only multicast forwarder on this multi-access network.  
A DR must be elected in a multi-access network, no matter this network connects to multicast sources or 
to receivers. The receiver-side DR sends join messages to the RP. The source-side DR sends register 
messages to the RP. 
A DR is elected on a multi-access subnet by means of comparison of the priorities and IP addresses 
carried in hello messages. An elected DR is substantially meaningful to PIM-SM. PIM-DM itself does not 
require a DR. However, if IGMPv1 runs on any multi-access network in a PIM-DM domain, a DR must be 
elected to act as the IGMPv1 querier on that multi-access network.  
IGMP must be enabled on a device that acts as a rece iver-side DR before receivers attached to this device 
can join multicast groups through this DR. 
For more information about IGMP, see  Configuring IGMP (available only on the HP 5500 EI) .
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    							 121 
Figure 40 DR election 
 
 
As shown in Figure 40, the  DR election process is as follows: 
1. Routers on the multi-access network send hello messages to one another. The hello messages 
contain the router priority for DR election. The router with the highest DR priority will become the 
DR. 
2. In the case of a tie in the router priority, or if any router in the network does not support carrying 
the DR-election priority in hello messages, the ro uter with the highest IP address will win the DR 
election. 
When the DR fails, a timeout in receiving a hello message triggers a new DR election process among the 
other routers. 
RP discovery 
The RP is the core of a PIM-SM domain. For a small-sized, simple network, one RP is enough for 
forwarding information throughout the network, and you can statically specify the position of the RP on 
each router in the PIM-SM domain. In most cases, however, a PIM-SM network covers a wide area and 
a huge amount of multicast traffic must be forwarded through the RP. To lessen the RP burden and 
optimize the topological structure of the RPT, you  can configure multiple candidate-RPs (C-RPs) in a 
PIM-SM domain, among which an RP is dynamicall y elected through the bootstrap mechanism. Each 
elected RP serves a different multicast group range.  For this purpose, you must configure a bootstrap 
router (BSR). The BSR serves as the administrative  core of the PIM-SM domain. A PIM-SM domain can 
have only one BSR, but can have multiple candidate-BSRs (C-BSRs). If the BSR fails, a new BSR is 
automatically elected from the C-BSRs to avoid service interruption. 
 
  NOTE: 
•  An RP can serve multiple multicast groups or all multicast groups. Only one RP can serve a given 
multicast group at a time. 
•   A device can serve as a C-RP and a C-BSR at the same time.  
 
As shown in  Figure 41, ea ch C-RP periodically unicasts its advertisement messages (C-RP-Adv messages) 
to the BSR. A C-RP-Adv message contains the address of the advertising C-RP and the multicast group 
range that it serves. The BSR collects these advert isement messages and chooses the appropriate C-RP 
information for each multicast group to form an RP-set, which is a database of mappings between 
Join message
Ethernet
Ethernet
RP
DR DR
Hello message
Register message
Source Receiver
Receiver
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    							 122 
multicast groups and RPs. The BSR then encapsulates the RP-set in the bootstrap messages (BSMs) that it 
periodically originates and floods the bootstrap messages to the entire PIM-SM domain. 
Figure 41  BSR and C-RPs 
 
 
Based on the information in the RP-sets, all routers in the network can calculate the location of the 
corresponding RPs based on the following rules:  
1. The C-RP with the highest priority wins.  
2. If all the C-RPs have the same priority, their  hash values are calculated through the hashing 
algorithm. The C-RP with the largest hash value wins.  
3.  If all the C-RPs have the same priority and hash va lue, the C-RP that has the highest IP address wins.   
The hashing algorithm used for RP calculation is Value (G, M, C
i) = (1 10 3 51524 5  *  (  (110 3 51524 5  *  ( G  
& M) + 12345) XOR C
i) + 12345) mod 231.  
Values in the hashing algorithm 
 
Value Description 
Value   Hash value 
G  IP address of the multicast group 
M  Hash mask length  
Ci IP address of the C-RP  
&  Logical operator of and 
XOR Logical operator of exclusive-or 
Mod Modulo operator, which gives the remainder of an integer division
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    							 123 
RPT building 
Figure 42 RPT building in a PIM-SM domain  
 
 
As shown in Figure 42, the pr ocess of building an RPT is as follows:  
1. When a receiver joins multicast group G, it us es an IGMP message to inform the directly 
connected DR. 
2.  After getting the receiver information, the DR se nds a join message, which is forwarded hop by 
hop to the RP that corresponds to the multicast group.  
3.  The routers along the path from the DR to the RP fo rm an RPT branch. Each router on this branch 
generates a (*, G) entry in its forwarding table.  The asterisk means any multicast source. The RP is 
the root of the RPT, and the DRs are the leaves of the RPT. 
The multicast data addressed to the multicast group G flows through the RP, reaches the corresponding 
DR along the established RPT, and finally is delivered to the receiver. 
W h e n  a  re c e ive r  i s  n o  l o n g e r  i n t e re s t e d  i n  t h e  m u l t icast data addressed to multicast group G, the directly 
connected DR sends a prune message,  which goes hop by hop along the RPT to the RP. After receiving 
the prune message, the upstream node deletes the inte rface that connects to this downstream node from 
the outgoing interface list and determines whether it  has receivers for that multicast group. If not, the 
router continues to forward the prun e message to its upstream router. 
Multicast source registration 
The purpose of multicast source registration will inform the RP about the existence of the multicast source.
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Figure 43 Multicast source registration 
 
 
As shown in Figure 43, the  multicast source registers with the RP as follows: 
1. The multicast source S sends the  first multicast packet to multicast group G. After receiving the 
multicast packet, the DR that dire ctly connects to the multicast source encapsulates the packet in a 
PIM 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 packet down th e RPT, and sends an (S, G) join message hop 
by hop toward the multicast source. Thus, the routers along the path from the RP to the multicast 
source constitute an SP T branch. Each router 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 multicast data from the multicast so urce travels along the established SPT to the RP. 
Then the RP forwards the data along the RPT to th e receivers. When the multicast traffic arrives at 
the RP along the SPT, the RP sends a register-stop  message to the source-side DR by unicast to stop 
the source registration process. 
 
  NOTE: 
The RP is configured to initiate an SPT switchover as described in this section. Otherwise, the source-side
DR keeps encapsulating multicast data in register me ssages, and the registration process will not stop 
unless no outgoing interfaces exist in the (S, G) entry on the RP.  
 
Switchover to SPT 
In a PIM-SM domain, a multicast group corresponds to one RP and RPT. Before the SPT switchover occurs, 
the source-side DR encapsulates all multicast data dest ined 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 receiver side. The RP acts as a transfer 
station for all multicast packets. The whole process involves the following issues:  
•   The source-side DR and the RP need to implement complicated encapsulation and de-encapsulation 
of multicast packets.  
•   Multicast packets are delivered along a path that might not be the shortest one. 
•   An increase in multicast traffic adds a great burden on the RP, increasing the risk of failure.  
Source
Server Host A
Host B
Host C ReceiverReceiver
Multicast packets
SPT
Join message
Register message RP
DR
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