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Cisco IOS IP Configuration Guide, Release 12.2
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About Cisco IOS Software Documentation
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Using Cisco IOS Software
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IP Overview
- Part 1: IP Addressing Services
- Part 2: IP Routing Protocols
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Part 3: IP Multicast
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Configuring IP Multicast Routing
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Configuring Source Specific Multicast
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Configuring Bidirectional PIM
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Configuring Multicast Source Discovery Protocol
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Configuring PGM Host and Router Assist
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Configuring Unidirectional Link Routing
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Using IP Multicast Tools
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Configuring Router-Port Group Management Protocol
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Configuring DVMRP Interoperability
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Feedback
Table Of Contents
Configuring Multicast Source Discovery Protocol
Requesting Source Information from an MSDP Peer
Controlling Source Information That Your Router Originates
Controlling Source Information That Your Router Forwards
Using TTL to Limit the Multicast Data Sent in SA Messages
Controlling Source Information That Your Router Receives
Configuring a Default MSDP Peer
Configuring an MSDP Mesh Group
Including a Bordering PIM Dense Mode Region in MSDP
Configuring an Originating Address Other Than the RP Address
Monitoring and Maintaining MSDP
Configuring Multicast Source Discovery Protocol
This chapter describes the Multicast Source Discovery Protocol (MSDP) feature. For a complete description of the MSDP commands in this chapter, refer to the "Multicast Source Discovery Protocol Commands" chapter of the Cisco IOS IP Command Reference, Volume 3 of 3: Multicast publication. To locate documentation of other commands in this chapter, use the command reference master index, or search online.
MSDP is a mechanism to connect multiple Protocol Independent Multicast sparse mode (PIM-SM) domains. MSDP allows multicast sources for a group to be known to all rendezvous points (RPs) in different domains. Each PIM-SM domain uses its own RPs and need not depend on RPs in other domains. An RP runs MSDP over TCP to discover multicast sources in other domains.
An RP in a PIM-SM domain has an MSDP peering relationship with MSDP-enabled routers in another domain. The peering relationship occurs over a TCP connection, where primarily a list of sources sending to multicast groups is exchanged. The TCP connections between RPs are achieved by the underlying routing system. The receiving RP uses the source lists to establish a source path.
The purpose of this topology is to have domains discover multicast sources in other domains. If the multicast sources are of interest to a domain that has receivers, multicast data is delivered over the normal, source-tree building mechanism in PIM-SM.
MSDP is also used to announce sources sending to a group. These announcements must originate at the RP of the domain.
MSDP depends heavily on BGP or MBGP for interdomain operation. We recommend that you run MSDP in RPs in your domain that are RPs for sources sending to global groups to be announced to the internet.
To identify the hardware platform or software image information associated with a feature, use the Feature Navigator on Cisco.com to search for information about the feature or refer to the software release notes for a specific release. For more information, see the "Identifying Supported Platforms" section in the "Using Cisco IOS Software" chapter.
How MSDP Works
Figure 78 illustrates MSDP operating between two MSDP peers. PIM uses MSDP as the standard mechanism to register a source with the RP of a domain.
When MSDP is configured, the following sequence occurs. When the first data packet of a source is registered by the first hop router, that same data packet is decapsulated by the RP and forwarded down the shared tree. That packet is also reencapsulated in a Source-Active (SA) message that is immediately forwarded to all MSDP peers. The SA message identifies the source, the group the source is sending to, and the address or the originator ID of the RP, if configured. If the peer is an RP and has a member of that multicast group, the data packet is decapsulated and forwarded down the shared-tree in the remote domain.
The PIM designated router (DR) directly connected to the source sends the data encapsulated in a PIM register message to the RP in the domain.
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The DR sends the encapsulated data to the RP only once per source, when the source goes active. If the source times out, this process happens again when it goes active again. This situation is different from the periodic SA message that contains all sources that are registered to the originating RP. These messages have no data.
Each MSDP peer receives and forwards the SA message away from the originating RP to achieve peer-RPF flooding. The concept of peer-RPF flooding is with respect to forwarding SA messages. The router examines the BGP or MBGP routing table to determine which peer is the next hop toward the originating RP of the SA message. Such a peer is called an "RPF peer" (Reverse Path Forwarding peer). The router forwards the message to all MSDP peers other than the RPF peer.
If the MSDP peer receives the same SA message from a non-RPF peer toward the originating RP, it drops the message. Otherwise, it forwards the message on to all its MSDP peers.
When an RP for a domain receives an SA message from an MSDP peer, it determines if it has any group members interested in the group the SA message describes. If the (*, G) entry exists with a nonempty outgoing interface list, the domain is interested in the group, and the RP triggers an (S, G) join toward the source.
Figure 78 MSDP Running Between RP Peers
Benefits
MSDP has the following benefits:
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Prerequisites
Before configuring MSDP, the addresses of all MSDP peers must be known in BGP or MBGP. If that does not occur, you must configure MSDP default peering when you configure MSDP.
MSDP Configuration Task List
To configure an MSDP peer and various MSDP options, perform the tasks described in the following sections. The tasks in the first section are required; the tasks in the remaining sections are optional.
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See the "MSDP Configuration Examples" section later in this chapter for configuration examples.
Configuring an MSDP Peer
You enable MSDP by configuring an MSDP peer to the local router.
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To configure an MSDP peer, use the following commands in global configuration mode as needed. The second command is optional.
Caching SA State
By default, the router does not cache source/group pairs from received SA messages. Once the router forwards the MSDP SA information, it does not store it in memory. Therefore, if a member joins a group soon after an SA message is received by the local RP, that member will need to wait until the next SA message to hear about the source. This delay is known as join latency.
If you want to sacrifice some memory in exchange for reducing the latency of the source information, you can configure the router to cache SA messages. To have the router cache source/group pairs, use the following command in global configuration mode:
Router(config)# ip msdp cache-sa-state [list access-list]
Creates SA state (cache source/group pairs). Those pairs that pass the access list are cached.
An alternative to caching the SA state is to request source information from a peer, which is described in the following section, "Requesting Source Information from an MSDP Peer." If you cache the information, you need not trigger a request for it.
Requesting Source Information from an MSDP Peer
Local RPs can send SA requests and get immediate response for all active sources for a given group. By default, the router does not send any SA request messages to its MSDP peers when a new member joins a group and wants to receive multicast traffic. The new member just waits to receive the next periodic SA message.
If you want a new member of a group to learn the current, active multicast sources in a connected PIM-SM domain that are sending to a group, configure the router to send SA request messages to the specified MSDP peer when a new member joins a group. Doing so reduces join latency, but requires some memory.
Note that information can be requested only from caching peers.
To configure this feature, use the following command in global configuration mode:
Repeat the preceding command for each MSDP peer that you want to supply you with SA messages.
An alternative to requesting source information is to cache the SA state, which is described in the section "Caching SA State" earlier in this chapter. If you cache the information, you need not trigger a request for it.
Controlling Source Information That Your Router Originates
There are two ways to control the multicast source information that originates with your router. You can control the following:
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To control which sources you will advertise, see the following section, "Redistributing Sources." To control whom you will provide source information to, see the section "Controlling Source Information That Your Router Forwards" later in this chapter.
Redistributing Sources
SA messages are originated on RPs to which sources have registered. By default, any source that registers with an RP will be advertised. The "A flag" is set in the RP when a source is registered. This flag indicates that the source will be advertised in an SA unless it is filtered with the following command.
To further restrict which registered sources are advertised, use the following command in global configuration mode. The access list or autonomous system path access list determines which (S, G) pairs are advertised.
Router(config)# ip msdp redistribute [list access-list] [asn as-access-list] [route-map map-name]
Advertises (S, G) pairs that pass the access list or route map to other domains.
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Filtering SA Request Messages
By default, only routers that are caching SA information can respond to SA request messages. By default, such a router honors all SA request messages from its MSDP peers. That is, it will supply the IP addresses of the sources that are active.
However, you can configure the router to ignore all SA request messages from an MSDP peer. Or, you can honor only those SA request messages from a peer for groups described by a standard access list. If the access list passes, SA request messages will be accepted. All other such messages from the peer for other groups will be ignored.
To configure one of these options, use either of the following commands in global configuration mode:
Controlling Source Information That Your Router Forwards
By default, the router forwards all SA messages it receives to all of its MSDP peers. However, you can prevent outgoing messages from being forwarded to a peer by using a filter or by setting a time-to-live (TTL) value. These methods are described in the following sections.
Using an MSDP Filter
By creating an MSDP filter, you can do one of the following:
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To apply an MSDP filter, use the following commands in global configuration mode as needed:
Using TTL to Limit the Multicast Data Sent in SA Messages
You can use TTL to control what data will be encapsulated in the first SA message for every source. For example, you could limit internal traffic to a TTL of 8. If you want other groups to go to external locations, you would need to send those packets with a TTL greater than 8.
To establish a TTL threshold, use the following command in global configuration mode:
Router(config)# ip msdp ttl-threshold {peer-address | peer-name} ttl-value
Limits which multicast data will be encapsulated in the first SA message to the specified MSDP peer.
Controlling Source Information That Your Router Receives
By default, the router receives all SA messages its MSDP RPF peers send to it. However, you can control the source information you receive from MSDP peers by filtering incoming SA messages. In other words, you can configure the router not to accept them.
You can do one of the following to control the source information you receive from MSDP peers:
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To apply a filter, use the following commands in global configuration mode as needed:
Configuring a Default MSDP Peer
An MSDP peer of the local router is probably a BGP peer also. However, if you do not want to have or cannot have a BGP peer, you could define a default MSDP peer from which to accept all SA messages. The default MSDP peer must be a previously configured MSDP peer. Configure a default MSDP peer when you are not BGP- or multiprotocol BGP-peering with an MSDP peer. If a single MSDP peer is configured, a router will always accept all SA messages sent to it from that peer.
Figure 79 illustrates a scenario where default MSDP peers might be used. In the figure, a customer that owns Router B is connected to the internet via two Internet service providers (ISPs), one that owns Router A and the other that owns Router C. They are not running BGP or MBGP between them. In order for the customer to learn about sources in the ISP domain or in other domains, Router B identifies Router A as its default MSDP peer. Router B advertises SA messages to both Router A and Router C, but accepts SA messages either from Router A only or Router C only. If Router A is first in the configuration file, it will be used if it is up and running. If Router A is not running, then and only then will Router B accept SA messages from Router C.
The ISP will also likely use a prefix list to define which prefixes it will accept from the customer router. The customer will define multiple default peers, each having one or more prefixes associated with it.
The customer has two ISPs to use. The customer defines both ISPs as default peers. As long as the first default peer identified in the configuration is up and running, it will be the default peer and the customer will accept all SA messages it receives from that peer.
Figure 79 Default MSDP Peer Scenario
Router B advertises SAs to Router A and Router C, but uses only Router A or Router C to accept SA messages. If Router A is first in the configuration file, it will be used if it is up and running. If Router A is not running, then and only then will Router B accept SAs from Router C. This is the behavior without a prefix list.
If you specify a prefix list, the peer will be a default peer only for the prefixes in the list. You can have multiple active default peers when you have a prefix list associated with each. When you do not have any prefix lists, you can configure multiple default peers, but only the first one is the active default peer as long as the router has connectivity to this peer and the peer is alive. If the first configured peer goes down or the connectivity to this peer goes down, the second configured peer becomes the active default, and so on.
To specify a default MSDP peer, use the following command in global configuration mode:
Router(config)# ip msdp default-peer {peer-address | peer-name} [prefix-list list]
Defines a default MSDP peer.
See the section "Default MSDP Peer" later in this chapter for a sample configuration.
Configuring an MSDP Mesh Group
An MSDP mesh group is a group of MSDP speakers that have fully meshed MSDP connectivity between one another. Any SA messages received from a peer in a mesh group are not forwarded to other peers in the same mesh group. Thus, you reduce SA message flooding and simplify peer-RPF flooding. The following command is used when multiple RPs are within a domain. It is especially used to send SA messages across a domain.
You can configure multiple mesh groups (with different names) in a single router.
To create a mesh group, use the following command in global configuration mode for each MSDP peer in the group:
Router(config)# ip msdp mesh-group mesh-name {peer-address | peer-name}
Configures an MSDP mesh group and indicates that an MSDP peer belongs to that mesh group.
Shutting Down an MSDP Peer
If you want to configure many MSDP commands for the same peer and you do not want the peer to go active, you can shut down the peer, configure it, and later bring it up.
You might also want to shut down an MSDP session without losing configuration information for the peer.
When a peer is shut down, the TCP connection is terminated and not restarted.
To shut down a peer, use the following command in global configuration mode:
Router(config)# ip msdp shutdown {peer-name | peer address}
Administratively shuts down the specified MSDP peer.
Including a Bordering PIM Dense Mode Region in MSDP
You might have a router that borders a PIM-SM region with a dense mode region. By default, sources in the dense mode region are not included in MSDP. You could configure this border router to send SA messages for sources active in the dense mode region. If you do so, it is very important to also configure the ip msdp redistribute global configuration command to apply to only local sources. Not configuring this command can result in (S, G) state remaining long after a source in the dense mode domain has stopped sending.
To configure the border router to send SA messages for sources active in the dense mode region, use the following command in global configuration mode:
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Configuring an Originating Address Other Than the RP Address
If you want to change the originator ID for any reason, use the ip msdp originator-id global configuration command in this section. For example, you might change the originator ID in one of these cases:
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To allow an MSDP speaker that originates an SA message to use the IP address of its interface as the RP address in the SA message, use the following command in global configuration mode:
Router(config)# ip msdp originator-id type number
Configures the RP address in SA messages to be the address of the originating router's interface.
Monitoring and Maintaining MSDP
To monitor MSDP SA messages, peers, state, or peer status, use the following commands in EXEC mode as needed:
To clear MSDP connections, statistics, or SA cache entries, use the following commands in EXEC modeas needed:
To enable Simple Network Management Protocol (SNMP) monitoring of MSDP, use the following commands in global configuration mode:
For more information about network monitoring using SNMP, refer to the "Configuring Simple Network Management Protocol (SNMP)" chapter in the Cisco IOS Configuration Fundamentals Configuration Guide.
MSDP Configuration Examples
This section contains the following MSDP configurations examples:
Default MSDP Peer
The following example is a partial configuration of Router A and Router C in Figure 79. Each of these ISPs may have more than one customer like the customer in Figure 79 that use default peering (no BGP or MBGP). In that case, they may have similar configurations. That is, they will only accept SAs from a default peer if the SA is permitted by the corresponding prefix list.
Router A Configuration
ip msdp default-peer 10.1.1.1ip msdp default-peer 10.1.1.1 prefix-list site-a ge 32ip prefix-list site-b permit 10.0.0.0/8Router C Configuration
ip msdp default-peer 10.1.1.1 prefix-list site-a ge 32ip prefix-list site-b permit 10.0.0.0/8Logical RP
The following example configures a logical RP using an MSDP mesh group. The four routers that are logical RPs are RouterA, RouterB, RouterC, and RouterD. RouterE is an MSDP border router that is not an RP. Figure 80 illustrates the logical RP environment in this example; the configurations for routers A, B, and E follow the figure.
It is important to note the use of the loopback interface and how those host routes are advertised in Open Shortest Path First (OSPF). It is also important to carefully choose the OSPF router ID loopback so the ID does not use the logical RP address.
In this example, all the logical RPs are on the same LAN, but this situation is not typical. The host route for the RP address is advertised throughout the domain and each PIM designated router (DR) in the domain joins to the closest RP. The RPs share (S, G) information with each other by sending SA messages. Each logical RP must use a separate originator ID.
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All networks are 171.69.0.0. The RP address is 10.10.10.10 on Loopback 10 on all RPs. BGP connections are 192.168.1.x on Loopback 0. Loopback 0 is put into BGP with network 192.168.1.3 mask 255.255.255.255 NLRI unicast multicast.
Figure 80 Logical RP Using MSDP
RouterA Configuration
!hostname RouterA!ip routing!ip subnet-zeroip multicast-routing!!interface Loopback0ip address 192.168.1.2 255.255.255.255no shutdown!interface Loopback10ip address 10.10.10.10 255.255.255.255no ip directed-broadcastip pim sparse-dense-modeno shutdown!interface Ethernet1/2description LANethernet2ip address 171.69.2.2 255.255.255.0ip pim sparse-dense-modeno shutdown!interface Ethernet4/0/0description LANethernet3ip address 171.69.3.2 255.255.255.0ip pim sparse-dense-modeno shutdown!router ospf 10network 171.69.0.0 0.0.255.255 area 0network 10.10.10.10 0.0.0.0 area 0network 192.168.1.2 0.0.0.0 area 0!router bgp 1no synchronizationnetwork 171.69.0.0 nlri unicast multicastnetwork 192.168.1.2 mask 255.255.255.255 nlri unicast multicastneighbor 192.168.1.3 remote-as 1 nlri unicast multicastneighbor description routerBneighbor 192.168.1.3 next-hop-selfneighbor 192.168.1.3 update-source loopback0neighbor 192.168.1.4 remote-as 1 nlri unicast multicastneighbor description routerCneighbor 192.168.1.4 update-source loopback0neighbor 192.168.1.5 remote-as 1 nlri unicast multicastneighbor description routerDneighbor 192.168.1.5 next-hop-selfneighbor 192.168.1.5 update-source loopback0neighbor 192.168.1.6 remote-as 1 nlri unicast multicastneighbor description routerEneighbor 192.168.1.6 update-source Loopback0neighbor 192.168.1.6 next-hop-self!!ip msdp peer 192.168.1.3 connect-source loopback 0ip msdp peer 192.168.1.5 connect-source loopback 0ip msdp peer 192.168.1.4 connect-source loopback 0ip msdp peer 192.168.1.6 connect-source Loopback0ip msdp mesh-group inside-test 192.168.1.3ip msdp mesh-group inside-test 192.168.1.4ip msdp mesh-group inside-test 192.168.1.5ip msdp mesh-group outside-test 192.168.1.6ip msdp cache-sa-stateip msdp originator-id loopback0!ip classlessip pim send-rp-disc scope 10ip pim send-rp-anno loopback 10 scope 10!RouterB Configuration
!hostname RouterB!ip routing!ip multicast-routingip dvmrp route-limit 20000!interface Loopback0ip address 192.168.1.3 255.255.255.255no shutdown!interface Loopback10ip address 10.10.10.10 255.255.255.255ip pim sparse-dense-modeno shutdown!interface Ethernet2description LANethernet 0ip address 171.69.0.3 255.255.255.0ip pim sparse-dense-modeno shutdown!interface Ethernet3description LANethernet 2ip address 171.69.2.3 255.255.255.0ip pim sparse-dense!router ospf 10network 171.69.0.0 0.0.255.255 area 0network 10.10.10.10 0.0.0.0 area 0network 192.168.1.3 0.0.0.0 area 0!router bgp 1no synchronizationnetwork 171.69.0.0 nlri unicast multicastnetwork 192.168.1.3 mask 255.255.255.255 nlri unicast multicastneighbor 192.168.1.2 remote-as 1 nlri unicast multicastneighbor description routerAneighbor 192.168.1.2 update-source loopback0neighbor 192.168.1.4 remote-as 1 nlri unicast multicastneighbor description routerCneighbor 192.168.1.4 update-source loopback0neighbor 192.168.1.5 remote-as 1 nlri unicast multicastneighbor description routerDneighbor 192.168.1.5 update-source loopback0neighbor 192.168.1.5 soft-recon in!ip msdp peer 192.168.1.2 connect-source loopback 0ip msdp peer 192.168.1.5 connect-source loopback 0ip msdp peer 192.168.1.4 connect-source loopback 0ip msdp mesh-group inside-test 192.168.1.2ip msdp mesh-group inside-test 192.168.1.4ip msdp mesh-group inside-test 192.168.1.5ip msdp cache-sa-stateip msdp originator-id loopback0!ip classlessip pim send-rp-disc scope 10ip pim send-rp-anno loopback 10 scope 10!RouterE Configuration
!hostname RouterE!ip routing!ip subnet-zeroip routingip multicast-routingip dvmrp route-limit 20000!interface Loopback0ip address 192.168.1.6 255.255.255.255no shutdown!interface Ethernet2description LANethernet 3ip address 171.69.3.6 255.255.255.0ip pim sparse-dense-modeno shutdown!interface Ethernet5description LANethernet 6ip address 192.169.1.6 255.255.255.0ip pim sparse-dense-modeip multicast boundary 20no shutdown!router ospf 10network 171.69.0.0 0.0.255.255 area 0network 192.168.1.6 0.0.0.0 area 0default-information originate metric-type 1!router bgp 1no synchronizationnetwork 171.69.0.0 nlri unicast multicastnetwork 192.168.1.6 mask 255.255.255.255 nlri unicast multicastnetwork 192.168.1.0neighbor 192.168.1.2 remote-as 1 nlri unicast multicastneighbor 192.168.1.2 update-source Loopback0neighbor 192.168.1.2 next-hop-selfneighbor 192.168.1.2 route-map 2-intern outneighbor 192.169.1.7 remote-as 2 nlri unicast multicastneighbor 192.169.1.7 route-map 2-extern outneighbor 192.169.1.7 default-originate!ip classlessip msdp peer 192.168.1.2 connect-source Loopback0ip msdp peer 192.169.1.7ip msdp mesh-group outside-test 192.168.1.2ip msdp cache-sa-stateip msdp originator-id Loopback0!access-list 1 permit 192.168.1.0access-list 1 deny 192.168.1.0 0.0.0.255access-list 1 permit any!route-map 2-extern permit 10match ip address 1!route-map 2-intern deny 10match ip address 1!
