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CX11x, CX31x, CX710 (Earlier Than V6.03), and CX91x Series Switch Modules V100R001C10 Configuration Guide 12

The documents describe the configuration of various services supported by the CX11x&CX31x&CX91x series switch modules The description covers configuration examples and function configurations.
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Principles

Principles

This section describes the implementation of MSDP.

MSDP Peers

The first task for implementing inter-domain multicast is to establish MSDP peer relationships.

RPs in PIM-SM domains set up MSDP peer relationships. MSDP peers exchange SA messages that carry (S, G) information generated when source DRs register to the RPs. Message exchange among MSDP peers ensures that SA messages sent by any RP can be received by all the other RPs.

As shown in Figure 8-72, MSDP can be deployed on other PIM routers apart from the RPs. MSDP peers established on different PIM routers have different functions.

Figure 8-72 Locations of MSDP peers

  • Establish MSDP peers on RPs.

    Classification

    Location

    Function

    MSDP peer on the source end

    Closest to the multicast source (generally the source RP, such as RP1)

    The source RP creates SA messages, sends SA messages to remote MSDP peers, and advertises multicast source information registered to the local RP.

    The MSDP peer on the source end must be configured on an RP. Otherwise, the MSDP peer on the source end cannot advertise multicast source information.

    MSDP peer on the receiver end

    Closest to the receiver (such as RP3)

    After receiving SA messages, the MSDP peer on the receiver end joins an SPT with the multicast source being the root according to source information contained in SA messages. After receiving multicast data from this source, the peer forwards multicast data along the rendezvous point tree (RPT) to local receivers.

    The MSDP peer on the receiver end must be configured on an RP. Otherwise, the MSDP peer on the receiver end cannot receive multicast source information from other domains.

    Intermediate MSDP peer

    Having multiple remote MSDP peers (such as RP2)

    The intermediate MSDP peer forwards SA messages received from a remote MSDP peer to other remote MSDP peers.

  • Establish MSDP peers on common PIM routers but not the RPs.

    These MSDP peers (such as RouterA and RouterB) only forward SA messages they receive.

NOTE:

To ensure that all RPs on a network share source information and the number of devices configured with MSDP is minimized, it is recommended that you configure MSDP only on the RPs on the network.

MSDP Packets

MSDP packets are encapsulated in TCP packets and are in the format of Type Length Value (TLV), as shown in Figure 8-73.

Figure 8-73 Format of MSDP packets
  • Type: indicates the packet type. Table 8-78 lists types of MSDP packets.
  • Length: indicates the packet length.
  • Value: indicates the packet content that depends on the packet type.
Table 8-78 Types of MSDP packets

Value

MSDP Packet Type

Function

Major Information Contained

1

Source-Active

(SA)

Carries multiple groups of (S, G) entries and is transmitted among several RPs.

  • IP address of the source RP.
  • Number of (S, G) entries contained in an SA message.
  • List of active (S, G) entries in the domain.

Encapsulates PIM-SM multicast data.

  • IP address of the source RP.

  • PIM-SM multicast data.

2

Source-Active Request

(SA-Req)

Actively requests (S, G) list of a specified group G to reduce the delay for joining the group.

Address of the group G.

3

Source-Active Response

(SA-Resp)

Responds to SA request messages.

  • IP address of the source RP.
  • Number of (S, G) entries contained in an SA-Resp message.
  • List of active (S, G) entries in the domain.

4

KeepAlive

Maintains MSDP peer connections. The Keepalive packet is sent only when no other protocol packet is exchanged between MSDP peers.

-

5

Reserved

This packet type is reserved and used as Notification messages at present.

-

6

Traceroute in Progress

Traces and detects the RPF path along which SA messages are transmitted.

  • IP address of the source RP.
  • List of active (S, G) entries in the domain.
  • Search ID
  • Returned search information
  • Number of hops

7

Traceroute Reply

As described in Table 8-78, SA messages carry (S, G) information and encapsulate multicast packets. MSDP peers share (S, G) information by exchanging SA messages. If an SA message contains only (S, G) information, remote users may not receive multicast data because the (S, G) entry has already timed out when reaching the remote domain. If multicast data packets are encapsulated in an SA message, remote users can still receive multicast data when the (S, G) entry times out.

When a new user joins the group, the user must wait for the SA message sent by the MSDP peer in the next period because SA messages are sent periodically. To reduce the delay for the new user to join the source SPT, MSDP supports SA request and response messages of Type 2 and Type 3 to improve the update efficiency of active source information.

Process of Setting Up MSDP Peer Relationships

Setting Up MSDP Peer Relationships

MSDP peers use port 639 to set up TCP connections.

After MSDP is enabled on two devices and they are specified as MSDP peers to each other, the devices compare their IP addresses. The device with the smaller IP address starts the ConnectRetry timer and initiates a TCP connection to the other device. The device with the larger IP address monitors whether a TCP connection is set up on the port 639. The MSDP peer relationship is set up after a TCP connection is set up. MSDP peers maintain the TCP connection by exchanging Keepalive messages.

Figure 8-74 Process of setting up an MSDP peer relationship

As shown in Figure 8-74, an MSDP peer relationship is set up between RouterA and RouterB in a process as follows:

  1. In initial state, the MSDP session status of the two routers are Down.
  2. After MSDP is enabled and they are specified as MSDP peers to each other, the routers compare their IP addresses used to set up a TCP connection.
    • RouterA has a smaller IP address. Therefore, it enters the Connect state, initiates a TCP connection to RouterB, and starts the ConnectRetry timer. This timer determines the interval for retrying setting up the TCP connection.
    • RouterB has a larger IP address. Therefore, it enters the Listen state and waits for a connection initiated by the peer.
  3. After a TCP connection is set up, the MSDP session status of the two ends become Up.
  4. MSDP peers send Keepalive messages to each other to request the peer to maintain the MSDP connection status.
MSDP Authentication

To improve MSDP security, MSDP peers perform TCP connection authentication. You must configure the same encryption algorithm and password on the two ends of an MSDP peer relationship. Otherwise, the TCP connection cannot be set up between MSDP peers. MSDP supports two encryption modes: MD5 and Keychain. The two modes are mutually exclusive, and you can configure only one of them between MSDP peers.

Inter-domain Multicast Source Information Transmission Among Domains

As shown in Figure 8-75, the PIM-SM network is divided into four PIM-SM domains. There is an active multicast source (Source) in the domain PIM-SM1, and RP1 knows the existence of this source after the source registers to RP1. Domains PIM-SM2 and PIM-SM3 want to know the exact position of this source to receive multicast data from it. MSDP peer relationships must be set up between RP1 and RP2, and between RP2 and RP3.

Figure 8-75 Inter-domain multicast source information transmission

Multicast source information is transmitted among domains under the following process:

  1. When Source in PIM-SM1 sends the first multicast packet to the multicast group, the designated router DR1 encapsulates multicast data to a register message and sends the message to RP1. RP1 then obtains information about this multicast source.
  2. As a source RP, RP1 creates SA messages and periodically sends SA messages to its peer RP2. SA messages contain the multicast source address S, the group address G, and the address of the source RP1 that creates the SA message.
  3. After receiving SA messages, RP2 performs a reverse path forwarding (RPF) check. RP2 forwards messages that pass the RPF check to RP3, and checks whether there is a member of group G in the local domain. Because PIM-SM2 contains no receiver of group G, RP2 does not perform any other action.
  4. After RP3 receives the SA message, it performs an RPF check on the message. The check succeeds. Because a member of group G locates in PIM-SM3, RP3 generates a (*, G) entry using IGMP.
  5. RP3 creates an (S, G) entry and sends a Join message with (S, G) information to Source hop by hop. A multicast path (the shortest path tree SPT) from Source to RP3 is then set up. After multicast data reaches RP3 along the SPT, RP3 forwards the data to the receiver along the RPT.
  6. After the receiver DR3 receives multicast data from Source, it determines whether to initiate an SPT switchover.

Controlling the Forwarding of SA Messages

To control SA message transmission among MSDP peers and prevent loops, configure rules for filtering SA messages. After the rules are configured, the device receives and forwards only SA messages that are transmitted along a correct path and match the rules. In addition, you can configure an MSDP mesh group for the MSPD peers to prevent them from flooding SA messages.

RPF Rules of SA Messages

To prevent SA messages from being circularly forwarded among MSDP peers, MSDP performs the RPF check on received SA messages. MSDP strictly controls the incoming SA messages, and discards the SA messages that do not comply with the RPF rules.

After receiving SA messages, the MSDP-enabled device determines the RPF peer of the source RP based on Multicast RPF Routing Information Base (MRIB). The RPF peer is the next hop along the optimal path from the RP that creates SA messages. If an SA message is sent by an RPF peer, the message is accepted and forwarded to other MSDP peers. MRIB includes MBGP, multicast static route, and BGP and IGP unicast routing.

Apart from the rule mentioned above, the device complies with the following rules when forwarding SA messages:

  • Rule 1: If the peer that sends the SA message is the source RP, the SA message is accepted and forwarded to other peers.

  • Rule 2: If the peer that sends the SA message is a static RPF peer, the SA message is accepted. One router can set up MSDP peer relationships with multiple routers simultaneously. You can select one or more peers from these remote peers as a static RPF peer or RPF peers.

  • Rule 3: If a router has only one remote MSDP peer, the remote peer automatically becomes the RPF peer. The router accepts the SA message sent by this remote peer.

  • Rule 4: If a peer and the local router are in the same mesh group, the SA message sent by this peer is accepted. The SA message is not forwarded to members of this mesh group but all the other peers outside the mesh group.

  • Rule 5: If the route that reaches the source RP spans multiple ASs, only the SA message sent by a peer in the next hop AS is accepted. If this AS has multiple remote MSDP peers, the SA message sent by the peer with the largest IP address is accepted.

  • Rule 6: If an SA message is sent from an MSDP peer that is a route advertiser or the next hop of a source RP, the receiving router permits the SA message. If a network has multiple equal-cost routes to a source RP, the router permits SA messages sent from all MSDP peers on the equal-cost routes.

The application of rules 5 and 6 depends on route types.
  • If a route to a source RP is a BGP or an MBGP route:
    • If an MSDP peer is an EBGP or MEBGP peer, rule 5 applies.
    • If an MSDP peer is an IBGP or MIBGP peer, rule 6 applies.
    • If an MSDP peer is not a BGP or an MBGP peer and the route to the source RP is an inter-AS route, rule 5 applies. Rule 6 applies in other cases.
  • If a route to a source RP is not a BGP or an MBGP route:
    • If IGP or multicast static routes exist and an SA message is sent from an MSDP peer that is the next hop of a source RP, rule 6 applies.
    • If no routes exist, the router discards SA messages sent from MSDP peers.
MSDP Mesh Group

When there are multiple MSDP peers on a network, SA messages are flooded among these MSDP peers. The MSDP peer needs to perform the RPF check on each received SA message, causing a heavy burden to the system. Configuring multiple MSDP peers to join the same mesh group can reduce the number of SA messages transmitted among these MSDP peers.

Mesh group members can be located in one or more PIM-SM domains. They can also be located in the same AS or multiple ASs.

You must set up MSDP peer relationships between any two members of the mesh group and the two members must identify each other as the member of the same mesh group. As shown in Figure 8-76, RouterA, RouterB, RouterC, and RouterD belong to the same mesh group. On each router, you must set up MSDP peer relationships with the other three routers.

Figure 8-76 Networking diagram of MSDP peer relationships among mesh group members

After mesh group members receive SA messages, they check the source of these SA messages.

  • If the SA message is sent by a certain MSDP peer outside the mesh group, the member performs the RPF check on the SA message. If the message passes the RPF check, the member forwards this message to all the other members in the mesh group.

  • If the SA message is sent by a member of the mesh group, the member directly accepts the message without performing the RPF check. In addition, it does not forward the message to other members in the mesh group.

Filtering SA Messages

By default, MSDP does not filter SA messages. SA messages sent from a domain are transmitted to all MSDP peers on the network.

However, (S, G) entries in some PIM-SM domains guide the forwarding within local PIM-SM domains. For example, some local multicast applications use global multicast group addresses or some multicast sources use private addresses 10.x.x.x. If SA messages are not filtered, these (S, G) entries are transmitted to other MSDP peers. To address this problem, configure rules (ACL rules are often used) for filtering SA messages, and apply these rules when creating, forwarding, or receiving SA messages.

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Updated: 2019-08-09

Document ID: EDOC1000041694

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