SA Message Receipt
SA messages are only accepted from the MSDP RPF peer that is in the best path back toward the originator.
The same SA message arriving from other MSDP peers must be ignored or SA loops can occur.
Deterministically selecting the MSDP RPF peer for an arriving SA message requires knowledge of the MSDP
topology. However, MSDP does not distribute topology information in the form of routing updates. MSDP
infers this information by using (M)BGP routing data as the best approximation of the MSDP topology for
the SA RPF check mechanism. An MSDP topology, therefore, must follow the same general topology as the
BGP peer topology. Besides a few exceptions (such as default MSDP peers and MSDP peers in MSDP mesh
groups), MSDP peers, in general should also be (M)BGP peers.
How RPF Check Rules Are Applied to SA Messages
The rules that apply to RPF checks for SA messages are dependent on the BGP peerings between the MSDP
peers:
•
Rule 1: Applied when the sending MSDP peer is also an interior (M)BGP peer.
•
Rule 2: Applied when the sending MSDP peer is also an exterior (M)BGP peer.
•
Rule 3: Applied when the sending MSDP peer is not an (M)BGP peer.
RPF checks are not performed in the following cases:
•
If the sending MSDP peer is the only MSDP peer, which would be the case if only a single MSDP peer
or a default MSDP peer is configured.
•
If the sending MSDP peer is a member of a mesh group.
•
If the sending MSDP peer address is the RP address contained in the SA message.
How the Software Determines the Rule to Apply to RPF Checks
The software uses the following logic to determine which RPF rule to apply to RPF checks:
•
Find the (M)BGP neighbor that has the same IP address as the sending MSDP peer.
•
If the matching (M)BGP neighbor is an internal BGP (iBGP) peer, apply Rule 1.
•
If the matching (M)BGP neighbor is an external BGP (eBGP) peer, apply Rule 2.
•
If no match is found, apply Rule 3.
The implication of the RPF check rule selection is as follows: The IP address used to configure an MSDP
peer on a device must match the IP address used to configure the (M)BGP peer on the same device.
Rule 1 of RPF Checking of SA Messages in MSDP
Rule 1 of RPF checking in MSDP is applied when the sending MSDP peer is also an i(M)BGP peer. When
Rule 1 is applied, the RPF check proceeds as follows:
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
200 OL-32598-01
Configuring MSDP
SA Message Origination Receipt and Processing
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