MSDP uses SA (Source Active) messages that contain S,G (Source Group) information for RPs (Rendezvous Points) in PIM sparse domains. Thanks to MSDP, RPs can learn about multicast sources in remote PIM sparse domains. With a default MSDP configuration, all SA messages are advertised and received between MSDP peers.
On your network, there are probably a couple of S,G states that should stay within your network and that don’t have to be advertised to MSDP peers on remote networks. For example:
- Local applications that use multicast and that are only used on the LAN.
- Multicast traffic that uses private addresses as the source.
- Multicast groups in the private 126.96.36.199/8.
By enabling MSDP SA filtering of some S,G states we:
- Reduce the number of MSDP SA messages that are exchanged between MSDP peers.
- Reduce the size of the MSDP SA cache.
- Don’t leak information about S,G state information that remote peers shouldn’t know about.
To demonstrate MSDP SA filtering, I use this topology:
Here’s what we have:
- R1 and H1 are one LAN1, R2 and H2 are on LAN2.
- R1 and R2 are connected to each other with a private WAN connection.
- R1 is the RP in LAN1.
- R2 is the RP in LAN2.
- R1 and R2 are MSDP peers.
- H1 and H2 are only used to ping different multicast groups to trigger MSDP SA messages.
Want to take a look for yourself? Here you will find the startup configuration of each device.
hostname H1 ! no ip routing ! no ip cef ! interface GigabitEthernet0/1 ip address 192.168.1.1 255.255.255.0 ! ip default-gateway 192.168.1.254 ! end
hostname H2 ! no ip routing ! no ip cef ! interface GigabitEthernet0/1 ip address 192.168.2.2 255.255.255.0 ! ip default-gateway 192.168.2.254 ! end
hostname R1 ! no ip domain lookup ip multicast-routing ip cef ! interface Loopback0 ip address 188.8.131.52 255.255.255.255 ip pim sparse-mode ! interface GigabitEthernet0/1 ip address 184.108.40.206 255.255.255.0 ! interface GigabitEthernet0/2 ip address 192.168.1.254 255.255.255.0 ip pim sparse-mode ! ip pim rp-address 220.127.116.11 ip msdp peer 18.104.22.168 connect-source GigabitEthernet0/1 ip msdp originator-id GigabitEthernet0/1 ! end
hostname R2 ! no ip domain lookup ip multicast-routing ip cef ! interface Loopback0 ip address 22.214.171.124 255.255.255.255 ip pim sparse-mode ! interface GigabitEthernet0/1 ip address 126.96.36.199 255.255.255.0 ! interface GigabitEthernet0/2 ip address 192.168.2.254 255.255.255.0 ip pim sparse-mode ! ip pim rp-address 188.8.131.52 ip msdp peer 184.108.40.206 connect-source GigabitEthernet0/1 ip msdp originator-id GigabitEthernet0/1 ! end
Let’s take a look at our MSDP peering:
R1#show ip msdp peer MSDP Peer 220.127.116.11 (?), AS ? Connection status: State: Up, Resets: 0, Connection source: GigabitEthernet0/1 (18.104.22.168) Uptime(Downtime): 00:03:09, Messages sent/received: 4/4 Output messages discarded: 0 Connection and counters cleared 00:04:09 ago SA Filtering: Input (S,G) filter: none, route-map: none Input RP filter: none, route-map: none Output (S,G) filter: none, route-map: none Output RP filter: none, route-map: none SA-Requests: Input filter: none Peer ttl threshold: 0 SAs learned from this peer: 0 Number of connection transitions to Established state: 1 Input queue size: 0, Output queue size: 0 MD5 signature protection on MSDP TCP connection: not enabled Message counters: RPF Failure count: 0 SA Messages in/out: 0/0 SA Requests in: 0 SA Responses out: 0 Data Packets in/out: 0/0
As you can see above, nothing is filtered at all. This means that all S,G state entries are exchanged through MSDP. Let’s try a quick ping from H1 to see if this is true:
H1#ping 22.214.171.124 Type escape sequence to abort. Sending 1, 100-byte ICMP Echos to 126.96.36.199, timeout is 2 seconds: .
The ping fails since there is no listener for this multicast group but it doesn’t matter. This adds an entry in the multicast routing table that will be exchanged through MSDP. Let’s check R2:
R2#show ip msdp sa-cache MSDP Source-Active Cache - 1 entries (192.168.1.1, 188.8.131.52), RP 184.108.40.206, AS ?,00:00:23/00:05:41, Peer 220.127.116.11
Above, we see that R2 has received an entry for 18.104.22.168 with RP 22.214.171.124 in its MSDP SA cache.
Let’s try to filter some things. I’ll create the following access-list on both MSDP routers:
R1 & R2 (config)#ip access-list extended MSDP_SA_FILTER
Let’s look at some example of what we could filter now.