The Cisco router does not have a mechanism to limit the multicast forwarding cache. However, the risk associated with this requirement can be fully mitigated by configuring the router to: 1. Filter PIM register messages. 2. Rate limiting the number of PIM register messages. 3. Accept MSDP packets only from known MSDP peers. Step 1: Verify that the RP router is configured to filter PIM register messages for any undesirable multicast groups and sources. The example below will deny any multicast streams for groups 239.5.0.0/16 and allow from only sources 10.1.2.6 and 10.1.2.7. ipv4 access-list PIM_REGISTER_FILTER 10 deny ipv4 any 239.5.0.0 0.0.255.255 20 permit ipv4 host 10.1.2.6 any 30 permit ipv4 host 10.1.2.7 any 40 deny ipv4 any any … … … router pim address-family ipv4 rp-address 2.2.2.2 accept-register PIM_REGISTER_FILTER Step 2: Verify that the router is configured to rate limiting the number of PIM register messages as shown in the example below. router pim address-family ipv4 allow-rp group-list FILTER_PIM_JOINS rp-address 10.2.2.2 accept-register PIM_REGISTER_FILTER maximum register-states 250 Note: The maximum register-states command is used to set an upper limit for PIM register states. When the limit is reached, PIM discontinues route creation from PIM register messages. If not configured, the default is 2000 which would be an overage for a small to average size multicast deployment. Step 3: Review the router configuration to determine if there is a receive path or interface filter to only accept MSDP packets from known MSDP peers as shown in the example below. Step 3a: Determine which interfaces would be peering MSDP with an external router by the configured peer addresses as shown in the example below. router msdp peer x.14.2.1 remote-as nn ! peer x.15.3.5 remote-as nn ! ! Step 3b: Verify that interfaces used for MSDP peering have an inbound ACL as shown in the example. interface GigabitEthernet0/0/0/1 ipv4 address x.14.2.2 255.255.255.252 ipv4 access-group EXTERNAL_ACL_INBOUND ingress Step 3c: Verify that the ACL restricts MSDP peering to only known sources. ipv4 access-list EXTERNAL_ACL_INBOUND 10 permit tcp host x.1.28.2 host x.1.28.8 eq 639 20 deny tcp any host x.1.28.8 eq 639 log 30 permit tcp host x.1.28.2 host x.1.28.8 eq bgp 40 permit tcp host x.1.28.2 eq bgp host x.1.28.8 50 permit pim host x.1.28.2 host x.1.28.8 60 permit tcp any any established … … … 140 deny ipv4 any any log Note: MSDP connections is via TCP port 639 If the RP router is not configured to filter PIM register messages, rate limiting the number of PIM register messages, and accept MSDP packets only from known MSDP peers, this is a finding.