| 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 18.104.22.168/16 and allow from only sources 10.1.2.6 and 10.1.2.7.
ipv4 access-list PIM_REGISTER_FILTER
10 deny ipv4 any 22.214.171.124 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
Step 2: Verify that the router is configured to rate limiting the number of PIM register messages as shown in the example below.
allow-rp group-list FILTER_PIM_JOINS
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.
Step 3b: Verify that interfaces used for MSDP peering have an inbound ACL as shown in the example.
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.