The Cisco multicast Rendezvous Point (RP) switch must be configured to limit the multicast forwarding cache so that its resources are not saturated by managing an overwhelming number of Protocol Independent Multicast (PIM) and Multicast Source Discovery Protocol (MSDP) source-active entries.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-221056	CISC-RT-000820	SV-221056r863379_rule		Low

Description
MSDP peering between networks enables sharing of multicast source information. Enclaves with an existing multicast topology using PIM-SM can configure their RP switches to peer with MSDP switches. As a first step of defense against a denial-of-service (DoS) attack, all RP switches must limit the multicast forwarding cache to ensure that switch resources are not saturated managing an overwhelming number of PIM and MSDP source-active entries.

Description

MSDP peering between networks enables sharing of multicast source information. Enclaves with an existing multicast topology using PIM-SM can configure their RP switches to peer with MSDP switches. As a first step of defense against a denial-of-service (DoS) attack, all RP switches must limit the multicast forwarding cache to ensure that switch resources are not saturated managing an overwhelming number of PIM and MSDP source-active entries.

STIG	Date
Cisco IOS XE Switch RTR Security Technical Implementation Guide	2024-06-06

Details

Check Text ( C-22771r507600_chk )
The Cisco switch 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 switch 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 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 x.1.2.6 and x.1.2.7. ip pim rp-address 10.1.12.3 ip pim accept-register list PIM_REGISTER_FILTER … … … ip access-list extended PIM_REGISTER_FILTER deny ip any 239.5.0.0 0.0.255.255 permit ip host x.1.2.6 any permit ip host x.1.2.7 any deny ip any any Step 2: Verify that the RP is configured to rate limiting the number of PIM register messages as shown in the example below: ip pim rp-address 10.2.2.2 ip pim register-rate-limit nn Step 3: Review the switch 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: Verify that interfaces used for MSDP peering have an inbound ACL as shown in the example. interface GigabitEthernet1/1 ip address x.1.28.8 255.255.255.0 ip access-group EXTERNAL_ACL_INBOUND in ip pim sparse-mode Step 3b: Verify that the ACL restricts MSDP peering to only known sources. ip access-list extended EXTERNAL_ACL_INBOUND permit tcp any any established permit tcp host x.1.28.2 host x.1.28.8 eq 639 deny tcp any host x.1.28.8 eq 639 log permit tcp host x.1.28.2 host 10.1.28.8 eq bgp permit tcp host x.1.28.2 eq bgp host x.1.28.8 permit pim host x.1.28.2 pim host x.1.28.8 … … … deny ip any any log Note: MSDP connections is via TCP port 639. If the RP switch 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.

Check Text ( C-22771r507600_chk )

The Cisco switch 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 switch 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 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 x.1.2.6 and x.1.2.7.

ip pim rp-address 10.1.12.3
ip pim accept-register list PIM_REGISTER_FILTER
…
…
…
ip access-list extended PIM_REGISTER_FILTER
deny ip any 239.5.0.0 0.0.255.255
permit ip host x.1.2.6 any
permit ip host x.1.2.7 any
deny ip any any

Step 2: Verify that the RP is configured to rate limiting the number of PIM register messages as shown in the example below:

ip pim rp-address 10.2.2.2
ip pim register-rate-limit nn

Step 3: Review the switch 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: Verify that interfaces used for MSDP peering have an inbound ACL as shown in the example.

interface GigabitEthernet1/1
ip address x.1.28.8 255.255.255.0
ip access-group EXTERNAL_ACL_INBOUND in
ip pim sparse-mode

Step 3b: Verify that the ACL restricts MSDP peering to only known sources.

ip access-list extended EXTERNAL_ACL_INBOUND
permit tcp any any established
permit tcp host x.1.28.2 host x.1.28.8 eq 639
deny tcp any host x.1.28.8 eq 639 log
permit tcp host x.1.28.2 host 10.1.28.8 eq bgp
permit tcp host x.1.28.2 eq bgp host x.1.28.8
permit pim host x.1.28.2 pim host x.1.28.8
…
…
…
deny ip any any log

Note: MSDP connections is via TCP port 639.

If the RP switch 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.

Fix Text (F-22760r507601_fix)
The risk associated with this requirement can be fully mitigated by configuring the switch to filter PIM register messages, rate limiting the number of PIM register messages, and accept MSDP packets only from known MSDP peers. Step 1: Configure the switch to filter PIM register messages received from a multicast DR 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 x.1.2.6 and x.1.2.7. SW1(config)#ip access-list extended PIM_REGISTER_FILTER SW1(config-ext-nacl)#deny ip any 239.5.0.0 0.0.255.255 SW1(config-ext-nacl)#permit ip host x.1.2.6 any SW1(config-ext-nacl)#permit ip host x.1.2.7 any SW1(config-ext-nacl)#deny ip any any SW1(config-ext-nacl)#exit SW1(config)#ip pim accept-register list PIM_REGISTER_FILTER SW1(config)#end Step 2: Configure the RP to rate limit the number of multicast register messages. SW1(config)#ip pim register-rate-limit nn Step 3: Configure the receive path or interface ACLs to only accept MSDP packets from known MSDP peers. SW1(config)#ip access-list extended EXTERNAL_ACL_INBOUND SW1(config-ext-nacl)#permit tcp any any established SW1(config-ext-nacl)#permit tcp host x.1.28.2 host x.1.28.8 eq 639 SW1(config-ext-nacl)#deny tcp any host x.1.28.8 eq 639 SW1(config-ext-nacl)#permit tcp host x.1.28.2 host x.1.28.8 eq bgp SW1(config-ext-nacl)#permit tcp host x.1.28.2 eq bgp host x.1.28.8 SW1(config-ext-nacl)#permit pim host x.1.28.2 host x.1.28.8 … … … SW1(config-ext-nacl)#deny ip any any

Fix Text (F-22760r507601_fix)

The risk associated with this requirement can be fully mitigated by configuring the switch to filter PIM register messages, rate limiting the number of PIM register messages, and accept MSDP packets only from known MSDP peers.

Step 1: Configure the switch to filter PIM register messages received from a multicast DR 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 x.1.2.6 and x.1.2.7.

SW1(config)#ip access-list extended PIM_REGISTER_FILTER
SW1(config-ext-nacl)#deny ip any 239.5.0.0 0.0.255.255
SW1(config-ext-nacl)#permit ip host x.1.2.6 any
SW1(config-ext-nacl)#permit ip host x.1.2.7 any
SW1(config-ext-nacl)#deny ip any any
SW1(config-ext-nacl)#exit
SW1(config)#ip pim accept-register list PIM_REGISTER_FILTER
SW1(config)#end

Step 2: Configure the RP to rate limit the number of multicast register messages.

SW1(config)#ip pim register-rate-limit nn

Step 3: Configure the receive path or interface ACLs to only accept MSDP packets from known MSDP peers.

SW1(config)#ip access-list extended EXTERNAL_ACL_INBOUND
SW1(config-ext-nacl)#permit tcp any any established
SW1(config-ext-nacl)#permit tcp host x.1.28.2 host x.1.28.8 eq 639
SW1(config-ext-nacl)#deny tcp any host x.1.28.8 eq 639
SW1(config-ext-nacl)#permit tcp host x.1.28.2 host x.1.28.8 eq bgp
SW1(config-ext-nacl)#permit tcp host x.1.28.2 eq bgp host x.1.28.8
SW1(config-ext-nacl)#permit pim host x.1.28.2 host x.1.28.8
…
…
…
SW1(config-ext-nacl)#deny ip any any