HP MSR Series Configuration Manual page 308

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In the outbound direction, if a permit statement is matched, IPsec considers that the packet
requires protection and continues to process it. If a deny statement is matched or no match is
found, IPsec considers that the packet does not require protection and delivers it to the next
function module.
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In the inbound direction:
Non-IPsec packets that match a permit statement are dropped.
IPsec packets destined for the device itself are de-encapsulated. By default, the
de-encapsulated packets are compared against the ACL rules. Only those that match a
permit statement are processed. Other packets are dropped. If ACL checking for
de-encapsulated IPsec packets is disabled, the de-encapsulated packets are not compared
against the ACL rules and are directly processed by other modules.
When defining ACL rules for IPsec, follow these guidelines:
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Permit only data flows that need to be protected and use the any keyword with caution. With the
any keyword specified in a permit statement, all outbound traffic matching the permit statement
will be protected by IPsec. All inbound IPsec packets matching the permit statement will be
received and processed, but all inbound non-IPsec packets will be dropped. This will cause all
the inbound traffic that does not need IPsec protection to be dropped.
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Avoid statement conflicts in the scope of IPsec policy entries. When creating a deny statement,
be careful with its match scope and match order relative to permit statements. The policy
entries in an IPsec policy have different match priorities. ACL rule conflicts between them are
prone to cause mistreatment of packets. For example, when configuring a permit statement for
an IPsec policy entry to protect an outbound traffic flow, you must avoid the situation that the
traffic flow matches a deny statement in a higher priority IPsec policy entry. Otherwise, the
packets will be sent out as normal packets. If they match a permit statement at the receiving
end, they will be dropped by IPsec.
The following example shows how an improper statement causes unexpected packet dropping. Only
the ACL-related configuration is presented.
Assume Router A is connected to subnet 1.1.2.0/24 and Router B is connected to subnet 3.3.3.0/24,
and the IPsec policy configuration on Router A and Router B is as follows:
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IPsec configuration on Router A:
acl advanced 3000
rule 0 permit ip source 1.1.1.0 0.0.0.255 destination 2.2.2.0 0.0.0.255
rule 1 deny ip
acl advanced 3001
rule 0 permit ip source 1.1.2.0 0.0.0.255 destination 3.3.3.0 0.0.0.255
rule 1 deny ip
#
ipsec policy testa 1 isakmp <---IPsec policy entry with a higher priority
security acl 3000
ike-profile aa
transform-set 1
#
ipsec policy testa 2 isakmp <---IPsec policy entry with a lower priority
security acl 3001
ike-profile bb
transform-set 1
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IPsec configuration on Router B:
acl advanced 3001
rule 0 permit ip source 3.3.3.0 0.0.0.255 destination 1.1.2.0 0.0.0.255
rule 1 deny ip
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