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9.1

Cisco Router CBAC & PPTP

Asked by MrPrince in Network Routers

Tags: Cisco Router, CBAC, PPTP, ACL, Firewall

Hi,

Im having problems configuring CBAC on a Cisco 871 router -  12.4(22)T. Im getting traffic in and out of the box but certain protocols dont seem to work, specifically PPTP and ICMP. Below are the pertinent parts of my config:

ip inspect name Global_IE tcp
ip inspect name Global_IE udp
ip inspect name Global_IE icmp
ip inspect name Global_IE pptp
!
interface Vlan10
ip address 172.16.0.1 255.255.255.252
 ip access-group Vestibule_Outbound in
 no ip redirects
 no ip unreachables
 no ip proxy-arp
 ip nbar protocol-discovery
 ip flow ingress
 ip flow egress
 ip nat inside
 ip virtual-reassembly
!
interface FastEthernet0
 switchport access vlan 10
 duplex full
 speed 100
!
interface FastEthernet4
 ip address dhcp
 ip access-group Inbound in
 no ip redirects
 no ip unreachables
 no ip proxy-arp
 ip nbar protocol-discovery
 ip flow ingress
 ip flow egress
 ip nat outside
 ip ips Global_IPS in
 ip ips Global_IPS out
 ip inspect Global_IE out
 ip virtual-reassembly
 duplex auto
 speed auto
!
ip access-list extended Vestibule_Outbound
 permit ip any any
 deny   ip any any log
!
ip access-list extended Inbound
remark Permit DHCP
 permit udp any eq bootps any eq bootpc
deny   ip any any log
!


My understanding of the above configuration is that:

1. PPTP traffic  is allowed by the Vestibule_Outbound ACL inbound to VLAN 10
2. The traffic leaves the fa4 interface since no Outbound ACL is applied.
3. CBAC opens a temporary ACE at the top of the Inbound ACL to allow the return traffic (GRE)

If I let SDM do the configuration I end up with an ACE of permit tcp any any eq gre on the Inbound ACL.  The same for ICMP & NTP. Although this works doesnt this simply open up a hole in the network? Isnt CBAC supposed to allow return traffic dynamically rather than having a specific ACE? Im having the same problem with ICMP too.

Can anyone explain the processing rules that CBAC uses? Ive read conflicting statements that CBAC:

1. First checks the traffic against an ACL then inspects it, if configured to, and finally amends the corresponding ACL to allow the return traffic.

2. Inspects the traffic first then amends the corresponding ACL to allow the return traffic.

Im confused. What is the correct way to accomplish this?

Thanks.
 
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[+][-]07/01/09 10:11 PM, ID: 24760587Expert Comment

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[+][-]07/02/09 05:48 AM, ID: 24762679Accepted Solution

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About this solution

Zone: Network Routers
Tags: Cisco Router, CBAC, PPTP, ACL, Firewall
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Solution Provided By: asavener
Participating Experts: 1
Solution Grade: A
 
[+][-]07/02/09 03:38 PM, ID: 24768444Author Comment

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