Issues configuring NAT

[gianlucaplatania]

Hello everyone,

I was trying to make some nat exercises on CML but I have issues with the inside interfaces. I made a 5 switches topology using routed interfaces and connecting one of them to my local gateway for external connectivity. I set the interface of this switch pointing to the internet with the ip nat outside command, and all the other interfaces with the ip nat inside cNAT ommand. I established an ACL allowing all the networks inside my LAN  with various permit statements and set the ip nat source list NAT interface gi1/0 overload (where gi1/0 is my interface towards the outside). Still, I can only reach the outside from the switch that is directly connected to the outside and the adjacet one (the one on its right). All switches are pingable between each other and have at least one routed interface using ospf. Every network was put in the access list. Directly connected switch can ping the outside with any source ip from different networks, the other one can use 2/3 source addresses. The rest can't ping the outside. Can you help me?

[balaji.bandi]

Can you post the show run (the device doing NAT for inside and outside)

[gianlucaplatania]

This is the configuration of the switch doing nat: 

Current configuration : 3983 bytes
!
! Last configuration change at 21:19:10 UTC Thu Jul 15 2021
!
version 15.2
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
service compress-config
!
hostname CORE1
!
boot-start-marker
boot-end-marker
!
!
!
no aaa new-model
no process cpu extended history
no process cpu autoprofile hog
!
!
!
!
!
!
!
!
ip cef
no ipv6 cef
!
!
!
spanning-tree mode pvst
spanning-tree extend system-id
!
no cdp run
!
!
!
!
!
!
!
!
!
!
!
!
!
!
interface Port-channel1
no switchport
ip address 172.16.0.1 255.255.255.248
ip nat inside
ip ospf 1 area 0
!
interface GigabitEthernet0/0
no switchport
no ip address
negotiation auto
no cdp enable
channel-group 1 mode desirable
!
interface GigabitEthernet0/1
no switchport
no ip address
negotiation auto
no cdp enable
channel-group 1 mode desirable
!
interface GigabitEthernet0/2
no switchport
ip address 10.0.1.2 255.255.255.252
ip nat inside
ip ospf 1 area 0
negotiation auto
no cdp enable
!
interface GigabitEthernet0/3
no switchport
ip address 10.0.2.6 255.255.255.252
ip nat inside
ip ospf 1 area 0
negotiation auto
no cdp enable
!
interface GigabitEthernet1/0
no switchport
ip address 192.168.1.12 255.255.255.0
ip nat outside
negotiation auto
!
interface GigabitEthernet1/1
negotiation auto
!
interface GigabitEthernet1/2
negotiation auto
!
interface GigabitEthernet1/3
negotiation auto
!
interface GigabitEthernet2/0
negotiation auto
!
interface GigabitEthernet2/1
negotiation auto
!
interface GigabitEthernet2/2
negotiation auto
!
interface GigabitEthernet2/3
negotiation auto
!
router ospf 1
default-information originate
!
ip nat inside source list NAT interface GigabitEthernet1/0 overload
ip forward-protocol nd
!
ip http server
ip http secure-server
!
ip route 0.0.0.0 0.0.0.0 192.168.1.254
ip ssh server algorithm encryption aes128-ctr aes192-ctr aes256-ctr
ip ssh client algorithm encryption aes128-ctr aes192-ctr aes256-ctr
!
ip access-list standard NAT
permit 10.0.0.0 0.0.0.255
permit 10.0.1.0 0.0.0.3
permit 10.0.2.0 0.0.0.3
permit 10.0.2.4 0.0.0.3
permit 172.16.0.0 0.0.0.7
!
!
no service-routing capabilities-manager
!

[balaji.bandi · Accepted solution]

I do not see any config issue  - you mentioned you have routes all the nodes, maybe you ping p2p interface it reaches works as expected.

- can you post-show ip route output to understand.

-  I do not recall maybe if this CML cisco IOL, there is some issue around 15.2 for the NAT and CEF

- no ip cef and test (this is not required to do in the real gear, because of IOL issue we may need to do this).

- Instead of PO - try only 1 interface NAT inside and test ( again this is not an a limitation on Physical gear).

- also check on end-node when you do traceroute, can this reach to your internet-connected end router?

- also check nat translation on the router connected to internet give you some information.

[gianlucaplatania]

Wow, CEF caused the issue and I solved with no ip cef on the edge switch. Thank you so much! Now I would like to know how cef interacts with nat being able to make it stop working correctly if not working.

Thanks for your

[balaji.bandi]

glad and good know it works as per my suggestions- That is the bug of the IOL image, if you use anything  above or 15.7 was fixed that issue,

Note : real world with physical equipment you do not need to do this all excercises .

[Giuseppe Larosa]

Hello @gianlucaplatania ,

in real world switches do not support NAT unless we are talking of high end models (in the past Cat6500, Cat 6800)

Your internet edge device should be a SW based router. Change the device connecting to the internet with a router and it should work.

However, also on real swich devices NAT commands can be issued without a warning because Cisco IOS XE CLI parser is too much "unified". So this may not be an issue of CML.

Hope to help

Giuseppe

[gianlucaplatania]

Hey Giuseppe thanks for your help! It was a CEF issue so I solved the problem. Anyway, which are the cisco OS that do not support NAT? Does NX-OS support it porperly?

[Giuseppe Larosa]

Hello Gianluca,

if you disable CEF on a production switch you can get an error message telling you that you cannot disable it or the switch will crash for very high CPU.

As I have written before for NAT you should use a router or a firewall.

and no NX-OS is not enough to have NAT working you need appropriate high end hardware.

IMHO this kind of exercises are not really good they are misleading. With real devices your NAT on low end switches would not work.

Attempt to use a SW based router instead and you will see it should support NAT with CEF enabled.

Hope to help

Giuseppe

[gianlucaplatania]

Thank you so much this was really interesting. I will keep it in mind and change my topology. I'm intrigued by the idea of using a firewall for the task, is it something common in real world implementations?

[gianlucaplatania]

I tried to put a Firewall on the edge, but I can't seem to configure a working NAT. I connected to the switch that was previously doing nat with a 11.0.0.0/30 p2p and used hte previous 192.168.1.12 to go towards the outside. Firewall can ping both 8.8.8.8 and anything inside the LAN.

This is the config:

ASA Version 9.15(1)1
!
hostname ciscoasa
enable password ***** pbkdf2
service-module 0 keepalive-timeout 4
service-module 0 keepalive-counter 6
names
no mac-address auto

!
interface GigabitEthernet0/0
nameif inside
security-level 100
ip address 11.0.0.2 255.255.255.252
!
interface GigabitEthernet0/1
nameif outside
security-level 0
ip address 192.168.1.12 255.255.255.0
!
interface GigabitEthernet0/2
shutdown
no nameif
no security-level
no ip address
!
interface GigabitEthernet0/3
shutdown
no nameif
no security-level
no ip address
!
interface GigabitEthernet0/4
shutdown
no nameif
no security-level
no ip address
!
interface Management0/0
no management-only
shutdown
no nameif
no security-level
no ip address
!
ftp mode passive
object network obj_any
subnet 0.0.0.0 0.0.0.0
pager lines 23
mtu outside 1500
mtu inside 1500
no failover
no failover wait-disable
icmp unreachable rate-limit 1 burst-size 1
no asdm history enable
arp timeout 14400
no arp permit-nonconnected
arp rate-limit 8192
!
object network obj_any
nat (inside,outside) dynamic interface
router ospf 1
router-id 12.12.12.12
network 11.0.0.0 255.255.255.252 area 0
log-adj-changes
default-information originate
!
route outside 0.0.0.0 0.0.0.0 192.168.1.254 1
timeout xlate 3:00:00
timeout pat-xlate 0:00:30
timeout conn 1:00:00 half-closed 0:10:00 udp 0:02:00 sctp 0:02:00 icmp 0:00:02
timeout sunrpc 0:10:00 h323 0:05:00 h225 1:00:00 mgcp 0:05:00 mgcp-pat 0:05:00
timeout sip 0:30:00 sip_media 0:02:00 sip-invite 0:03:00 sip-disconnect 0:02:00
timeout sip-provisional-media 0:02:00 uauth 0:05:00 absolute
timeout tcp-proxy-reassembly 0:01:00
timeout floating-conn 0:00:00
timeout conn-holddown 0:00:15
timeout igp stale-route 0:01:10
user-identity default-domain LOCAL
aaa authentication login-history
no snmp-server location
no snmp-server contact
crypto ipsec security-association pmtu-aging infinite
crypto ca trustpoint _SmartCallHome_ServerCA
no validation-usage
crl configure
crypto ca trustpool policy
auto-import
crypto ca certificate chain _SmartCallHome_ServerCA
certificate ca 0509
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921610d8 9e2747fb 3b21e3f8 eb1d5b02 03010001 a381b030 81ad300f 0603551d
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30090603 55040613 02424d31 19301706 0355040a 13105175 6f566164 6973204c
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quit
telnet timeout 5
ssh stricthostkeycheck
ssh timeout 5
ssh version 2
ssh key-exchange group dh-group14-sha256
console timeout 0
console serial
threat-detection basic-threat
threat-detection statistics access-list
no threat-detection statistics tcp-intercept
dynamic-access-policy-record DfltAccessPolicy
!
class-map inspection_default
match default-inspection-traffic
!
!
policy-map type inspect dns migrated_dns_map_1
parameters
message-length maximum client auto
message-length maximum 512
no tcp-inspection
policy-map global_policy
class inspection_default
inspect dns migrated_dns_map_1
inspect ftp
inspect h323 h225
inspect h323 ras
inspect ip-options
inspect netbios
inspect rsh
inspect rtsp
inspect skinny
inspect esmtp
inspect sqlnet
inspect sunrpc
inspect tftp
inspect sip
inspect snmp
policy-map type inspect dns migrated_dns_map_2
parameters
message-length maximum client auto
message-length maximum 512
no tcp-inspection
!
service-policy global_policy global
prompt hostname context
no call-home reporting anonymous
call-home
profile License
destination address http https://tools.cisco.com/its/service/oddce/services/DDCEService
destination transport-method http
profile CiscoTAC-1
no active
destination address http https://tools.cisco.com/its/service/oddce/services/DDCEService
destination address email callhome@cisco.com
destination transport-method http
subscribe-to-alert-group diagnostic
subscribe-to-alert-group environment
subscribe-to-alert-group inventory periodic monthly
subscribe-to-alert-group configuration periodic monthly
subscribe-to-alert-group telemetry periodic daily
Cryptochecksum:dd8ff72bf9a5d8c738c93a50f4b218e5

[Georg Pauwen]

Hello,

what network is on the inside ? Can you ping the inside network from the ASA ?

Try and add a route on the ASA pointing to your inside network:

route inside x.x.x.x y.y.y.y 11.0.0.1

[gianlucaplatania]

ASA can ping anything inside my network and outside. Routes are put throught OSPF

S* 0.0.0.0 0.0.0.0 [1/0] via 192.168.1.254, outside
O 10.0.0.0 255.255.255.0 [110/12] via 11.0.0.1, 01:17:03, inside
O 10.0.1.0 255.255.255.252 [110/11] via 11.0.0.1, 01:17:03, inside
O 10.0.1.4 255.255.255.252 [110/12] via 11.0.0.1, 01:17:03, inside
O 10.0.2.0 255.255.255.252 [110/12] via 11.0.0.1, 01:17:03, inside
O 10.0.2.4 255.255.255.252 [110/11] via 11.0.0.1, 01:17:03, inside
C 11.0.0.0 255.255.255.252 is directly connected, inside
L 11.0.0.2 255.255.255.255 is directly connected, inside
O 172.16.0.0 255.255.255.248 [110/11] via 11.0.0.1, 01:17:03, inside
C 192.168.1.0 255.255.255.0 is directly connected, outside
L 192.168.1.12 255.255.255.255 is directly connected, outside

[Georg Pauwen]

Hello,

do you see any NAT translations at all on the ASA (show xlate) ?

Try and specify the actual subnets instead of 'any':

object network obj_inside_networks
subnet 10.0.0.0 255.0.0.0
subnet 172.16.0.0 255.255.0.0
subnet 11.0.0.0 255.255.255.252

!

object network obj_inside_networks
nat (inside,outside) dynamic interface

[gianlucaplatania]

Hello, thank you for your answer. Show xlate is showing the following result:

ciscoasa# show xlate
0 in use, 0 most used

After putting your more specific piece of configuration, the result seems to be the same. What am I missing?