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Ask the Expert: IPsec VPN

Community Manager
Community Manager

Read the bioWith Marcin Latosiewicz

Welcome to the Cisco Support Community Ask the Expert conversation. This is an opportunity to Get an update on IPsec VPN with Cisco expert Marcin Latosiewicz who will answer questions on the topic of best practices when implementing IPsec VPNs on IOS and ASA. Marcin Latosiewicz is a Customer Support Engineer at the Cisco Technical Assistance Center in Belgium, which over four years of experience with Cisco Security products and technologies including IPSec, VPN, internetworking appliances, network and systems security, internet services and Cisco networking equipment.

Remember to use the rating system to let Marcin know if you have received an adequate response. 

Marcin might not be able to answer each question due to the volume expected during this event. Remember that you can continue the conversation on the Security sub-community   discussion forum shortly after the event.   This event lasts through December 9th, 2011. Visit this forum often to view responses to your questions and the questions of other community members.


Hi Marcy,

I set up in a Cisco 1801 a site-to-site vpn between site A and site  B, also I set up the Cisco 1801 as a Easy VPN server. Everything works  fine ...

The site-to-site vpn traffic from A to B has to do a source NAT to address to be able to arrive to B site.

The remote vpn traffic is arriving to the LAN of site A

The problem that i am facing is that i don't know how to set up the Cisco 1801 to manage the remote vpn traffic to site B. I think i have to do a source nat of the remote vpn traffic to address but i don't know how, should i use a ip nat inside or a nat outside command?

Thanks in advanced


What you need to remember is that NAT is done before encryption and after decryption.

So if you NAT traffic you need to most likely take it into consideration for routing and/or VPN.

Let's take this scenario:

X - one network

Y - network two

Z - network I would like X to be visible as on when traversing to Y.

A - device with X on LAN

B - device with Y on LAN

X -----A --- (internet) ---- B ---- Y

Scenario 1.

What I would typically do is on A.

ip nat inside source static X Z /24 [route-map RMAP1]

Route-map I can use to make sure this translation is only done when going from X to Y.

And when specifying interesting traffic for VPN I woud do on A:

permit ip Z Y

while on B:

permit ip Y Z

Scenario 2)

On B:

ip nat outside source static X Z /24

ACL for VPN on A:

permit X Y

ACL for VPN on B:

permit Y X

Let me know if this answers your question, I might have gotten your scneario wrong.


Hi Marcin,

Thanks for your reply,

I think i expolian myself not very clear. The site-to-site VPN with nat is already in use , that part is ok. The problem that i have is that i Also set up the router as an Easy VPN for remote users.

The traffic of these remote uses is the one that i want to route to site B through the site-to-site VPN.

So remote uses traffic has to be natted into xxxxx in orden to be accepted by site B.


(Remote users- -----Easy VPN----ROUTER (site A) --------(nat xxxxxx)---------SITE B

The principal problem that i have is that i don't know where to do the nat for the remote users VPN traffic to be translated into address xxxxxx when is routing to site B.

I am not sur e if this escenario is possible.



Gotcha, much more clear now.

What I would suggest is switching to DVTI deployment on router A in your topology.

This will allow you to enable "ip nat inside" on virtual-template and then use normal source NAT for remote users going out to the internet or to site B.

Example confrim (- NAT)  is here:

(I made the assumption you have "ip nat outside" on your WAN-facing interface)



The DVTI was the answer, thank you so much.

Best regards,


From your description it could be a problem with ISP, although I can tell you that it's rare nowadays.

My immediate suggestion is to perform a sniffer trace (wireshark or whathaveyou) on the interface associated with 3g dongle.

You should ESP or UDP/4500 packets leaving, but we're interested if you see anything going back.

What CAN be a problem is the packets coming back but are corrupted (not that uncommon over cellular networks).

If you don't see any return packets coming in you can confirm on the ASA that that you see encapsulations and decapsulations in "show crypto ipsec sa peer IP_ADD_RE_SS". If the values are non-zero it mean that we're processing traffic, if any (or both) are zero, we have some problem on the ASA or on the route to ASA.



Hi Marcin

I want to assign static IPs to  users that login to IPSec VPN using Group Authentication in ASA 8.2.   They authenticate through a Windows RADIUS server.  Right now, they are  connecting just fine and pulling an IP from the pool I have configured  in the IPSec policy. 

What would the best way to assign static IPs through VPN??

Thank you,



Truth be told, assigning statically to everyone does not scale for large deployments, although it's a neat control mechanism in small and medium setups.

You need to modify/double check two settings.

1) You need to make sure ASA can accept IP addresses for VPN users from AAA servers.

vpn-addr-assign aaa

2) Framed-IP-address RADIUS attribute can be sent from RADIUS as an AV pair (


For more information about supported attributes on ASA.

You can also check how you can assign statically from ASA itself (with local AAA auth)

Hope this helps,


Good day Marc.

Please I have a problem that needs urgent help. I have a T1 card installed on my 1841 router but my providers are giving me an E1 link and their complain is that it will not work with the E1 line. But i think it should be possible to use an E1 link with a T1 interface card depending on the configuration. Am really lost. Can you help me?

Good day marc,

i have three asa 5505 and they all share the same problem, vpn over  ipsec does work sometimes for week very good, but suddenly it can stop working (clients can always connect but cant ping or connect to remote recourses).

Two of asa's are running 7.2 and yesterday i updated one to 8.42, but no help.

Most of the time when vpn is not working, client can only ping once a remote server. No matter if client is using 3g or wired connection.

Ping statistics for

    Packets: Sent = 4, Received = 1, Lost = 3 (75% loss),

Approximate round trip times in milli-seconds:

    Minimum = 11ms, Maximum = 11ms, Average = 11ms

then suddenly, after 5min or 2 days..vpn connection all the time open or after reconnect,

Ping statistics for

    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:

    Minimum = 12ms, Maximum = 16ms, Average = 13ms

Client use xp and windows difference.

Second problem is that remote dns name's does not work, so i cannot use for example mapped home folder with server name, i have to use those with server's ip..

Running configuration (of 8.42)

Result of the command: "sh run"

: Saved


ASA Version 8.4(2)


hostname ciscoasa

domain-name domain

enable password *** encrypted

passwd ***  encrypted



name 213.139.x.x ulkoip description gw


interface Ethernet0/0

switchport access vlan 2


interface Ethernet0/1


interface Ethernet0/2


interface Ethernet0/3


interface Ethernet0/4


interface Ethernet0/5


interface Ethernet0/6


interface Ethernet0/7


interface Vlan1

nameif inside

security-level 100

ip address

ospf cost 10


interface Vlan2

nameif outside

security-level 0

ip address 213.139.x.x

ospf cost 10


interface Vlan3

no forward interface Vlan1

nameif dmz

security-level 50

ip address

ospf cost 10


boot system disk0:/asa842-k8.bin

ftp mode passive

clock timezone EEST 2

clock summer-time EEDT recurring last Sun Mar 3:00 last Sun Oct 4:00

dns domain-lookup inside

dns server-group DefaultDNS



domain-name domain

same-security-traffic permit inter-interface

same-security-traffic permit intra-interface

object network obj-


object network obj-


object network obj_any-01


object network obj_any-02


object network obj-


object network obj_any-03


object network obj_any-04


object-group network obj_any

access-list inside_nat0_outbound extended permit ip

access-list as extended permit ip

access-list domainVPN_splitTunnelAcl standard permit

access-list domainVPN_splitTunnelAcl_1 standard permit

access-list domainVPN_splitTunnelAcl_2 standard permit

access-list kissa_splitTunnelAcl standard permit

access-list Sisaverkko standard permit

access-list tunneliryhma_splitTunnelAcl standard permit

access-list inside_nat0_outbound_1 extended permit ip any any

access-list inside_test extended permit icmp any host

access-list Outside_In extended permit icmp any any unreachable

access-list Outside_In extended permit icmp any any time-exceeded

access-list Outside_In extended permit icmp any any echo-reply

pager lines 24

logging enable

logging asdm informational

mtu inside 1500

mtu outside 1500

mtu dmz 1500

ip local pool VPNpooli2 mask

icmp unreachable rate-limit 1 burst-size 1

icmp permit any inside

icmp permit host inside

icmp permit any outside

icmp permit host outside

asdm image disk0:/asdm-645-206.bin

no asdm history enable

arp timeout 14400

nat (inside,any) source static obj- obj- destination static obj- obj- no-proxy-arp

nat (inside,inside) source static any any no-proxy-arp route-lookup


object network obj_any-01

nat (inside,outside) dynamic interface

object network obj_any-02

nat (inside,outside) dynamic obj-

object network obj_any-04

nat (dmz,outside) dynamic obj-

route outside ulkoip 1

timeout xlate 3:00:00

timeout conn 1:00:00 half-closed 0:10:00 udp 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

dynamic-access-policy-record DfltAccessPolicy

user-identity default-domain LOCAL

eou allow none

url-cache dst 10

http server enable

http inside

http inside

no snmp-server location

no snmp-server contact

snmp-server enable traps snmp authentication linkup linkdown coldstart

crypto ipsec ikev1 transform-set ESP-DES-SHA esp-des esp-sha-hmac

crypto ipsec ikev1 transform-set ESP-3DES-SHA esp-3des esp-sha-hmac

crypto ipsec ikev1 transform-set ESP-AES-256-MD5 esp-aes-256 esp-md5-hmac

crypto ipsec ikev1 transform-set ESP-DES-MD5 esp-des esp-md5-hmac

crypto ipsec ikev1 transform-set ESP-AES-192-MD5 esp-aes-192 esp-md5-hmac

crypto ipsec ikev1 transform-set ESP-3DES-MD5 esp-3des esp-md5-hmac

crypto ipsec ikev1 transform-set ESP-AES-256-SHA esp-aes-256 esp-sha-hmac

crypto ipsec ikev1 transform-set ESP-AES-128-SHA esp-aes esp-sha-hmac

crypto ipsec ikev1 transform-set ESP-AES-192-SHA esp-aes-192 esp-sha-hmac

crypto ipsec ikev1 transform-set ESP-AES-128-MD5 esp-aes esp-md5-hmac

crypto dynamic-map outside_dyn_map 1 set pfs

crypto dynamic-map outside_dyn_map 1 set ikev1 transform-set ESP-3DES-SHA

crypto dynamic-map outside_dyn_map 20 set pfs

crypto dynamic-map outside_dyn_map 20 set ikev1 transform-set ESP-3DES-SHA

crypto dynamic-map outside_dyn_map 40 set pfs

crypto dynamic-map outside_dyn_map 40 set ikev1 transform-set ESP-3DES-SHA

crypto dynamic-map outside_dyn_map 60 set pfs

crypto dynamic-map outside_dyn_map 60 set ikev1 transform-set ESP-3DES-SHA

crypto dynamic-map outside_dyn_map 80 set pfs

crypto dynamic-map outside_dyn_map 80 set ikev1 transform-set ESP-3DES-SHA

crypto dynamic-map outside_dyn_map 100 set pfs

crypto dynamic-map outside_dyn_map 100 set ikev1 transform-set ESP-3DES-SHA

crypto dynamic-map outside_dyn_map 120 set pfs

crypto dynamic-map outside_dyn_map 120 set ikev1 transform-set ESP-3DES-SHA

crypto dynamic-map outside_dyn_map 140 set pfs

crypto dynamic-map outside_dyn_map 140 set ikev1 transform-set ESP-3DES-SHA

crypto dynamic-map outside_dyn_map 160 set pfs

crypto dynamic-map outside_dyn_map 160 set ikev1 transform-set ESP-3DES-SHA

crypto dynamic-map SYSTEM_DEFAULT_CRYPTO_MAP 65535 set ikev1 transform-set ESP-AES-128-SHA ESP-AES-128-MD5 ESP-AES-192-SHA ESP-AES-192-MD5 ESP-AES-256-SHA ESP-AES-256-MD5 ESP-3DES-SHA ESP-3DES-MD5 ESP-DES-SHA ESP-DES-MD5

crypto map outside_map 65535 ipsec-isakmp dynamic outside_dyn_map

crypto map outside_map interface outside

crypto isakmp nat-traversal 30

crypto ikev1 enable outside

crypto ikev1 policy 10

authentication pre-share

encryption 3des

hash sha

group 2

lifetime 86400

telnet timeout 5

ssh timeout 5

console timeout 0

management-access inside

dhcpd domain domain


dhcpd address inside

dhcpd dns interface inside

dhcpd wins interface inside

dhcpd domain domain interface inside

dhcpd enable inside


threat-detection basic-threat

threat-detection statistics host

threat-detection statistics port

threat-detection statistics protocol

threat-detection statistics access-list

no threat-detection statistics tcp-intercept


enable outside

group-policy domainVPN internal

group-policy domainVPN attributes

vpn-tunnel-protocol ikev1 l2tp-ipsec

split-tunnel-policy tunnelspecified

split-tunnel-network-list value Sisaverkko

address-pools none

username Vantaa password *** encrypted

username Vantaa attributes

service-type remote-access

username Hannes password ***  encrypted privilege 15

username Hannes attributes

vpn-group-policy domainVPN

username poysant password ***  encrypted

username poysant attributes

vpn-group-policy domainVPN

tunnel-group domainVPN type remote-access

tunnel-group domainVPN general-attributes

address-pool (inside) VPNpooli2

address-pool VPNpooli2

default-group-policy domainVPN

tunnel-group domainVPN ipsec-attributes

ikev1 pre-shared-key *****

isakmp keepalive disable


class-map inspection_default

match default-inspection-traffic



policy-map type inspect dns preset_dns_map


  message-length maximum 4096

policy-map global_policy

class inspection_default

  inspect ftp

  inspect h323 h225

  inspect h323 ras

  inspect rsh

  inspect rtsp

  inspect esmtp

  inspect sqlnet

  inspect skinny 

  inspect sunrpc

  inspect xdmcp

  inspect sip 

  inspect netbios

  inspect tftp

  inspect dns preset_dns_map

  inspect ip-options

  inspect icmp

class class-default

  user-statistics accounting


service-policy global_policy global

prompt hostname context

compression anyconnect-ssl

call-home reporting anonymous prompt 2


profile CiscoTAC-1

  no active

  destination address http

  destination address email

  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


: end


Neither of the questions is really on best practices in IPsec VPN, but I can give this a shot.

Problem 1)

I would suggest opening a TAC case to get to the bottom of this, but here are a few things you can check on your own:

- Check if clients behind NAT and clients with public IP address are affected in the same way.

- Check if ASA is processing those packets and sending replies (show crypto ipsec sa, is a good place to start).

- Since the problem persists between two very distinct ASA versions and on three devices, I would be also interested if same ISP involved.

- Are all clients affected when the problem starts or only a few ones?

- Is there anything in topology that might try to understand ESP or UDP/4500 packets (some firewalls and broadband routers are known to "inspect" VPN flows).

Problem 2)

It's not clear to me whether you have problems with name resolution or with connection after name resolution is done.

For example did you check if the names resolve properly in "nslookup" and whethere there is a difference when looking up "server" and "server.mydomain.tld". If you inspect "ipconfig /all" do you see proper DNS and domain suffic applied to interface?




The problem you mention is not related to IPsec VPN best practices :-)

What I would suggest is to open up a TAC case specifying:

1) Information about WIC/module you're using for E1/T1?

2) Purpose of E1/T1 (uplink to ISP for data, voice trunk)?

3) software information about router.

4) "show diag", "show inv", "show logg", "show tech outputs".



Hi Marcin,

I have been a regular viewer of your documents in the forum and appreciate your contribution . Can you please throw some light on how to evaluate performance or calculate the throughput across Site to Site VPN .For eg: usually we calculate the throughput of firewall (ASA) from the output of show interface and using the statistics over there (bytes / sec , pps etc) .But how do we know how much VPN traffic is contributing and if the firewall is getting overwhelmed because of the VPN traffic and not because of the clear text traffic




Thanks for interest in our docs, I hope you're getting meaningful information out of it :-)

Now regarding your question I see several levels here.

1) Are IPsec flows contributing to any soft of "oversubscription" of the ASA.

The answer is "yes, they can". IPsec packets will still occupy interface buffers. But it's treated like any other frame in the buffer.

Packet encryption and decryption is handled by special accelerator engine, so that part should not overwhelm the CPU (which is the shared resource for entire platform).

The best way to monitor whether it is IPsec that is causing the problems is monitor the connection table, to see if there is not abnormally high amount of connections related to particular tunnel.

For example:

show conn detail address

2) Now regarding calculating of throughput.

There are two possible answers here.

a) Maximum throughput.

In which case I suggest running a iperf with UDP of 1400 bytes which should give you a good enough max throughput via tunnel.

Link to iperf

b) Plotting current throughput of IPsec and IKE on ASA.

Best to monitor:





(Please note that tunnel index is not persitent - I can't find related enhancement request, will update this post when/if found)

Note that if you want to know what OIDs are supported on ASA you can do.

show snmp-server oidlist  !it's a hidden command.

c) Plotting current throughput on IOS.

We already recommend using virtual interfaces. (Tunnel, DVTI or SVTI).

You can monitor tunnel bandwidth by reading interface stats.

Remember to make ifindex persistent over reload

PE2_872(config)#snmp-server ifindex persist

Please note that we can probably write a whole book chapter on this topic I just wanted to provide you a place to start.



Tomas Fidler

Hello Marcin,

I have an question.

My written security policy says that vpn client should be able to connect just from known locations.


               Home office,

               Specific branch,

               customers host network.

I know IPs (ranges, IP addresses aso.) but I dont know how to limit this user to connect from his home office, and another from another home office network.

So my questions are:

     1. How to limit ability to login into VPN (IPsec or better SSL VPN) just from specific IPs.

     2. How to limit ability to login into VPN just from specific IPs, specified per user.

Thank you,


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