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New Member

Cisco 877 ADSL with Additional routed external IP address

Hi All,

Company 1 - We have a Cisco 877 ADSL Router in operation that is doing Nat for a small office with a private internal range and DHCP. And a site to site VPN. All is great. There is one External IP that has a 24Bit mask.

Company 2 – will be moving in soon next door and will be using the same internet connection and will be bringing with them their own Cisco Ethernet Router.

Ideally I plan to give Company 2 their own External IP and plug their External Ethernet port into FE4 of the Cisco 877.

My issue is the Cisco router config – as everything is working fine for Company 1 I’d like to do the minimum config changes required.

I have been told by my ISP they can provide a single routable address.

I have been looking into what config on the 877 is needed but I am unsure exactly. I am thinking IRB?

Any comments greatly appreciated.

Thanks,

Craig.

16 REPLIES
Super Bronze

Re: Cisco 877 ADSL with Additional routed external IP address

Yes, if your ISP can provide a single routable IP towards the existing external IP that would work out for you.

Your current company 1 877 router can be configured with NATing for this new routable IP for company 2 LAN.

The current 877 router should already have proxy arp enabled on the outside interface, and once the ISP routes the new external IP to your existing 877, then you can configure the NAT, eg:


access-list 101 permit ip any

ip nat pool companyB-pool

ip nat inside source list 101 pool companyB-pool

The above is assuming that you currently have the NAT with ACL specific to company A LAN (instead of "permit ip any any" for the ACL on NAT statement).

Hope that answers your question.

New Member

Re: Cisco 877 ADSL with Additional routed external IP address

Hi Jennifer thanks for your reply.

Would this allow me to configure the new external routable IP directly on the WAN interface of company 2’s router?

Super Bronze

Re: Cisco 877 ADSL with Additional routed external IP address

No, in the scenario that I advised earlier, you don't have to use company 2 router anymore. Everything will be terminated on company 1 router, and by routing the new external IP towards company 1 router.

New Member

Re: Cisco 877 ADSL with Additional routed external IP address

Sorry I should have mentioned that it is a specific requirement by company 2 that they have an external IP configured directly on WAN interface of their router – one of the reasons being that they are using telephony features on it and will be using SIP.

So they need to make sure that the packets are identified correctly by their sip provider by having an external IP directly configured on the outside interface of their router Cisco UC500.

Super Bronze

Re: Cisco 877 ADSL with Additional routed external IP address

In that case, just terminate the ISP directly on company 2 router as they already are. To keep it simple, just have 2 ISP connections, ie: 1 to company 1 router and the second to company 2 router if you are going to use company 2 router anyway.

New Member

Re: Cisco 877 ADSL with Additional routed external IP address

The issue is timing and cost – we’re not going be able to install a line and have adsl enabled in time here in the UK.

Is there any way it can be done?

Super Bronze

Re: Cisco 877 ADSL with Additional routed external IP address

Just re-reading your first post, did you say you have /24 public IP for company 1? That means, you have spare public ip addresses within company 1 /24 public IP?

How is company 1 connected to the ISP? via ethernet?, directly? or via switch?

Hall of Fame Super Gold

Re: Cisco 877 ADSL with Additional routed external IP address

halijenn wrote:

Just re-reading your first post, did you say you have /24 public IP for company 1? That means, you have spare public ip addresses within company 1 /24 public IP?

How is company 1 connected to the ISP? via ethernet?, directly? or via switch?

Jennifer,

the OP has clearly stated:

I have been told by my ISP they can provide a single routable address.

Consequently, he can only provide port-forwarding to the router for the other company.

If that is not enough for their needs, they will have to subscribe an additional ADSL circuit, that is very economical.

New Member

Re: Cisco 877 ADSL with Additional routed external IP address

paolo - the ISP can provide additional routed IP addresses. I have checked and they can provide as many as we want – but they are routable the WAN interface will remain the same current IP / 28 Mask.

Hall of Fame Super Gold

Re: Cisco 877 ADSL with Additional routed external IP address

If the ISP can assign additional IP addresses, then you can give one to the 2nd router, by static NAT or sometime even without, using an approproate configuration.

New Member

Re: Cisco 877 ADSL with Additional routed external IP address

Paolo , static Nat was my original thought but it has to be a routable ip directly assigned to the external interface of the router.

The reason for this is that company 2 using Cisco router UC500 will be using sip for call termination inbound and outbound. With static Nat outbound calls will fail as the header of the sip packets will contain the original private IP addresses and not the external IP address and therefore outbound calls fail due to authentication.

Hall of Fame Super Gold

Re: Cisco 877 ADSL with Additional routed external IP address

Craig,

Cisco NAT has ALG (Application Layer Gateway), meaning it will analyze each SIP packet, and replace addresses as necessary-

Ans, as mentioned above, it may also be possible to assign the address to a 2nd router, without NAT.

New Member

Re: Cisco 877 ADSL with Additional routed external IP address

Is ALG (Application Layer Gateway) enabled by default when using static Nat? The reason why I’m asking is that I have deployed a number of Cisco voice systems with SIP and have always had this issue and have had to assign the external IP on the outside interface of the router. Shame that ADSL here in the UK is widespread, all Cisco routers with voice e.g. UC500 are Ethernet based so I have to use an ADSL modem as well..

I will look into ALG but ideally I’d like to assign the IP direct on the router.

Hall of Fame Super Gold

Re: Cisco 877 ADSL with Additional routed external IP address

SIP NAT ALG is enabled by default, and in my direct experience, it works very well.

I agree that for VoIP, it is better to avoid NAT altogether, so this 2nd company has to make a choice, if they want their own circuit, subscribe for one.

New Member

Re: Cisco 877 ADSL with Additional routed external IP address

Sorry I made a mistake in my original post.

Company 1 – connected to Internet via ADSL Cisco 877 Router with one IP assigned / 32 Mask

New Member

Re: Cisco 877 ADSL with Additional routed external IP address

In case someone else needs to do this, it’s possible using a BVI interface and an additional vlan.

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