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Category: PPP

L2TP default routing: solutions

There are three tools that can (according to a CCIE friend of mine) solve any networking-related problems: GRE tunnels, PBR and VRFs. The solutions to the L2TP default routing challenge nicely prove this hypothesis; most of them use at least one of those tools.

Policy-based routing on virtual template interface. Use the default route toward the Internet and configure PBR with set default next-hop on the virtual template interface. The PBR is inherited by all virtual access interfaces, ensuring that the traffic from remote sites always passes the network core (and the firewall, if needed).

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Small steps to large complexity

Imagine you have a large retail network: your remote offices use ISDN to dial into the central site and upload/download whatever periodic reports they have. Having a core router connected to an ISDN PRI interface is the perfect solution:

A few years later, ISDN is becoming too slow for your increased traffic needs and you want to replace it with DSL or VPN-over-Internet solution. Your Service Provider offers you PPPoE forwarding with L2TP. This is a perfect solution as it allows you to minimize the changes:

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PPPoE testbed

Following the ADSL QoS discussions, I decided to test “a few” things in the lab. I didn’t want to build a huge lab with DSL modems and DSLAM and decided to emulate an end-to-end DSL network with routers.

Step 1: Create a PPPoE session between a client (SOHO router) and a server (NAS). I’d never configured it before, so I’ve visited uncle Google first. It gave me tons of useless hits (no wonder) and a few somewhat useful ones. All of them used the old VPDN-centric PPPoE syntax. It seems I’ve stumbled across another “niche spot”: PPPoE testbed implemented with recent IOS configuration syntax (bba-group).

Read more in the “PPPoE Testbed” article in the CT3 wiki.

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OSPF breaks when faced with overlapping IP addresses

A while ago cciepursuit described his problems with PPP-over-Frame Relay. Very probably his problems were caused by a static IP address assigned to the virtual template interface (this address gets cloned to all virtual access interfaces and IOS allows you to have the same IP address on multiple WAN point-to-point links). I recreated very similar (obviously seriously broken) scenario in my lab using point-to-point subinterfaces over Frame Relay to simplify the setup.

This is not something you’d want to do in your production network.

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OSPF ignores subnet mask mismatch on point-to-point links

The common wisdom says that the subnet mask mismatch will stop the OSPF adjacency from forming (I’ve included a sample debugging printout in yesterday’s post). In reality, the subnet mask is checked only on the multi-access interfaces and is ignored on point-to-point links. The source of this seemingly weird behavior is Section 10.5 of RFC 2328 which says:

The generic input processing of OSPF packets will have checked the validity of the IP header and the OSPF packet header. Next, the values of the Network Mask, HelloInterval, and RouterDeadInterval fields in the received Hello packet must be checked against the values configured for the receiving interface. Any mismatch causes processing to stop and the packet to be dropped. In other words, the above fields are really describing the attached network's configuration. However, there is one exception to the above rule: on point-to-point networks and on virtual links, the Network Mask in the received Hello Packet should be ignored.
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Internet Access Russian Dolls

When the local Telco installed my blindingly fast 20 Mbps Internet-over-fiber-cable service, I was expecting to use DHCP on the router’s outside interface to connect to the Internet. After all, they’re running switched Ethernet VLANs over the fiber cable, and using DHCP seemed a logical choice. Imagine my surprise when I had to configure PPP-over-Ethernet (PPPoE) – it was as if I would be using a DSL connection, not a fiber-optic cable.

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PPP default route

One of those readers that prefer to remain anonymous has left an interesting comment to my post “Almost-dynamic routing over ADSL interfaces”:
You do not need the route "ip route 0.0.0.0 0.0.0.0 Dialer0 10 track 100" and the tracking if you configure "ppp ipcp route default" on the dialer interface. Works the same way... :-)
You might be wondering why Cisco's engineers decided to pollute IOS with yet another feature. The problem they had was the way PPP over Frame Relay is implemented: it uses virtual interfaces and although you have a very static connection, you cannot bind a static interface name to it. A dynamic interface (with potentially changing name) is cloned from the virtual template every time the PPP-over-Frame-relay session is started. Obviously you cannot configure a static default route pointing to it in advance, so you need yet another feature to do it (I'll not even try to figure out how to create non-default static routes pointing to cloned interface).
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Reduce IP addressing errors in lab environment

One of the most tedious tasks in the initial lab setup (at least for me) is the IP address configuration, which usually includes a number of typos and mixups on the WAN links. You can simplify then WAN address configuration if you configure only one end of the WAN link and let PPP do the rest. For example, you could use the following configuration to configure WAN link on your core router …
hostname Core-2
!
interface Serial1/0
 description link to POP
 ip address 10.0.2.1 255.255.255.252
 encapsulation ppp
 peer default ip address 10.0.2.2
… and use IPCP negotiation on the POP router to pick up the WAN IP address:
hostname POP
!
interface Serial1/0
 description link to Core-1
 ip address negotiated
 encapsulation ppp

You should not configure no peer neighbor-route on the router that gets dynamic IP address, as the subnet mask is not assigned with IPCP; you need the IPCP-generated host routes if you want to do hop-by-hop telnet between the routers.

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Remove unwanted PPP peer route

When IOS started supporting dynamic allocation of IPCP (IP over PPP) addresses, it also got the mechanism to insert a dynamic host route toward the neighbor's IP address once the IP addresses were negotiated. This mechanism makes perfect sense in dynamic address allocation environments, as the subnet mask is not negotiated with IPCP. Without a host route toward the other end of the PPP link, there would be no easy way to reach the IP prefixes reachable via the PPP link.
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Configure DNS servers through IPCP

After I've fixed the default routing in my home office, I've stumbled across another problem: the two ISPs I'm using for my primary and backup link have DNS servers that reply solely to the DNS requests sent from their own IP address range:

When the traffic is switched from the primary to the backup ISP, I therefore also need to switch the DNS servers. Fortunately, this is quite easy to do on a router; you just need to configure ppp ipcp dns request on the dialer interface and the router starts asking for the DNS server address as part of the IPCP negotiation.
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Emulate dialup links with serial lines

I had to figure out various PPP parameters (and associated Cisco IOS behavior) and didn't have real dial-up equipment in my lab setup. I could have gone with PPPoE, but it turned out it's way simpler to emulate dialup connections (at least the PPP negotiations work as expected) on fixed serial lines. This is the minimum setup you need on the “caller” side …
interface Serial1/0
 ip address negotiated
 encapsulation ppp
 ppp authentication pap optional
 ppp pap sent-username client password 0 client
… and this is the “server”-side configuration:
interface Serial1/0
 ip address 10.0.0.33 255.255.255.252
 encapsulation ppp
 peer default ip address 10.0.0.34
 ppp authentication pap callin
!
username client password client
To trigger PPP negotiations, shut down and re-enable the serial interface on either side.

Note: As I'm using PAP authentication, I could use the more secure username secret configuration command, which would not work with CHAP.

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