Direct DSL Connections between Cisco routers

For some time I have a small test network with a number of old Cisco routers (mainly 2500 series). Recently I decided to purchase a c1841 through Marktplaats as the IOS versions of the 2500 is really ancient and is very limited for more complex setups nowadays (latest IOS version is 12.3)

With the c1841 I also obtained two SHDSL cards, specifically:

  • a G.SHDSL WIC (WIC-1SHDSL-V3)
  • a 2-pair G.SHDSL HWIC (HWIC-2SHDSL)

These cards are described in detail on Cisco’s website. It turned out that the G.SHDSL HWIC card is supported in my main router (a c1921), so I decided to connect my test network to my main router using a DSL connection using these two cards to fully separate the test network from my main networks (and for the fun of it). Cisco had a good guide available to set this up (see Configuring Cisco G.SHDSL HWICs in Cisco Access Routers and Setup back to back CPE connection and ) but as it turned out not to be totally trivial I decided to document my setup here as well.

Cabling

The nice thing about (SH)DSL is that it uses standard phone (CAT-4) cables that can be up to several kilometers long. For my test lab I started off with a standard (2-wire) phone cable with 6-pin RJ-11 connectors. (SH)DSL uses a straight connection where one of the ends should be put in CPE (subscriber) mode and the other one in CPO (office) mode.

As the 2-pair G.SHDSL HWIC (HWIC-2SHDSL) interface has two ports and I temporarily had two c1841 routers with a G.SHDSL WIC (WIC-1SHDSL-V3) I also created a splitter cable as per the Cisco G.SHDSL documentation (diagram below) to establish two DSL connections.

SHDSL RJ-11 splitter schematic
SHDSL RJ-11 splitter schematic

Dual DSL connection with 2 wires using a splitter cable

With the above cable it was quite easy to establish DSL connections between my c1921 router with the 2-pair G.SHDSL HWIC (HWIC-2SHDSL) card and two c1841with an G.SHDSL WIC (WIC-1SHDSL-V3) adapter. Unfortunately the 2-pair G.SHDSL HWIC (HWIC-2SHDSL) can only operate in CPE (client) mode so c1921 was running in CPE mode for both lines and the two c1841s both need to be set to CPO (office) mode. This setup looks like this:

c1921 with 2 DSL connections to 2 c1841s

Below is the configuration used:

c1921 with 2 DSL connections to 2 c1841s:

controller SHDSL 0/0/0
  dsl-group 0 pairs 0
  !
  dsl-group 1 pairs 1
  !
!
interface ATM0/0/0
  no ip address
  no atm ilmi-keepalive
  pvc 0/35
    encapsulation aal5snap
  !
  pvc 8/35
    encapsulation aal5mux ppp dialer
    dialer pool-member 2
  !
!
interface ATM0/0/1
  no ip address
  no atm ilmi-keepalive
  pvc 0/35
    encapsulation aal5snap
  !
  pvc 8/35
    encapsulation aal5mux ppp dialer
    dialer pool-member 3
  !
!
interface Dialer1
  ip unnumbered GigabitEthernet0/0
  encapsulation ppp
  dialer pool 2
  dialer-group 2
!
interface Dialer2
  ip unnumbered GigabitEthernet0/0
  encapsulation ppp
  dialer pool 3
  dialer-group 3
!
dialer-list 2 protocol ip permit
dialer-list 3 protocol ip permit

Obviously in a real-live setting additional statements will be required to ensure that firewall, nat, etc. are also correct but that is not the intention of this description. The above sets up the SHDSL adapter in 2-line more and defines two ppp connections over ATM and uses the IP address of the main GigabitEthernet interface also for the dialer interfaces. The corresponding configuration on the two c1841s is:

c1841 with DSL connections to c1921 (two times):

controller DSL 0/0/0
 mode atm
 line-term co
 dsl-mode shdsl symmetric annex B
!
interface ATM0/0/0
 no ip address
 no atm ilmi-keepalive
 pvc 0/35 
  encapsulation aal5snap
 !
 pvc 8/35 
  encapsulation aal5mux ppp dialer
  dialer pool-member 1
 !
!
interface Dialer0
 ip unnumbered Loopback0
 encapsulation ppp
 dialer pool 1
 dialer-group 1
!
ip route 0.0.0.0 0.0.0.0 Dialer 0
!
dialer-list 1 protocol ip permit
!

This (re)uses the IP address of the Loopback0 interface for the dialer interface. Thanks to the default route all 3 Cisco’s will be able to reach each other. During the simple speed tests I was able to do I noticed that the DSL connection could almost reach it’s 2Mbit maximum throughput, even from one c1841 through to c1921 to the other c1841. For more complex routing I would recommend not using static routes but using a routing protocol like EIGRP (which I am using as well and will describe later).

Single DSL connection with 4 wires using a straight cable

As one of the c1841s was a loaner, I then decided to setup the permanent connection slightly different using a 4-wire cable and configured the c1921 and c1841 slightly differently to utilise all 4 wires of the cable. The benefit of this setup is that it doubles the connection speed to approx. 4.5Mbit (still not really amazing considering that my internet connection over fiber cable is 50Mbit, but a bit more). This setup looks like this:

Single connection with 4 wires using a straight cable

Below is the configuration used:

c1921 4-wire DSL connections to c1841:

controller SHDSL 0/0/0
 dsl-group auto 
  shdsl 4-wire mode enhanced
 !
!
interface ATM0/0/0
  no ip address
  no atm ilmi-keepalive
  pvc 0/35
    encapsulation aal5snap
  !
  pvc 8/35
    encapsulation aal5mux ppp dialer
    dialer pool-member 2
  !
!
interface Dialer1
  ip unnumbered GigabitEthernet0/0
  encapsulation ppp
  dialer pool 2
  dialer-group 2
!

This setup is pretty similar to that before apart from how the SHDSL controller is configured and there is also (due to this configuration) only 1 ATM interface (so also only Dialer interface needed). However, as the controller is changed it is required to issue the following command to remove the previous dsl-group definitions to switch from the previous setup:

controller SHDSL 0/0/0
  no dsl-group 0
  no dsl-group 1

Before this new configuration can be entered, which also removes all configuration of the ATM interfaces (the Dial interfaces are unaffected). The corresponding configuration on the c1841 is:

c1841 4-wire DSL connections to c1921:

controller DSL 0/0/0
 mode atm
 line-term co
 line-mode 4-wire enhanced
 dsl-mode shdsl symmetric annex B
!
interface ATM0/0/0
 no ip address
 no atm ilmi-keepalive
 pvc 0/35 
  encapsulation aal5snap
 !
 pvc 8/35 
  encapsulation aal5mux ppp dialer
  dialer pool-member 1
 !
!
interface Dialer0
 ip unnumbered Loopback0
 encapsulation ppp
 dialer pool 1
 dialer-group 1
!
ip route 0.0.0.0 0.0.0.0 Dialer 0
!
dialer-list 1 protocol ip permit
!

With this new configuration early measurements indeed confirm the bandwith is higher, but not really doubled. As this is a connection to my test network, the performance is not really an issue but I like the idea of having physical cable that I can remove easily and the fact that my test network is not connected to my main switch / network in any way.

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