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We are testing an LTE configuration with a Cisco C1113-8PLTEEA router. We already have a wired internet connection in a specific VRF called DSL. As far as i understand the LTE backup, we do have to have the cellular interface in the same VRF in order for the routing (track object) to switch from the wired to the wireless connection. And there lies my problem. The cellular stops working as soon as I put it into a vrf. Lets have a look at the working configuration:

 controller Cellular 0/2/0
 lte sim data-profile 16 attach-profile 2 slot 0
 lte modem link-recovery wait-timer 5
 profile id 16 apn corporate.provider.ch authentication pap_chap username abc password def

 interface Cellular0/2/0
   ip address negotiated
   dialer in-band
   dialer idle-timeout 0
   dialer watch-group 2
   dialer-group 2
   ipv6 enable
   pulse-time 1

ip access-list standard 2
 10 permit any
dialer-list 2 protocol ip permit

ip route 0.0.0.0 0.0.0.0 Cellular0/2/0

So if I add the vrf at the Layer3 interface, the connection never comes up. Here is the adjusted configuration, i marked the changed lines with arrows.

 controller Cellular 0/2/0
 lte sim data-profile 16 attach-profile 2 slot 0
 lte modem link-recovery wait-timer 5
 profile id 16 apn corporate.provider.ch authentication pap_chap username abc password def

 interface Cellular0/2/0
   ip vrf forwarding DSL <-----------new
   ip address negotiated
   dialer in-band
   dialer idle-timeout 0
   dialer watch-group 2
   dialer-group 2
   ipv6 enable
   pulse-time 1

ip access-list standard 2
 10 permit any
dialer-list 2 protocol ip permit

ip route vrf DSL 0.0.0.0 0.0.0.0 Cellular0/2/0 <-----------adjusted

I havent found any VRF commands in the other areas. All the manuals I found, dont work with VRFs, so im a bit lost. Has anybody ever done this? what am i missing?

Here is the logging output without VRFs. I do have 2 IP SLA that ping google and show as soon as the connection is up.

Oct 26 10:57:11.377: %PARSER-5-CFGLOG_LOGGEDCMD: User:admjos  logged command:shutdown 
Oct 26 10:57:38.657: %PARSER-5-CFGLOG_LOGGEDCMD: User:admjos  logged command:no shutdown 
Oct 26 10:57:40.653: %LINK-3-UPDOWN: Interface Cellular0/2/0, changed state to down
Oct 26 10:58:04.586: %LINK-3-UPDOWN: Interface Cellular0/2/0, changed state to up
Oct 26 10:58:05.584: %LINEPROTO-5-UPDOWN: Line protocol on Interface Cellular0/2/0, changed state to up
Oct 26 10:58:08.378: %TRACK-6-STATE: 100 ip sla 100 reachability Down -> Up
Oct 26 10:58:08.378: %TRACK-6-STATE: 200 ip sla 200 reachability Down -> Up

And here is the output if I confiugure the VRF:

Oct 26 10:29:12.180: %PARSER-5-CFGLOG_LOGGEDCMD: User:admjos  logged command:no shutdown 
Oct 26 10:29:12.954: %SYS-5-CONFIG_I: Configured from console by admjos on vty0 (172.16.225.82)
Oct 26 10:29:14.178: %LINK-3-UPDOWN: Interface Cellular0/2/0, changed state to down

The interface basically just stays at the down state.

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  • So I take it the cellular doesn't obtain an IP address when the vrf forwarding command is applied, correct? – ditrapanij Oct 26 '20 at 9:42
  • @ditrapanij: correct – Mario Jost Oct 26 '20 at 9:51
  • Thanks. Give me a few minutes, labbing this up on a C1117. – ditrapanij Oct 26 '20 at 9:52
  • Couldn't replicate. You may need to hardreset the modem by shutting the cellular interfaces and going into controller cell0/2/0 and running lte radio off/no lte radio off – ditrapanij Oct 26 '20 at 22:56
  • Just got it to work, thanks for your help anyways. This gave me the security that there was no obvious flaw with my LTE configuration... – Mario Jost Oct 27 '20 at 13:56
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Just got it to work. The trick is to have something that generates traffic and triggers the cellular interface to connect. Didnt relize that the IP SLA i had configured on the global vrf made the connection work for me. So basically this fixed it for me:

ip sla 1000
 icmp-echo 8.8.8.8
  vrf DSL
  threshold 500
  timeout 1000
  frequency 4
ip sla schedule 1000 life forever start-time now

You can verify this by looking at the reason at the show dialer command:

roTST01#show dialer                  

Ce0/2/0 - dialer type = DIALER CWAN
Idle timer (never), Fast idle timer (20 secs)
Wait for carrier (30 secs), Re-enable (15 secs)
Dialer state is data link layer up
Dial reason: ip (s=172.20.220.1, d=8.8.8.8)
Time until disconnect never
Current call connected 00:45:41
Connected to lte

Dial String      Successes   Failures    Last DNIS   Last status
lte                      2          0    00:45:41       successful   Default

Sending a manual ping does often not work in this kind of cases, as the router does not have a valuable IP yet.

roTST01#ping vrf DSL 8.8.8.8
% VRF DSL does not have a usable source address 

So essentially, the IP SLA just takes a source IP from any other interface that it can find. In my case, the interface was not even in the same VRF.

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