Solved: CUCM 8.6 DHCP server question on TFTP

George Banuelos · ‎12-05-2013

Hi,

The CUCM 8.6 DHCP server is being used to provide ip addressing to 2 different sites with a total of six scopes. The question is that the DHCP scopes do not have the TFTP server address (which is the subscriber) in option 150. All the phones work fine and I was surprised to see that the scopes had the TFTP address missing. How is it that the phones are getting their TFTP loads? Shouldn't the DHCP scopes need to include the subscriber IP address in the TFTP option 150?

Thank you!

"I believe that the greatest work of God has not been in the miraculous, nor in the spectacular, incomprehensible Creation, but rather I believe it was accomplished on the Cross where He Himself purchased the redemption of His Bride... the salvation of m

William Bell · ‎12-05-2013

George,

You have two fields for Option 150 (IIRC). One for primary and one for secondary. As you have discovered, you can assign those parameters at the DHCP server level or at the scope level. Scope level overrides server level. Put another way, the scope will inherit the server DHCP options if you have don't have the option set at the scope level.

To provide redundancy, you can put the second subscriber in the 'Secondary TFTP Server' field of either the DHCP server or the scope. I don't see the value in duplicating configurations across six scopes unless you are trying to minimize latency for the TFTP traffic to the phone (which is a good enough reason).

If, OTOH, you are going to have two TFTP servers for every site AND the order you assign those servers is the same then I would just set both option 150 params at the server level and let the scopes inherit. Otherwise, you can override at the scope level. The key point is the parameters aren't cumulative.

HTH.

-Bill (http://ucguerrilla.com)

HTH -Bill (b) http://ucguerrilla.com (t) @ucguerrilla

Please remember to rate helpful responses and identify

View solution in original post

William Bell · ‎12-05-2013

George,

So in that case, having the option 150 of the added scope for the new office should have the new subscriber as the primary TFTP server - correct?

Correct. The scope options will override the server options.

Quick question on bandwidth for replication... to keep the off-site subscriber replication in mint condition, would QoS be needed and what is the typical bandwidth requirement?

As one can imagine, there are lots of different sessions stood up between CUCM cluster nodes. Cisco calls this traffic Inter Cluster Communication Signaling (ICCS). There are two classes of traffic. One for realtime traffic and one for non-realtime traffic. An example of real-time is when you go off hook on your phone and I can see your status via BLF or a shared line (remote in use) status update. If you and I were on different cluster nodes, the communication of your line state is considered real time. Non-real time traffic includes db replication.

Key point to understand is that CUCM nodes do not use the DB configuration for real time call processing. When data is written to the database, a service (I believe it is still DBLHelper) kicks in gear and communicates the state change to other processes and UCM nodes. The state change gets loaded into cache. That is real time. Later (maybe ms later), the db change is replicated. That is non-realtime.

By default, CUCM classifies realtime ICCS as CS3 (DSCP 24) and on-realtime as Best Effort/BE (DSCP 0). If your access switches are set to trust DSCP from the CUCM nodes then you are already classifying traffic in accordance with Cisco's best practice. You just need to preserve and treat the traffic accordingly as it traverses your network.

HTH.

-Bill (http://ucguerrilla.com)

HTH -Bill (b) http://ucguerrilla.com (t) @ucguerrilla

Please remember to rate helpful responses and identify

View solution in original post

George Banuelos · ‎12-05-2013

update... i just checked the DHCP server settings for the publisher and noticed where the TFTP option 150 field is populated with the subscriber address... so this answers part of my question... the other unanswered part is: should the individual scopes also have the subscriber IP address referenced in the TFTP option 150? and can this reference be a different subscriber in the event a 2nd subscriber is added to the cluster?

"I believe that the greatest work of God has not been in the miraculous, nor in the spectacular, incomprehensible Creation, but rather I believe it was accomplished on the Cross where He Himself purchased the redemption of His Bride... the salvation of m

William Bell · ‎12-05-2013

George,

You have two fields for Option 150 (IIRC). One for primary and one for secondary. As you have discovered, you can assign those parameters at the DHCP server level or at the scope level. Scope level overrides server level. Put another way, the scope will inherit the server DHCP options if you have don't have the option set at the scope level.

To provide redundancy, you can put the second subscriber in the 'Secondary TFTP Server' field of either the DHCP server or the scope. I don't see the value in duplicating configurations across six scopes unless you are trying to minimize latency for the TFTP traffic to the phone (which is a good enough reason).

If, OTOH, you are going to have two TFTP servers for every site AND the order you assign those servers is the same then I would just set both option 150 params at the server level and let the scopes inherit. Otherwise, you can override at the scope level. The key point is the parameters aren't cumulative.

HTH.

-Bill (http://ucguerrilla.com)

HTH -Bill (b) http://ucguerrilla.com (t) @ucguerrilla

Please remember to rate helpful responses and identify

George Banuelos · ‎12-05-2013

Thank you for the help. This is useful, since there is a possibility that a 2nd subscriber will be added to this cluster and that new subscriber will be off-site (on a different subnet). So in that case, having the option 150 of the added scope for the new office should have the new subscriber as the primary TFTP server - correct?

Quick question on bandwidth for replication... to keep the off-site subscriber replication in mint condition, would QoS be needed and what is the typical bandwidth requirement?

George

"I believe that the greatest work of God has not been in the miraculous, nor in the spectacular, incomprehensible Creation, but rather I believe it was accomplished on the Cross where He Himself purchased the redemption of His Bride... the salvation of m

William Bell · ‎12-05-2013

George,

So in that case, having the option 150 of the added scope for the new office should have the new subscriber as the primary TFTP server - correct?

Correct. The scope options will override the server options.

Quick question on bandwidth for replication... to keep the off-site subscriber replication in mint condition, would QoS be needed and what is the typical bandwidth requirement?

As one can imagine, there are lots of different sessions stood up between CUCM cluster nodes. Cisco calls this traffic Inter Cluster Communication Signaling (ICCS). There are two classes of traffic. One for realtime traffic and one for non-realtime traffic. An example of real-time is when you go off hook on your phone and I can see your status via BLF or a shared line (remote in use) status update. If you and I were on different cluster nodes, the communication of your line state is considered real time. Non-real time traffic includes db replication.

Key point to understand is that CUCM nodes do not use the DB configuration for real time call processing. When data is written to the database, a service (I believe it is still DBLHelper) kicks in gear and communicates the state change to other processes and UCM nodes. The state change gets loaded into cache. That is real time. Later (maybe ms later), the db change is replicated. That is non-realtime.

By default, CUCM classifies realtime ICCS as CS3 (DSCP 24) and on-realtime as Best Effort/BE (DSCP 0). If your access switches are set to trust DSCP from the CUCM nodes then you are already classifying traffic in accordance with Cisco's best practice. You just need to preserve and treat the traffic accordingly as it traverses your network.

HTH.

-Bill (http://ucguerrilla.com)

HTH -Bill (b) http://ucguerrilla.com (t) @ucguerrilla

Please remember to rate helpful responses and identify

George Banuelos · ‎12-05-2013

thank you for the replies.. very helpful!

"I believe that the greatest work of God has not been in the miraculous, nor in the spectacular, incomprehensible Creation, but rather I believe it was accomplished on the Cross where He Himself purchased the redemption of His Bride... the salvation of m