Cisco CCNP ISCW Exam Tutorial

Dead Peer Detection - Default And "On-Demand"

By Chris Bryant, CCIE #12933

With all things Cisco, we just have to have a keepalive, and with our IPSec peers, that keepalive is Dead Peer Detection.  (There's a second keepalive as well that I'll mention after the DPD discussion.)

I feel silly telling you what the DPD does, since if any networking feature has a "the name is the recipe" name, it's this one!  As with any keepalive, there are a few basics we need to know....

The CCNP exams generally aren't IOS-version specific, certainly not like the CCIE exams are, but you should know that DPD was introduced with IOS version 12.3(7)T.  Older IOS versions do not use DPD, obviously, and you may run into routers with earlier IOS versions out in the field.

According to Cisco's website, the following devices support DPD:

DPD can run in two different ways, the default setting and "on-demand".  The default setting is much like the routing protocol hellos we've studied in the past.  According to Cisco's website, the router will send a DPD Hello every 10 seconds "unless the router receives a hello message from the peer".  

As with routing protocols, the drawback of the regularly-scheduled hello packet is that it results in more packets to be processed - and in this case, encrypted and decrypted.  That's why DPD offers an on-demand configuration where a router will send a DPD Hello only in advance of sending traffic to a peer. 

The second keepalive method is simply the keepalive method of the routing protocol you're using over the VPN. Of course, that timer depends on whether you're running RIP, OSPF, or EIGRP - so you better review your CCNA studies and have those timers down cold!

We'll continue our discussion of VPNs in the next installment of my CCNP ISCW exam tutorial series -- and in the meantime, learn more about my ISCW Study Package and The Ultimate CCNP Study Package Bundle, which covers all four CCNP exams, and be totally prepared for CCNP exam success!

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com