Networking, VoIP and Tech

Quick Tip!

So I was trying to set up some fancy (legacy technology at this point) Frame Relay stuff for labbing OSPF.  I encountered:

R1(config)#int s1/2.1 multipoint 
% Warning: cannot change link type

Hmm weird I thought.  Before blaming this on GNS3 or a crappy image, I did some shallow digging on Google.  Apparently, a lot of newbies like me have run into this problem.  The solution is that Cisco subinterfaces default to being in point-to-point mode.  Once created, it cannot be changed to a multipoint interface without doing the following:

R1(config)#no int s1/2.1
Not all config may be removed and may reappear after reactivating the sub-interface

R1(config)#int s1/2.1 multipoint 

Hey I’m in!  Just a quick FYI for those who may be miffed by the idiosyncrasies of Cisco IOS.  This was on 12.4T.


Some discussion of MTU

MTU is the largest PDU that can be passed or transmitted, in bytes, by an interface on a specific protocol.

  • Default Layer 3 IPv4 MTU is 1500.
  • Jumbo Ethernet MTU up to 9216
  • WLAN MTU 7981
  • FDDI MTU 4352


Some caveats here for CCNP Studies so far:

  • Don’t mess with MTU unless you know what you’re doing.  Most MTU’s are left at 1504 or 1514 unless it’s a 10gig link.
  • Not required to match for EIGRP neighborship
  • Just to make sure, use #ping command with large packet sizes or df-bit set over a link to test MTU


I found some links for Cisco talks about MTU on IOS vs XR hardware.  Also another on MTU with ATM interfaces.  If anyone finds official documentation from Cisco about MTU, please let me know.




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