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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CCNP ROUTE – EIGRP Lab with Frame Relay


The original lab text is as follows, the video shows a slightly modified version.

Task 1

Configure hostnames and IP addressing on all routers as illustrated in the network topology.


Task 2

Configure EIGRP for AS 1 as illustrated in the topology. However, do NOT advertise the 150.x.x.x subnets on any router via EIGRP.


Task 3

Configure routers R2 and R4 to as EIGRP stub routers. These routers should advertise their 150.x.x.x/24 subnets as Internal EIGRP routes to R1.


Task 4

Configure R1 so that it advertises ONLY a default route to both R2 and R4 as an External EIGRP route. Next, verify your configuration using the appropriate commands. Additionally, also verify that R2 and R4 can ping each others’ 150.x.x.x/24 subnets from their LAN interfaces, as well as the subnet on R1s LAN.

Task 5

Configure EIGRP authentication for AS 1 so that all the routers use the password CCNP-2010; however, after 15 minutes, the routers should stop using this password and begin using the password CCNP-2011. The password CCNP-2011 should be used by all routers indefinitely. Verify your configuration using the appropriate commands.


Frame Relay Review Lab




We had a lot of issues setting up GNS 3 topology with Frame Relay switch in the middle.  This above lab is great practice.




I think we need to set a time for a demo so everyone won’t have the same issues doing FR labs like we did.


The below topology looks simple to configure in GNS3 but it’s actually quite a challenge for myself to get right.


Please look for video coming up to explain FR switches and GNS3.


Video Explaining EIGRP Network Command


Any feedback greatly appreciated.  This is my first screencast going over how the EIGRP network command matches the interfaces that you want to add into the EIGRP process.  It’s also how ACL matches packets based in IP.  At its core, it has to do with the wildcard mask and matching binary configurations whereas wildcard mask is a 0. to connect with me and comment.

#show interface descriptions


I like using descriptions in GNS 3 labs to tell the interfaces apart without cheating and looking at the labeled topology.

This command takes a potentially jumbled mess of info show in #show interface and organizes it nicely.

One more command in the toolbox.  This is giving #show ip protocols a run for its top spot in the category of my favorite show commands.

Keep on geeking out!