QApp: Cisco Router, Extended Ping, MTU Dont Fragment

  • 27 June 2024
  • 3 replies



Can a QApp ping the IP on the other end of a link with the “don’t fragment” bit set to that link’s MTU? Can this be done for every interface on the router?

3 replies

Discovering the IP address of the other side of the link seems to be a bit of a problem. I’m thinking it could be done via ARP table lookup, but that requires knowing the name of the VRF (where they are in use, which is frequently the case for WAN links here).


There are three data gathering operations that need to take place:


Interface data (including interface name, IP address, and MTU)

VRF data (show ip VRF)

ARP data (including interface name and associated IP addresses, based on VRF: show arp vrf NAME)


Guessing a GApp will be necessary to tie this together? Not sure how to execute commands based on output data from another command?

@Verbatim The first challenge is to find out the Neighbor device and its interfaces, to do this kindly use cdp/lldp and find out the neighboring router with parsing. After the neighbor router is found, we need to add its IP interfaces either by using internal APIs or by using another canvas, use this device as input and run CLI on it to find IP interfaces.

Pinging is easy, we just need to make sure we pass the commands in one line using options, we can make IP as variable which should be coming from neighboring router IP interfaces.


@akverma: thanks for the reply!


A solution relying on ARP is preferred over CDP / LLDP, because those two protocols are often not running on ISP / MPLS PE devices, and the links that are most likely to have MTU problems are those with ISP / MPLS provider on the other side of the link.




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