Thursday, September 18, 2014

The exact route limit on the Cisco 3850

In a previous post I wrote about the route limit on the Cisco 3850 platform. After the issue earlier this year with the Cisco routers hitting the limit of 512k routes in hardware I think I have to clarify how many routes the Cisco 3850 platform supports. Cisco only releases these numbers for their big routers. The specification states 27000 routes.

To see the exact number of routes you can query the router directly.

my-switch#sho platform tcam utilization asic all           

CAM Utilization for ASIC# 0

 Table                                              Max Values        Used Values

 --------------------------------------------------------------------------------

 Directly or indirectly connected routes            32768/7680        119/4538

As you can see the router support 32768 directly connected routes and 7680 indirectly connected routes. A directly connected route is a host connected via L2 to the switch (like a host on a directly connected subnet) whereas indirectly connected routes are routes that are routed via another IP address.

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Wednesday, September 17, 2014

How to properly downgrade your MirkoTik

It is not easy to downgrade your MikroTik router. In the documentation you’ll find that you can use the “/system package downgrade” to downgrade your router. But this often leaves you with a router that only partially works after the downgrade. IPSec related stuff is known to stop working after the downgrade.

So how to downgrade? I do it this way:

  1. Get a backup of your configuration using “/export compact”. If you already downgraded your firewall this will not work. You then have to extract just parts of the configuration like “/ip addr ex com” and “/ip route ex com” to get the basic stuff.
  2. Downgrade your router to the version you are aiming for.
  3. Create a script file basic.rsc and upload it to the router. This file should contain as little as possible. Just what you need to log on to the router again.
  4. Do a factory reset using “/system reset-configuration keep-users=yes run-after-reset=basic.rsc”.

You have to use the extension .rsc or the script won’t run. If there are any configuration errors in this file nothing is executed. That is why you should keep it as small as possible. Just add the lines needed to log into the router. You usually need some IP addresses, routes and perhaps VLAN configuration.

There is a bug on many versions of the RouterOS that is important to notice. If the script file is less than about 512 bytes the router can’t read it either. If your file is small  you will have to add some lines like “# dummy text to make this a big file” to the end of the file.

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RouterOS ICMP fragmentation needed bug

On several MikroTik routers, at least one tile and ppc platforms the router won’t send back “ICMP fragmentation needed” packets to the sender if the packet is to big to be sent out on another interface. This bug makes PMTU not work and many applications is broken because of this.

This issue is found on at RouterOS 6.19. By downgrading to 6.6 this issue disapperars.

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