17 Oct

When your ISP sends you BPDU frames…

As an end-user, you should never receive STP BPDU frames from the ISP. The workstation in enterprise networks should not either. It is always a resultĀ of misconfiguration or lack of knowledge from network engineers about basics of network security. BPDU can reveal information about your network that can be later used to compromise it. In the worst case, an attacker can impact your system by changing the spanning-tree topology and perform a Man-In-The-Middle attack.

I noticed that my ISP is sending me BPDU frames. Let’s see, using this case real-life scenario, what we can tell about his network.

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31 Aug

Reserved VLANs on NX-OS 5.2(1)

Internal VLANs are used for services like MPLS, FCoE, Multicast over GRE, enhancement to SPAN, etc. Some Features have special requirements like which VLAN can be reserved for them. Example of such service are Multicast VLAN which can only start with VLAN id’s that is multiple of 64.

Prior to release 5.2(1) the reserved VLAN range was 3968 to 4048, and 4094, and it was not configurable. After the upgrade user-defined VLANs might fall within the new reserved range which now range from 3968 to 4095 and is configurable. If that occurs, switch fallback to old range but the features that need the additional reserved VLANs won’t work propely. What you have to do is change range of reserved VLANs using command:

switch(config)# system vlan 2000 reserve
This will delete all configs on vlans 2000-2127. Continue anyway? (y/n) [no] y
Note: After switch reload, VLANs 2000-2127 will be reserved for internal use.
      This requires copy running-config to startup-config before
      switch reload. Creating VLANs within this range is not allowed.

Now the disadvantage if this is you have to reboot whole chassis. Simple switchover between supervisors on Nexus 7000 won’t be enough. Also, if you are using vPC if one switch is using new VLAN range and other one is still configured to use old range switches will not forward those VLANs on vPC peer-link. Hence, those VLANs will get suspended on vPC port-channel. But this should not affect any other VLANs in vPC.

15 Jul

Simple line card performance testing configuration (IOS XR)

It’s easy trick but I’m going to put it here anyway so anyone can use it and for me so I can easy find code when I need it šŸ™‚ This is “snake” that is made traffic from generator go through all ports on line cards and between two line cards. It’s made for testing purposes in lab environment if you need to verify performance capability of ASR9K router (or any other runnin IOS XR in this case) for customer.
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09 Jun

No ISSU on Nexus 7000 with vPC between VDCs on same chassis – follow up

A little while ago I wrote about unsupported scenario of upgrading NX-OS on Nexus 7000. It’s a scenario I found few months ago and forgot to write about it, but my other customer had same problem few weeks ago which refreshed my memory. Back then we got information from Cisco TAC that this scenario is not supported but no official information is available about this.

I got mail from Andras Toth from Cisco TAC lately pointing me that this limitation has been documented in NX-OS 5.1 release notes. Thanks Cisco for making it clear. And remember it applies to all NX-OS releases, not just 5.1.

20 May

Cannot write configuration on Nexus

One of my customers told me that he can’t save configuration on Nexus 7000 switch. It has been working for over 6 months with no problems.

switch1# copy running-config startup-config
Configuration update aborted: another request for config change is already in progress

It looks like a CSCtj44206 bug. In this case software switchover is not a solution as this request will be refused by system due to configuration lock. Solution, except full chassis reload, is performing hardware switchover by removing active supervisor from chassis. This will unlock configuration and let you perform ISSU to latest NX-OS where this bug is solved.

14 May

No ISSU on Nexus 7000 with vPC between VDCs on same chassis

If you are using multiple VDCs on your Nexus 7000 switch and you create vPC between two of them you can forget about upgrading NX-OS via ISSU. This scenario is not supported, but there is no information about that in documentation. If you try to perform upgrade you will see following error message:

Notifying services about the switchover.            [#                   ]   0% -- FAIL. Return code 0x401E007B (request was aborted by service).
Please issue "show install all failure-reason" to find the cause of the failure.

Failure recovery action::
"Standby will be rebooted to force netboot and image download".
Install has failed. Return code 0x401E007B (request was aborted by service).

Please identify the cause of the failure, and try 'install all' again.

n7k-switch# show install all failure-reason
Service "vpc" in vdc: 2 returned error: configuration lock can not be acquired for peer switch (x41B7004B)
Service "vpc" in vdc: 3 returned error: configuration lock can not be acquired for peer switch (x41B7004B)

The only way to upgrade is to download new software, replace bootvar and reboot whole chassis.

09 May

Detecting Unidirectional Link Failure on STP

UDLD is a nice mechanism detecting unidirectional transmission over fiber or copper link. But it tests only physical layer. Therefore if there is a problem with transmitting BPDUs over link UDLD won’t detect this problem which may cause loops in network.

IEEE 802.1D-2004 Rapid STP standard defines dispute mechanism that works similar to UDLD but on layer 2 of OSI model. Root bridge is sending Superiors BPDUs over it’s links and is waiting for replies from the neighbors. If an adjacent switch respond in a way suggesting it didn’t receive SBPDU, root bridge block its port, thus preventing the bridging loop.

Dispute mechanism works only for RSTP and MST BPDUs because those ones includes the role and state of the sending port.

Dispute can be found on Catalyst 6500 with IOS 12.2(33) SXI and above, Catalyst 4500 and 4900 since 12.2(52)SG release as well as on Nexus switches.

25 Aug

vPC peer-link on Nexus switches

On Catalyst 6500 platform we had VSS technology that provides redundancy of uplinks with single spanning-tree point. Pair of Nexus switches cannot be one logical devices but with vPC technology those can act as singe spanning-tree point. Idea of redundancy is pretty much similar – downstream switch have two bundled uplinks, one to each uplink switches. If one of links of switches fails traffic is still forwarded using second link. This technology on Nexus switches is called vPC (Virtual PortChannel).
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