Author Archives: admin

Verify port-based QoS configuration in hardware of Cisco 6509

It’s a pleasure to work with Cisco TAC. I would say this is the only way to instantly improve your troubleshooting skills in any area. I was involved in Cisco 6509 troubleshooting that turned to be an IOS software-to-hardware QoS configuration bug. With this post I will slightly cover the bug logic and will show you the way to confirm QoS configuration in Cisco 6509’s module hardware.
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Warning – MODE button!

You all know that fancy Mode button located on every single non-modular Catalyst switch (Cisco 3750/3560). It gives you a way to visualize switchports information – like duplex mode, speed, stack member, master switch, PoE status and so on. This magic button also provides you with the only available method to enter a recovery mode (if enable password had been lost). All these features are well known to any Cisco engineer – this is the basis of CCNA course that relates to L2/switches subject. Now, imagine another not-so-well-known feature which can put you in the middle of the nightmare… Read more …

CBAC and HTTP inspection IOS bug

I was doing some work for my friend recently. One of the requirements was to provide a local Internet breakout to the office. I ended up using Dynamic PAT, Static ACL (Inbound) and Content-Based Access Control or CBAC (Outbound). CBAC was configured to support Generic UDP and TCP protocols as well as HTTP, HTTPS, IMAP, POP3, SMTP, DNS, FTP and few others. Everything was working in line with the design with one strange exception. Read more …

eBGP Loopbacks: Multihop vs Disable Connected Check

By default, BGP checks if its configured external (or eBGP) peers are directly connected. If not, BGP will not initialize for those peers (will stuck in Init). Respectively, this is true when destination’s IP address belongs to a loopback interface, even though it is configured on the directly connected router. Moreover, default TTL value for eBGP packets is 1 (that is why the check is required). There are two ways to change router’s default behavior – configure eBGP multihop or disable the check for directly connected peers. They both do same thing for loopback interfaces, yet the logic is different. Read more …

EIGRP Offset Lists Logic

EIGRP Offset Lists allow to add a value (an offset) to the calculated integer metric (RD/FD) for a given prefix. How does this fit into the composite metric’s concept? We know EIGRP’s Update messages contain the following metric components – cumulative delay, least bandwidth, load and reliability; as well as non-metric components – hop count and MTU. So, where’s offset? Read more …