Cisco ZBF show commands
A note to self on how to check current sessions on a Cisco router that has Zone Based Firewall configured (ZBF)
BGP aggregation – suppress generation of anchor null0 local routes
A scenario came up when a colleague was working on a project where his router was peering with a 3rd party router over eBGP. The router was performing aggregation for 10.x.x.x subnets, summary-only as 10.x/16. We know that BGP will generate a null0 anchor route automatically for the summary address. Now a situation occurred where […]
BGP backdoor
This topic is about BGP’s ability to prefer another router (kinda de-prefer, actually). The following diagram will make it clear. In this situation, it may be a better choice to traverse from R1 to R2 via the point-to-point link instead of going over the ISP Cloud. Here R1 and R2 are eBGP peers only. The […]
BGP route-maps and prefix-lists
Lately, I have been struggling to understand the logic behind route-map and its functioning when used along with a prefix-list in order to filter prefixes. Basically configurations where the permit/deny of prefix-list couples with permit/deny of route-map. So I decided to lab it out in GNS3 – only allow 172.1.1.0/24 to pass through; filter everything […]
Hidden utility on Cisco IOS – TTCP
Ttcp can be used to test P2P links, however, the drawback of generating the traffic from router is high CPU usage as this is a control-plane operation. Therefore, it is not really recommended to use this utility to determine throughput for bigger interfaces. TTCP is a hidden undocumented and unsupported IOS command. For testing of […]
Testing ISDN circuits from Cisco IOS
The Datacenter NOC where I work requires the network team working in night shifts to test the working status of ISDN circuits as they are very prone to failure due to multiple reasons. Let’s find the WAN IP address of a remote branch by logging into the Regional Office (RO) router. Login to the remote […]
Everyday Cisco IOS CLI tips
Mistypes holding up work This is often an annoying problem. Mistype a command and the router thinks you just typed a host name. For example, here we just mistyped the word “show”. We didn’t want to telnet to a device named “shwo”. The way to handle this is to change the preferred transport method. Problem […]