Genie's Tech Blog

Where knowledge has no dimensions

CEF - Monitoring and Debugging

Hey Guys, Welcome back. Today I will continue my discussion on the old topic of Load Balancing and will discuss how to monitor and debug the CEF load balancing mechanism. A number of configurtion, exec and debug commands exist to monitor and debug the load balancing in a running system. We shall now see them one - by - one. 1. ip cef accounting load-balance-hash This configuration command enables the per hash packet counters which can be seen using the show ip cef <prefix> interna... [More]

CEF - The Polarization Effect

Hello Friends, In the last post we discussed about CEF Load Balancing. We also saw that per-destination based load balancing uses hash function to load balance the traffic. Since the hash function is used, it is very important that we also understand the concept of traffic polarization.  Consider a topology in which there are 4 paths in order to reach A0 to B0.   {R1, R2, R4}, {R1. R2, R5}, {R1, R3, R6} and {R1, R3, R7}. Now, considering R1, per-destination will assign the sessi... [More]

CEF Load-Balancing

Hello All, Today, I will be discussing some basic stuff about CEF (Cisco Express Forwarding) load balancing mechanisms and understanding how different algorithms are working. But, lets first start with defining what is load balancing. Load balancing is basically the ability to share the trafic to the destination IP prefix over all the active paths. CEF supports two methods of load balancing: 1. Per-packet load balancing 2. Per-destination load balancing We will now be discussi... [More]

MPLS G-ACh and GAL

As we read about Layer 2 VPN's, Pseudowires, etc, there comes a need for MPLS OAM (Operations, Administration and Maintenance) mechanisms that can we can use for fault detection, administration and multiple other functions over a PW and a LSP. These functions can be used between any two LERs or LSRs and specifically in a MPLS TP environment. There are existing tools which can serve similar functions: 1. VCCV - Virtual Circuit Connectivity Verification 2. BFD MPLS 3. LSP Ping 4. BFD VCCV Not... [More]

OSPF Multi-Area Adjacency

Hey Guys, I am back again with an interesting topic. OSPF Multi-Area Adjacency, But before we get onto this topic, lets get to the background of OSPF and why we need this feature. OSPF always prefers intra-area routes over the inter-area routes. This may cause suboptimal routing depending on which area the link is configured. In real life scenarios, Virtual Links are also not the feasible solution to solve the suboptimal routing issue and neither adding parallel additional physical or logical ... [More]

MPLS TE with Fast-Reroute (FRR) - Link Protection

Hey Guys, Today I have a very interesting topic to discuss. We are going to discuss about MPLS Traffic-engineering (TE) with Fast-Reroute (FRR). Actually, I came across some customer issue and I had to dig deeper into the platform level of 7600 to find out the root cause. But, then lately I realized that I should share the concept with everyone. So here it is..  MPLS Traffic-Engineering MPLS TE combines the ATM's Traffic engineering capabilities with IP's flexibility and Class-of-Service... [More]

NSF, NSR and GR

Hello Folks... Welcome back again. I was reading through some High Availability features and came across a good discussion on NSR (Non-Stop Routing), NSF (Non-Stop Forwarding) and GR (Graceful Restart). Just thought of sharing the same in simple words. But, first we'll understand the need for the same. Most of the high end routers separate the Control plan with the Forwarding plane with separate components viz. memory and processors. The control plane runs the routing protocols, maintains the ... [More]

Packet Capture on ASR1k

Hi Guys, Recently I came across a Cisco's Packet Capture tool on ASR1000 platform. You can capture the packet both in the ingress and the egress direction. The packet capture is done on the Quantum Flow Processor (QFP) Asic of the ESP card. Please note that the packet capture is shown on the IOS-XE version 3.3.1. Below is an example for the same. If you want to capture a packet between ASR1k and any other router say ASR1k ------- ASR9k (Please remember that the ASR9k is a IOS-XR based router... [More]

BFD : Concept

Introduction In both Enterprise and Service Provider networks, the convergence of business-critical applications onto a common IP infrastructure is becoming more common. Given the criticality of the data, these networks are typically constructed with a high degree of redundancy. While such redundancy is desirable, its effectiveness is dependent upon the ability of individual network devices to quickly detect failures and reroute traffic to an alternate path. This detection is now typically acc... [More]