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Bridging & Switching

Option AB – Inter-AS MPLS VPN – The whole story (5) – Updated Dec 2008

In late 2007, Cisco introduced a new Inter-AS option; Option AB – This feature was introduced in the 12.2(33)SRC code. This feature combines the the best aspects of Option 1 (10A) and Option 2 (10B) (named type a and type b as per RFC4364 section 10 “Multi-AS Backbone”). Remember that type a (Option 1 AKA 10A) was not scalable, since a separate interface/subinterface and an eBGP session are required per each VRF on the ASBR, while it is simple, secure and per VRF QoS capable (per VRF interface/subinterface). On the other hand type b (Option 2 AKA 10B) despite being …

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System MTU explained

By default any interface on the switch can only send or receive a frame of a maximum size 1500 byte. Some applications like (QinQ, MPLS over Ethernet) require increasing the MTU of the switches in your network to allow greater sizes because thier frames may exceed the default 1500 bytes size. You can increase the MTU of all interfaces operating at 10/100 Mbits using the command system mtu <vlaue>. This command will not affect GigaEthernet ports; the command system mtu jumbo may be used to alter the MTU of the GigaEthernet interfaces. You will need to reload the switch for …

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What is: BPDU filter?

BPDUs are the messages exchanged between switches to calculate the spanning tree topology. BPDU filter is a feature used to filter sending or receiving BPDUs on a switchport. It is extremely useful on those ports which are configured as portfast ports as there is no need to send or receive any BPDU messages on of these ports. BPDU filter can be configured globally or under the interface level. When configured globally all portfast enabled ports stop sending and receiving BPDUs, but if a BPDU is received on the port it gets out of the portfast state and normally participate in …

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How-to: Configure Private VLANs

This is a master piece post from Petr Lapukhov explaining Private VLANs; what are they ? and how to configure and troubleshoot them ?. I am sure you will never get confused by Private VLANs anymore. Private VLANs revisited Read and understand

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How-to: Configure Spanning Tree Protocol Part3

After explaining how STP works to ensure your network is free of bridging loops in previous posts, we are going directly to a configuration example. We will use a simple topology in the diagram below and configure the switches to accomplish the tasks listed below. Task1: Configure SW0 as a root bridge for both VLAN1,2. For SW0 to be a root switch you have to manipulate the BID to make it the lowest in the network. The command spanning-tree vlan 1,2 root primary is used to automatically set the priority. SW0(config)#spanning-tree vlan 1,2 root primary Task2: Configure SW1 as a …

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How-to: Configure Spanning tree protocol Part2

In the first post STP part1, we discussed the root bridge election process, that was the first step in calculating the spanning tree topology. Lets dive and explore step2. Step2: Best Path selection After choosing and agreeing on the root switch for the domain, each switch starts looking for the shortest path to the root of the tree. The path with the lowest cost is selected to be the only active path. Path selection is based on the port cost concept which is based on the port speed as shown in the table below Bandwidth Cost 4 Mbps 250 10 …

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