Tag Archives: 1.2a

1.2.a Evaluate proposed changes to a network

1.2.a [vi] Evaluate impact of new traffic on existing QoS design

There can at least be two major scenarios that you need to keep in mind when introducing new traffic to an existing QoS design or implementation.

New traffic could just go to default-class which may be totally undesired

New traffic ends up matching an existing class causing a mix of transport types in a given class. For example, if new traffic is UDP and existing traffic is TCP, it could lead to TCP starvation (also known as UDP dominance).

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1315-1321).  . Kindle Edition.

1.2.a Evaluate proposed changes to a network



1.2.a [v] Migrate spanning tree protocol

The Spanning Tree Protocol (STP) is a network protocol that ensures a loop-free topology for any bridged Ethernet local area network. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. Spanning tree also allows a network design to include spare (redundant) links to provide automatic backup paths if an active link fails, without the danger of layer-2 loops, or the need for manual enabling/ disabling of these backup links. Spanning Tree Protocol (STP) was originally, standardized as IEEE 802.1D most recently in 802.1d-1998, but deprecated as of 802.1d-2004 in favor of Rapid Spanning Tree Protocol (RSTP). RSTP creates a spanning tree within a network of connected layer-2 bridges (typically Ethernet switches), and disables those links that are not part of the spanning tree, leaving a single active path between any two network nodes. While STP can take up to 50 seconds to respond to a topology change, RSTP is typically able to respond to changes within 3 x Hello times (default hello interval is 2 seconds) or even within a few milliseconds of a physical link failure.

In 2001 , the IEEE introduced Rapid Spanning Tree Protocol (RSTP) as 802.1w. Cisco’s proprietary versions of Spanning Tree Protocol, Per-VLAN Spanning Tree (PVST) and Per-VLAN Spanning Tree Plus (PVST +), create a separate spanning tree for each VLAN.

Rapid Per-VLAN Spanning Tree (RPVST) creates a spanning tree for each VLAN, just like PVST/ PVST +. Multiple Spanning Tree Protocol (MSTP) is similar to Cisco’s Multiple Instances Spanning Tree Protocol (MISTP), and is an evolution of the Spanning Tree Protocol and the Rapid Spanning Tree Protocol.

PVST + to MST Migration:

It is difficult to convert all the switches in the enterprise network to MST at the same time. Because of the backward compatibility, you can convert it step by step. It is recommended to implement the changes in the scheduled maintenance window because the spanning tree reconfiguration can disrupt the traffic flow.

When you enable MST, it also enables RSTP. The spanning tree uplinkfast and backbonefast features are PVST + features, and it is disabled when you enable MST because those features are built within RSTP, and MST relies on RSTP. When you migrate from PVST to RPVST, port status cycles through block and learning before moving to forwarding.

STP to RSTP (802.1w) or MSTP (802.1s)

The IEEE has pretty much incorporated most of the Cisco’s RSTP and MISTP concepts into two standards, namely 802.1w (RSTP) and 802.1s (MST).

Configuration Steps:

● Identify point-to-point and edge ports, ensuring all switch-to-switch links, on which a rapid transition is desired, are full-duplex.

● Figure out how many instances are needed in the switched network (an instance translates to a logical topology)

● Decide what VLANs to map onto those instances, and carefully select a root and a back-up root for each instance.

● Choose a configuration name and a revision number that will be common to all switches in the network.

● Migrate the core first. Change the STP type to MST, and work your way down to the access switches. MST can interact with legacy bridges running PVST + on a per-port basis, so it is not a problem to mix both types of bridges as long as interactions are clearly understood.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1307-1312).  . Kindle Edition.


1.2.a Evaluate proposed changes to a network

1.2.a [iv] Adding multicast support

Traditional IP communication allows a host to send packets to a single host (unicast transmission) or to all hosts (broadcast transmission). IP multicast provides a third scheme, allowing a host to send packets to a group of hosts. These hosts are known as group members. Packets delivered to group members are identified by a single multicast group address . Multicast packets are delivered to a group using best-effort with UDP transport, just like IP/ UDP unicast packets. The multicast environment consists of senders and receivers. Any host, regardless of whether it is a member of a group or not, can send to a group. However, only the members of a group receive the data.

A multicast address is chosen for the receivers in a multicast group. Senders use that address as the destination address of a datagram to reach all members of the group. Membership in a multicast group is dynamic; hosts can join and leave at any time. There is no restriction on the location or number of members in a multicast group, and a host can be a member of more than one multicast group at a time. The Cisco IOS supports the following protocols to implement IP multicast routing:

● IGMP is used between hosts on a LAN and the routers on that LAN to track the multicast groups of which hosts are other members.

● Protocol Independent Multicast (PIM) is used between routers so that they can track which multicast packets to forward to each other and to their directly connected LANs.

● Distance Vector Multicast Routing Protocol (DVMRP) is used on the MBONE (the multicast backbone of the Internet). The Cisco IOS software supports PIM-to-DVMRP interaction.

● Cisco Group Management Protocol (CGMP) is used on routers connected to Catalyst switches to perform tasks similar to those performed by IGMP.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1268-1275).  . Kindle Edition.

1.2.a Evaluate proposed changes to a network

below link to stretch article:


1.2.a [iii] Routing protocol migration

There are two common approaches for migrating between routing protocols.

Use administrative distance (AD) to migrate the routing protocols

Use redistribution and a moving boundary

When using migration by AD method, two routing protocols are run at the same time. This approach assumes sufficient resources such as memory, CPU, and bandwidth are in place. The first step in migration by AD is to turn on the new protocol, but make sure that it has a higher AD than the older routing protocol so it is not preferred. This step enables the protocol and allows adjacencies or neighbors and routing databases to be formed but does not actually rely on the new routing protocol for routing decisions . When the new protocol is fully deployed, various checks can be done with show commands to confirm that everything is working as desired. During the final cutover, the AD is shifted for one of the two protocols so that the new routing protocol will now have a lower AD hence preferred. With migration by redistribution method, the migration is completed as a series of smaller steps. In each step , part of the network is converted to the new routing protocol. In a big network, the AD approach might be used to support this conversion. In a smaller network, an immediate cutover might suffice . To provide full connectivity during migration by redistribution, the boundary routers between the two parts of the network would have to bi-directionally redistribute between the two routing protocols. Filtering via tags would be one effective way to manage this situation.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1244-1253).  . Kindle Edition.

1.2.a Evaluate proposed changes to a network

1.2.a [ii] Migrate parts of a network to IPv6

There are both pre-deployment and deployment phases when it comes to migrating from IPv4 to IPv6. The specifics would really depend on the network that is being migrated however we can note down some broader areas that would need planning in each phase. Pre-Deployment:

● Establish the network starting point

● Network assessment

● Defining early IPv6 security guidelines and requirements


● Transport considerations for integration

● Campus IPv6 migration options

● WAN IPv6 migration options

● Advanced IPv6 services options

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1225-1237).  . Kindle Edition.