Chapter 10 - RSTP and EtherChannel Configuration Flashcards
Define Access (Layer) Switch
- The network architecture that end user devices connect to
- Usually situated at the edge of a LAN
- Generally will not connect to other access switches directly
- Generally handles port security, DAI, and QoS as traffic will be classified and marked closest to its entrance to the network
- Generally only uses Layer 2
Define Core (Layer) Switch
- Connects to Distribution switches
- Generally have much higher forwarding rates than Access and Distribution layer switches
- Avoids CPU intensive tasks such as QoS and security
- Connects to the internet in a 3 tier design
- Generally will only use Layer 3
Define Distribution (Layer) Switch
- A switch that connects switches together (core to access and access to access) to distribute frames throughout the LAN.
- They are usually layer 3 switches and handle things interVLAN routing.
- Connects to the internet in a 2 tier design
- Sometimes called Aggregation Layer
What is the need for multiple spanning tree topologies in a single broadcast domain?
Usually layer 2 load balancing. You could have a root bridge per VLAN and different blocked ports meaning that a single link isn’t oversaturated by carrying traffic for all VLANs.
What is PVST+?
- Per VLAN Spanning Tree.
- This is a Cisco proprietary standard.
- It allows for one STP topology per VLAN.
- You can also have RPVST+ which is the same as PVST+ except that it is based on RSTP.
What was the IEEEs answer to Cisco’s PVST+?
MSTP (Multiple Spanning Tree Protocol).
What form of spanning tree are the below based on?
- PVST+
- RPVST+
- MSTP
- RPVST+ - RSTP
- PVST+ - STP
- MSTP - RSTP
How many spanning tree topologies can you have in the below?
- STP
- PVST+
- RSTP
- RPVST+
- MSTP
- RSTP - 1 (CST - Common Spanning Tree)
- MSTP - 1 or more (it allows for as many MSTP instances (MSTIs) as network requires without needing one per VLAN)
- RPVST+ - 1 per VLAN
- PVST+ - 1 per VLAN
- STP - 1 (CST - Common Spanning Tree)
What are the IEEE standard numbers of the below?
- STP
- PVST+
- RSTP
- RPVST+
- MSTP
- RPVST+ - 802.1w
- MSTP - 802.1s
- PVST+ - 802.1D
- RSTP - 802.1w
- STP - 802.1D
What are the configuration parameters for enabling the below?
- PVST+
- RPVST+
- MSTP
- MSTP - mstp
- RPVST+ - rapid-pvst
- PVST+ - pvst
How was the original BID format changed to accomodate the introduction of VLANs into STP/RSTP?
Original: Priority field (0-65535) (16 bits) + System ID (MAC Address) (48 bits)
Post VLAN: Priority field (Multiple of 4096) (4 bits) + System ID Extension (VLAN ID) (12 bits) + System ID (MAC Address) (48 bits)
How many bits is the Priority field comprised of?
4-bits. This was originally 16-bits.
What is the command to configure primary and secondary root bridges in a spanning-tree topology?
spanning-tree vlan <vlan-id> root primary - Will set the priority dynamically to either 24576 by default or 4096 less than the current root bridge's priority.</vlan-id>
spanning-tree vlan <vlan-id> root secondary - Will set the priority of this switch dynamically to be 28672 as it assumes that this is 4096 less than the default value of the rest of the switches (32768) but it is also higher than the primary switch.</vlan-id>
What are some of the differences between RSTP/STP and RPVST+/PVST+?
- RSTP only creates one spanning tree topoloy (known as the Common Spanning Tree (CST)) whilst RPVST+ creates one tree per VLAN.
- RSTP sends one set of Hello BPDUs in the network regardless of how many VLANs there are. RPVST+ sense one set of messages per VLAN.
- RSTP and RPVST+ use different multicast destination addresses. RSTP uses 0180.c200.0000 (defined by IEEE) and RPVST+ uses 0100.0CCC.CCCD
(defined by Cisco). - When BPDUs are sent on VLAN trunks, RSTP sends the messages in the native VLAN with no VLAN header/tag. RPVST+ sends the messages meant for each VLAN with that VLANs header/tag (Sometimes known as BPDU Tunneling).
- RPVST+ adds an extra type-length value (TLV) to the BPDU that identifies the VLAN ID whereas RSTP does not.
- Both view the 16-bit priority field as including the 12-bit System ID Extension (VLAN ID) but RSTP will always have it set as 0000.0000.0000.
True or false. PVST+ and RPVST+ are Cisco proprietary standards.
True. MSTP, however, is an open IEEE standard.
True or False. STP and RSTP recognise VLAN tags.
False. They do not.
What commands allow you to configure Switch Priority per VLAN and Port Cost per VLAN?
spanning-tree vlan <vlan-ID> priority <priority value (multiple of 4096)> - Allows you to set the current switch's priority in a specified VLAN.</vlan-ID>
spanning-tree vlan <vlan-ID> cost <cost> - Allows you to set the STP/RSTP port cost for a specified VLAN. This can affect root cost.</cost></vlan-ID>
True or False. When talking about EtherChannels, the terms EtherChannel, PortChannel, LAG, and Channel-group are synonymous.
True.
How do you create a new EtherChannel on a switch?
- Enter the channel-group <channel-group> <mode> command in the interface configuration mode for each interface you wish to be part of this EtherChannel.</mode></channel-group>
- Need to use the same number channel-group for interfaces on the local switch that you want to join the EtherChannel.
- Can use different channel-group numbers between switches even if you want them to be part of the same EtherChannel.
What are the two protocols responsible for creating Dynamic EtherChannels? What do they do?
- These protocols dynamically negotiate the creation and maintenance of EtherChannels
- They are used to send messages to/receive messages from neighboring switches to advise which one is going to start the EtherChannel and whether certain interfaces can be added to the EtherChannel.
- PAgP (Port Aggregation Protocol) is a Cisco proprietary protocol
- LACP (Link Aggregation Control Protocol) is based on IEEE standard 802.3ad
What is a difference between PAgP and LACP?
- LACP allows for 16 links per EtherChannel whereas PAgP only allows for 8.
- Although, LACP will only have 8 of these links active at a time and will keep the rest in standby in case any of the active links go down.
Which keywords are used to enable which protocol (LACP or PAgP) after the channel-group <channel-group> mode command?</channel-group>
- desirable and auto will enable PAgP. Enabling desirable on a switch will mean this switch begins negotiations.
- active and passive will enable LACP. Enabling active on a switch will mean this switch begins negotiations.
- These keywords are used in place of the word ‘on’ after the mode parameter as ‘on’ uses neither LACP nor PAgP and will statically enable the EtherChannel rather than dynamically using one of the above protocols.
- Manually enabling the EtherChannel is not advised for various reasons. For example the switches will not send messages to advise when links have gone down so these will have to be manually checked and removed.
- Enabling a protocol on one end of an EtherChannel link and ‘on’ on the other will cause the link to fail.
True or false. A switch will check the settings of any new ports being added to the EtherChannel against that of the ports already in there.
True. The settings that are checked are:
- Speed
- Duplex
- Access/Trunk (all ports must be access or trunks, they can’t be mixed)
- If an access port, the access VLAN
- If a trunk port, the allowed VLAN list and the native VLAN
- STP interface settings (e.g. STP port cost)
- If a neighboring switch is being checked (instead of a port waiting to be added to an EtherChannel) then all of these apart from the STP interface settings must match.
- If these don’t match, the interface will not be added to the EtherChannel.
What is EtherChannel load distribution?
- This is the process by which a switch will decide what physical interface to forward frames out of when in an EtherChannel.
- It makes these decisions based on various numeric values found in layer 2, 3, and 4 headers.
- The fields it uses depend on the load distribution method (defined using the port-channel load-balance <method> command). The various methods can be:
- src-mac - Uses the source MAC address
- dst-mac - Uses the destination MAC address
- src-dst-mac - Uses both the source and destination MAC address
- src-ip - Uses the source IP address
- dst-ip - Uses the destination IP address
- src-dst-ip - Uses both the source and destination IP address
- src-port - Uses the source TCP or UDP port
- dst-port - Uses the destination TCP or UDP port
- src-dst-port - Uses the source and destination TCP or UDP port</method> - The method identifies fields in the message headers. Any messages that are part of the same connection or “flow” will have the same values in their fields and therefore will be forwarded over the same links. (e.g. src-mac will make sure that all messages with the same src-mac will traverse the same link).
The chosen method can be important as it:
- Causes all messages in a single connection (flow) to use the same link in a channel, meaning that the switch won’t reorder the frames if one of the links is busy but it decides to send the next frame over an empty link.
- Integrates the load distribution work into the ASIC (Application Specific Integrated Chip) responsible for hardware forwarding to ensure that the load distribution work is just as quick as the work necessary to forward any frame.
- To use all active links in an EtherChannel, adjusting to the addition and removal of links as necessary.
- Balance the traffic across links.
