Chapter 1) Implementing Routing in the Data Center Flashcards
Describe the role of OSPF Hello-Packets
Hello packets are sent out on each OSPF-enabled interface to discover other OSPF neighbor routers. In addition, OSPF uses hello packets for keepalive and bidirectional traffic.
What are the default OSPF Hello & Dead intervals?
Default Hello interval is 10 seconds
Default Dead interval is 4 times the hello interval (40 seconds)
What algorithm is used by routers in OSPF to built a routing table?
Dijkstra’s Shortest Path First (SPF) algorithm
What are the multicast addresses used in OSPF versions 2 & 3 for sending Hello messages to adjacent routers?
OSPF Version 2 (IPv4): 224.0.0.5
OSPF Version 3 (IPv6): FF02::05
What are addresses and subnet masks referred to as in OSPF version 3
Prefix, and Prefix Length respectively
Which version of OSPF supports BFD? (Bidirectional Forwarding Detection)
OSPF version 2 only
How often are OSPF LSAs flooded by default?
LSAs are flooded based on the link-state refresh time (every 30 minutes by default).
Define OSPF Opaque LSAs and the 3 LSA types that fall under this category
Opaque LSAs consist of a standard LSA header followed by application-specific information.
LSA type 9: Flooded to the local network.
LSA type 10: Flooded to the local area.
LSA type 11: Flooded to the local autonomous system
Define a Type 1 LSA in OSPF versions 2 & 3
Type 1 LSAs are referred to as Router LSAs in OSPF versions 2 & 3
- Sent from all OSPF-enabled routers and flooded into the local OSPF area
- Contains the costs of all links and a list of OSPF neighbors on the link
Define a Type 2 LSA in OSPF versions 2 & 3
Type 2 LSAs are referred to as Network LSAs in OSPF versions 2 & 3
- Sent only by the DR and flooded to all other routers in the local OSPF area
- Lists all routers in the multi-access network
Define a Type 3 LSA in OSPF version 2 & 3
Type 3 LSAs are referred to as Network Summary LSAs in OSPF version 2
Type 3 LSAs are referred to as Inter-Area Prefix LSAs in OSPF version 3
- LSA sent by the Area Border Router (ABR) to an external area
- Includes the link cost from the Area Border Router (ABR) to the local destination
Define a Type 4 LSA in OSPF versions 2 & 3
Type 4 LSAs are referred to as ASBR Summary LSAs in OSPF version 2
Type 4 LSAs are referred to as Inter-Area Router LSAs in OSPF version 3
- Sent by the Area Border Router (ABR) to an external area.
- This LSA advertises the link cost to the ASBR only.
Define a Type 5 LSA in OSPF versions 2 & 3
Type 5 LSAs are referred to as AS External LSAs in OSPF versions 2 & 3
- Generated by the ASBR.
- Includes the link cost to an external autonomous system.
- AS External LSAs are flooded throughout the autonomous system.
Define a Type 7 LSA in OSPF versions 2 & 3
Type 7 LSAs are referred to as NSSA External LSAs in OSPF versions 2 & 3
- Generated by the ASBR within a not-so-stubby area (NSSA)
- Includes the link cost to an external autonomous system destination.
- NSSA External LSAs are flooded only within the local NSSA.
Describe the LSA group pacing feature
Used to reduce high CPU or buffer usage. This feature groups LSAs with similar link-state refresh times and packs multiple LSAs into a single OSPF update message.
Describe the OSPF Max Age Timer
The Max Age timer represents the maximum interval that an LSA can remain in the LSDB without being updated via an OSPF UPDATE message. When the Max Age timer expires for an LSA, it is removed from the LSDB.
Describe an OSPF Area
A logical division of routers and links within an OSPF domain that creates separate subdomains.
- LSDB flooding is contained within an area
Define the term AS and the role of an ASBR
An Autonomous System (AS) is a a network controlled by a single organization or administrative entity. An Autonomous Systems Boundary Router (ASBR) is used to connects an OSPF area to a different Autonomous System (AS).
Describe the function of an OSPF Stub Area & TSA
A stub area is an area that does not allow AS External (type 5) LSAs and does not contain or connect to an ASBR (Autonomous System Boundary Router). A regular stub area receives summarized routes for areas outside of the stubby area. A TSA (Totally-Stubby-Area) will only receive a default summary route of 0.0.0.0 /24 for all areas outside of the TSA.
Describe the function of an OSPF NSSA & TNSSA
A Not-So-Stubby-Area (NSSA) is typically used to redistribute routes from other routing protocols such as RIP or BGP via the ASBR. A NSSA filters LSA types 4 & 5, whereas the Totally NSSA area filters types 3, 4, & 5.
Describe an OSPF Virtual Link
All OSPF areas must have a direct connection back to the backbone area (Area 0). However, in some topologies, a direct physical connection to the backbone may not be available. An OSPF virtual link can be used to connect a distant area to the backbone through another area.
What 2 authentication methods are supported by OSPF?
1) Simple password authentication
2) MD5 authentication digest
What routing algorithm is used by Border Gateway Protocol (BGP)?
A path-vector routing algorithm to exchange routing information between BGP speakers.
Describe BGP path selection
A single best-path is selected for a remote host or network. Each best-path carries well-known mandatory, well-known discretionary, and optional transitive attributes that are used in BGP best-path analysis.
Describe the function of BGP ECMP.
Equal-Cost Multipath is a load-balancing feature where traffic can be carried over multiple “best-paths” that tie for top place in routing metric calculations. This offers better bandwidth utilization.
What version of BGP is supported by Cisco NX-OS. What port and transport mechanism is utilized by BGP to establish BGP sessions?
Cisco NX-OS supports BGP version 4. BGP uses port 179 over TCP for establishing BGP sessions with peers.
What size Autonomous System numbers are supported by BGP?
16-bit and 32-bit.
What BGP peering types are supported by NX-OS?
BGP peers are not discovered automatically, they must be manually configured using one of the following:
1) Individual IPv4 or IPv4 address
2) IPv4 or IPv6 prefix peers for a single AS number
3) Dynamic AS number prefix peers
What size is a BGP router-ID and what is the order for how it is chosen?
Router-IDs are 32-bit values represented as an IPv4 address, but are not used in any routing. Cisco NX-OS chooses a router ID in the following order:
1) A manually configured router ID is chosen first.
2) The loopback interface is chosen as the router ID.
3) The highest IP address associated with a physical interface is then chosen.
What is the 1st step in the BGP Sequence for comparison of paths? (Assuming the path is valid)
Cisco NX-OS chooses the path with the highest WEIGHT.
What is the 2nd step in the BGP Sequence for comparison of paths?
Cisco NX-OS chooses the path with the highest local preference (LOCAL_PREF).
What is the 3rd step in the BGP Sequence for comparison of paths?
Cisco NX-OS chooses the path that is locally originated (LOCAL_PATH).
What is the 4th step in the BGP Sequence for comparison of paths?
Cisco NX-OS chooses the shorter AS_PATH.
What is the 5th step in the BGP Sequence for comparison of paths?
Cisco NX-OS chooses the path with the lowest origin type. IGP is preferable to EGP.
What is the 6th step in the BGP Sequence for comparison of paths?
Cisco NX-OS chooses the path with the lower multi-exit discriminator (MED).
What is the 7th step in the BGP Sequence for comparison of paths?
NX-OS chooses the path from the external peer over the internal peer. EGP is preferable to IGP.
What is the 8th step in the BGP Sequence for comparison of paths?
Cisco NX-OS chooses the path with the lower IGP metric.
What is the 9th step in the BGP Sequence for comparison of paths?
Cisco NX-OS uses the path that was selected by the best-path algorithm the last time that it was run.
What is the 10th step in the BGP Sequence for comparison of paths?
Cisco NX-OS selects the path with the shorter cluster length.
What is the 11th step in the BGP Sequence for comparison of paths?
Cisco NX-OS chooses the path received from the peer with the lower IP address.
What is the 1st step in the BGP Sequence for determining the order of path comparisons?
Cisco NX-OS partitions the paths into groups. Within each group, Cisco NX-OS compares the MED among all paths.
What is the 2nd step in the BGP Sequence for determining the order of path comparisons?
Cisco NX-OS determines the best path in each group by iterating through all paths in
the group and keeping track of the best one so far. A new best-path is chosen if it is better than the current best-path.
What is the 3rd step in the BGP Sequence for determining the order of path comparisons?
Cisco NX-OS forms a set of paths that contain the best path selected from each group
in step 2. Cisco NX-OS selects the overall best path from this set of paths by going
through them as in step 2.
What advantage is offered by Multiprotocol BGP (MBGP)?
Multiprotocol BGP (MBGP) can carry different sets of routes depending on the address family. For example, BGP can carry one set of routes for IPv4 unicast routing, one set of routes for IPv4 multicast routing, and one set of routes for IPv6 multicast routing.
What is the default BGP Keepalive interval and hold timer?
Cisco NX-OS uses the following by default:
Keepalive interval: 60 seconds.
Hold timer: 180 seconds.
Describe BFD and the operating mode supported by Cisco NX-OS?
Bidirectional Forwarding Detection (BFD) is a detection protocol designed to provide fast forwarding–path failure detection times for media types, encapsulations, topologies, and routing protocols.
Cisco NX-OS supports the BFD asynchronous mode, which sends BFD control packets between two adjacent devices to activate and maintain BFD neighbor sessions between the devices.
What are the 3 session parameters supported by BFD in Cisco NX-OS?
Desired minimum transmit interval: The interval at which this device wants to send BFD hello messages.
Required minimum receive interval: The minimum interval at which this device can accept BFD hello messages from another BFD device.
Detect multiplier: The number of missing BFD hello messages from another BFD device before this local device detects a fault in the forwarding path.
What is the definition and purpose of a multicast group?
A multicast IP address is referred to as a group. End devices subscribe to multicast groups to receive data.
Describe the purpose of a multicast channel.
A channel is a multicast IP address that includes the multicast group and the source of the multicast transmission.
What is the IANA (Internet Assigned Naming Authority) range of multicast messages for both IPv4 & IPv6?
The Internet Assigned Numbers Authority (IANA) has assigned:
224.0.0.0 through 239.255.255.255 as IPv4 multicast addresses.
IPv6 multicast addresses begin with 0xFF.
Describe the function of IGMP.
The Internet Group Management Protocol (IGMP) is used by hosts that want to receive
multicast data to request membership in multicast groups. Once the group membership
is established, multicast data for the group is directed to the LAN segment of the
requesting host.
What version(s) of IP (Internet Protocol) is supported by IGMP
IGMP only supports IPv4
What is the default version of IGMP used when Cisco NX-OS enables the IGMP process?
By default, IGMPv2 is the version used. IGMPv3 must be enabled manually on the interfaces where its functionality wants to be used.
What are the main key differences between IGMPv2 & IGMPv3?
IGMPv3 supports the following additional features:
- support for SSM (Source Specific Multicast) which can be used to build shortest path trees from receiver to source.
- hosts no longer perform report suppression, which means that hosts always send
IGMP membership reports when an IGMP query message is received.
What is the purpose of the IGMP designated querier?
An IGMP querier sends query messages to the all-hosts multicast group periodically to discover whether any hosts want to receive multicast data.
What IP address is used for the all-hosts multicast IP group?
The all-hosts multicast IP (group) is 224.0.0.1.
How is the IGMP querier for a multicast group elected?
The software elects an IGMP as the designated querier on the subnet if it has the lowest IP address. A router will drop its role as the querier if it receives a host query message from a router with a lower IP address.
What is the function of IGMPv2 membership report suppression?
Only one membership report per group needs to be sent to the router. As such, other hosts suppress their reports to reduce network traffic.
What is the TTL of a IGMP or MLD multicast message sent by a designated querier for a subnet?
The TTL (Time To Live) of this type of message is 1.
What type of message is sent by an IGMP host that no longer wishes to subscribe to a multicast group?
A host running IGMPv2 or later will send an IGMP LEAVE message to the designated querier.
What process occurs after a designated querier receives an IGMP LEAVE message from a host?
The designated querier will send out a query message to check if this is the last host that is subscribed to the multicast group. If a report is not received by a certain interval, then it is assumed that no other hosts are subscribed the group, and as such, the multicast group state is removed.
The configurable interval for the querier is referred to as the last member query response interval.
What process occurs after a designated querier receives an IGMP LEAVE message from a host?
The designated querier will send out a query message to check if this is the last host that is subscribed to the multicast group. If a report is not received by a certain interval, then it is assumed that no other hosts are subscribed the group, and as such, the multicast group state is removed.
The configurable interval for the querier is referred to as the last member query response interval.
Describe the purpose and function of Switch IGMP Snooping.
Switch IGMP Snooping is used to limit the amount of IGMP traffic being carried over a VLAN. The software listens for IGMP messages to determine which ports are multicast sources and receives. The software can then limit the amount of IGMP flooding to ports that are known IGMP hosts.
IGMP Snooping is enabled by default in Cisco NX-OS.
What version(s) of IP are supported by MLD?
MLD (Multicast Listener Discovery) only supports IPv6.
What protocol is MLD derived from?
MLDv1 is derived from IGMPv2
MLDv2 is derived from IGMPv3
What IP protocol message types are supported by IGMP and MLD?
IGMP supports IP protocol 2 message types.
MLD supports IP protocol 58 message types.
How is the MLD process started on a device?
The MLD process is started automatically on the device. You cannot enable MLD manually on an interface. MLD is enabled automatically when you perform one of the following configuration tasks on an interface:
- Enable PIM6.
- Statically bind a local multicast group.
- Enable link-local group reports.
What is the default version of MLD used when the process first starts?
By default, the software enables MLDv2 when it starts the MLD process. You can enable MLDv1 on interfaces where you want only its capabilities.
What are the main key differences between MLDv1 and MLDv2?
MLDv2 supports the following additional features:
- support for SSM (Source Specific Multicast) which can be used to build shortest path trees from receiver to source.
- hosts no longer perform report suppression, which means that hosts always send
MLD listener reports when an MLD query message is received.
What IP address range is reserved by the IANA for MLD multicast messages?
MLD utilizes link-local addresses in the range FF02::0/16.
Define the function of an MDT.
A Multicast Distribution Tree is used to represent the path that multicast data takes between the routers that connect sources and receivers.
Describe the function of a MDT source tree.
A MDT source tree is also referred to as a Shortest Path Tree (SPT). A source tree represents the shortest path that the multicast traffic takes through the network from source to receiver.
Define the function of a MDT source tree?
A MDT source tree is also referred to as a Shortest Path Tree (SPT). A source tree represents the shortest path that the multicast traffic takes through the network from source to receiver.
Describe the function of a MDT shared tree.
A MDT shared tree is also referred to as a RP Tree (RPT). A shared tree represents the
shared distribution path that the multicast traffic takes through the network from a shared root or rendezvous point (RP) to each receiver
Describe the function of a MDT bidirectional shared tree.
A bidirectional shared tree represents the shared distribution path that the multicast traffic directly takes through the network from each source to each receiver.
What is the key difference between a MDT shared tree and a bidirectional shared tree?
With a shared tree, multicast data must pass through the RP (Rendezvous Point) before reaching the receiver. Bidirectional shared trees allow data to be directly sent from source to receiver without having to first pass through the RP.
What is the key difference between a MDT shared tree and a bidirectional shared tree?
With a shared tree, multicast data must pass through the RP (Rendezvous Point) before reaching the receiver. Bidirectional shared trees allow data to be directly sent from source to receiver without having to first pass through the RP.
What is the version(s) of IP supported by PIM
Protocol Independent Multicast (PIM) supports the following:
- IPv4 for PIMv2
- IPv6 for PIMv6
Describe the difference between PIM sparse mode and dense mode.
PIM sparse mode: this is a “pull” model where we only forward multicast traffic when requested.
PIM dense mode: this is a “push” model where we flood multicast traffic everywhere and then prune it when it’s not needed.
What PIM mode is supported by Cisco NX-OS?
Only PIM sparse mode is supported by Cisco NX-OS. PIM dense mode is not available on Cisco NX-OS.
What multicast protocols are enabled by default on the Cisco NX-OS system?
Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) are enabled by default on Cisco NX-OS. The PIM sparse mode feature must be enabled manually before configuring multicast.
Describe the role of MSDP in regards to PIM.
Multicast Source Discovery Protocol (MSDP) is used to allow for the discovery of multicast sources in other PIM domains.
Describe the role of the DR in regards to PIM.
Designated Routers (DRs) are used to forward multicast traffic onto a LAN segment. When 2 or more routers are connected to the same segment, a single DR is elected between them.
What version of IP is supported for MSDP?
Only IPv4 is supported for Multicast Source Discovery Protocol.
How can sources be configured as peers if the reside in two separate PIM6 domains?
Multicast Source Discovery Protocol (MSDP) does not support IPv6 (PIM6). However, IPv6 peers can be configured using source-specific multicast (SSM) and multiprotocol BGP (MBGP). This will allow multicast forwarding between multiple PIM6 domains.
What is the default mode of multicast groups for Cisco NX-OS?
By default, Cisco NX-OS uses ASM (Any Source Multicast) as its Multicast Distribution Tree (MDT).
What PIM tree building mode(s) are supported by ASM?
Any Source Multicast (ASM) supports both shared and source trees. Protocol Independent Multicast (PIM) uses:
- shared trees to discover sources and receivers
- source trees to building the shortest path from source to receiver
What PIM tree building mode(s) are supported by Bidir
Bidirectional Shared Trees (Bidir) only supports shared trees.
Define a DF in regards to PIM Bidir.
The Designated Forwarder (DF) is the router that is connected to the receiver on the same LAN segment
What PIM tree building mode(s) are supported by SSM
Source Specific Multicast (SSM) only supports source trees.
What PIM tree building mode(s) require the configuration or discovery of a RP?
Any Source Multicast (ASM) and Bidirectional Shared Trees (Bidir) require the initial configuration of a Rendezvous Point (RP). However, Birdir can continue to carry multicast traffic whether or not the RP is active or connected.
Source Specific Multicast (SSM) does not require the configuration or discovery of an RP.
Describe the function of the PIM hello message and define the following:
1) The multicast group associated with a PIM hello message.
2) The default hello interval.
3) The default hold-time value.
PIM hello messages are used to establish adjacencies and elect a designated router (DR) for the LAN segment.
1) The multicast group (IP) associated with PIM hello messages is 224.0.0.13.
2) The default hello interval is 30 seconds.
3) The default hold-time value is typically 3.5 times the hello interval.
What is the process for electing a DR in PIM?
PIM begins by sending hellos containing information about the router.
1) The highest priority value is considered first.
2) If not, the router with the highest IP address is chosen.
Describe the function of the PIM Join-Prune message are where they are sent to.
Protocol Independent Multicast (PIM) join-prune messages are used by the designated router (DR) to create a tree that connects the source to the receiver. The DR sends the join-prune messages to the RP in the PIM domain if ASM or Bidir are being used. Otherwise, if SSM is the mode being used, the join-prune message will be sent to the source directly.
Describe the function of the PIM register message.
PIM register messages are unicast to the RP by designated routers that are directly connected to multicast sources. The PIM register message has the following functions:
- To notify the RP that a source is actively sending to a multicast group.
- To deliver multicast packets sent by the source to the RP for delivery down the shared tree.
Describe the role of a PIM RP.
A rendezvous point (RP) is a router that you select in a multicast network domain that acts as a shared root for a multicast shared tree.
Describe the role of a PIM RP.
A rendezvous point (RP) is a router that you select in a multicast network domain that acts as a shared root for a multicast shared tree.
What are the 4 types of PIM Rendezvous Points that can exist in a multicast network domain?
You can configure as many RPs as you like, and you can configure them to cover different group ranges:
1) Static RP: manually configured for a multicast group range
2) BSR (Bootstrap Router): ensures that all RPs in the multicast domain have the same cache
3) Auto RP: Cisco proprietary protocol
4) Anycast RP: used for creating redundant and load-balanced RPs using MSDP or PIM
Describe the process behind how multicast forwarding works.
Multicast forwarding uses the Reverse Path Forwarding (RPF) algorithm to determine how to forward traffic from source to receiver. When a receiver joins the multicast group, a path is built from the receiver to the source or the RP (SSM or ASM/Birdir respectively). The path from a source to receiver flows in the reverse direction from the path that was created when the receiver joined the group.
If the packet arrives on the interface leading to the source, the packet is forwarded out each interface in the outgoing interface (OIF) list for the group. Otherwise, the router drops the packet.
What is the default version of HSRP supported by Cisco NX-OS
By default, Cisco NX-OS uses HSRP by default, but an interface can be configured to use HSRP version 2.
What group ranges are supported by HSRP version 1 & version 2 respectively
HSRP version 1 supports group ranges 0 to 255
HSRP version 2 supports group ranges 0 to 4095
What multicast IP address is used by HSRP version 1 to send hello packets?
HSRP version 1 uses multicast IP 224.0.0.2.
What multicast IP address is used by HSRP version 1 to send hello packets?
HSRP version 1 uses multicast IP 224.0.0.2.
What multicast IP address is used by HSRP version 2 to send hello packets?
HSRP version 1 uses multicast IP 224.0.0.102.
What is the purpose of HSRP message digest 5 (MD5) algorithm authentication?
Protects against HSRP-spoofing software.
What port and transport protocol are used by HSRP to send and receive hello packets?
Port 1985 over UDP.
What MAC address range is used by HSRP version 1 for the virtual interface?
0000.0C07.ACxy, where xy is the HSRP group number in hexadecimal format.
What MAC address range is used by HSRP version 2 for the virtual interface?
0000.0C9F.Fxyz, where is the HSRP group number in hexadecimal format.
What is the purpose of the HSRP coup message?
Used by the standby router to take over the role as the active router.
What is the purpose of the HSRP resign message?
Sent by the active router, signifies that the standby router will be taking over responsibilities as the active router. Also sent by the active router when it is shut down.
What many VRRP groups are supported on a single router?
A single router can be joined to 255 VRRP groups?
What is the multicast address used for VRRP advertisements
VRRP uses address 224.0.0.18 for advertisements.
What IP protocol number was assigned by the IANA for VRRP messages?
112
What is the priority of the VRRP master for the group?
255
What is the default priority for a VRRP router?
100
What modes of authentication are supported for VRRP?
No authentication, or plain-text authentication.
What the options for VRRP tracking?
Object tracking, and native interface tracking.
