Chapter 1 - Notes

Question 1

What does OSPF utilizes for neighbor discovery?

Answer 1

hello packets (multicast IPv4 244.0.0.5 or IPv6 FF02::5)

Question 2

What is the OSPF default hello_interval set to?

Answer 2

If a router does not receive a hello packet within 40 seconds (OSPF dead-interval is usually a multiple of the hello interval), the neighbor is removed from the the local neighbor table.

Question 3

Explain the process of OSPF neighbor discovery.

Answer 3

When a neighbor is discovered, the two routers compare information in the hello packet to determine whether the routers have compatible configurations.

The neighbor routers attempt to establish adjacency, which means that the routers synchronize their link-state databases to ensure that they have identical OSPF routing information.

Adjacent routers share link-state advertisements (LSAs) that include information about the operational state of each link, the cost of the link, and any other neighbor information. When all OSPF routers have identical link-state databases, the network is converged.

Each router then uses Dijkstra’s Shortest Path First (SPF) algorithm to build its route table.

Question 4

What are the 6 key differences between OSPFv3 and OSPFv2 protocols?

Answer 4

“1. OSPFv3 provide support for IPv6 routing prefixes and the larger size IPv6 addresses, OSPF Hello address FF02::5.

2. LSAs in OSPFv3 are expressed as prefix and prefix length instead of address and mask.
3. The router ID and area ID are 32 bit numbers with no relationship to IPv6 addresses.
4. OSPFv3 uses link-local IPv6 for neighbor discovery and other features.
5. OSPFv3 can use the IPv6 authentication trailer (RFC 6506) or IPSec (RFC 4552) for authentication. However, neither are these options is supported on Cisco NX-OS.
6. Bidirectional Forwarding Detection (BFD) is supported in OSPFv2 only.”

Question 5

How can you control the flooding rate of LSA updates?

Answer 5

“LSA group pacing feature

LSA group pacing can reduce high CPU or buffer usage by grouping LSAs with similar link-state referesh times to allow OSPF to pack multiple LSAs into an OSPF update message.”

Question 6

OSPF Area

Answer 6

An area is a logical division of routers and links within an OSPF domain that creates subdomains. LSA flooding is contained within an area. The link-state database is limited to links within the area. You can assign an area ID to the interfaces within the defined area either by entering as a 32-bit value numnber or in dotted-decimal notation.

Question 7

What’s an ABR and what does it do?

Answer 7

If you have more than one area, one or more routers become area border routers (ABRs). An ABR connects to both the backbone area and at least one other defined area.

The ABR has a separate link-state database for each area to which it connects. The ABR sends Network Summary (type 3) LSAs from one connected area to the backbone area. The backbone area sends summarized information about one area to another area.

Question 8

What’s an ASBR?

Answer 8

OSPF defines another router type as an autonomous system boundary router (ASBR). This router connects an OSPF area to another autonomous system. An autonomous system is a network controlled by a single technical administration entity. OSPF can redistribute its routing information into another autonomous system or receive redistributed routes from another autonomous system.

Question 9

What’s a stub area?

Answer 9

You can limit the amount of external routing information that floods an area by making it a stub area. A stub area is an area that does not allow AS External (type 5) LSAs. These LSAs are usually flooded throughout the local autonomous system to propagate external route information. Stub areas have the following requirements:

• All routers in the stub area are stub routers.
• No ASBR routers exist in the stub area.
• You cannot configure virtual links in the stub area.

Stub areas use a default route for all traffic that needs to go through the backbone area to the external autonomous system.

Question 10

What’s an NSSA?

Answer 10

There is an option to allow OSPF to import autonomous system external routes within a stub area; this is a not-so-stubby area (NSSA). An NSSA is similar to a stub area, except that an NSSA allows you to import autonomous system (AS) external routes within an NSSA using redistribution. The NSSA ASBR redistributes these routes and generates NSSA External (type 7) LSAs that it floods throughout the NSSA.

You can optionally configure the ABR that connects the NSSA to other areas to translate this NSSA External LSA to AS External (type 5) LSAs. The ABR then floods these AS External LSAs throughout the OSPF autonomous system. Summarization and filtering are supported during the translation. You can, for example, use NSSA to simplify administration if you are connecting a central site using OSPF to a remote site that is using a different routing protocol. Before NSSA, the connection between the corporate site border router and a remote router could not be run as an OSPF stub area because routes for the remote site could not be redistributed into a stub area. With NSSA, you can extend OSPF to cover the remote connection by defining the area between the corporate router and remote router as an NSSA.

Question 11

What’s a virtual link?

Answer 11

All OSPF areas must physically connect to area 0 (backbone area). If one area cannot connect directly to area 0, you need a virtual link. Virtual links allow you to connect an OSPF area ABR to a backbone area ABR when a direct physical connection is not available.

You can also use virtual links to temporarily recover from a partitioned area, which occurs when a link within the area fails, isolating part of the area from reaching the designated ABR to the backbone area.

Question 12

DR and BDR

Answer 12

OSPF routers with the broadcast network type will flood the network with LSAs. The same link-state information needs to be sent from multiple sources. For this type, OSPF uses a single router, the designated router (DR), to control the LSA floods and represent the network to the rest of the OSPF area. If the DR fails, OSPF selects a backup designated router (BDR). If the DR fails, OSPF uses the BDR.

Network types are as follows:

• Point-to-point: A network that exists only between two routers. All neighbors on a point-to-point network establish adjacency, and there is no DR required.
• Broadcast: A network with multiple routers that can communicate over a shared medium that allows broadcast traffic, such as Ethernet. OSPF routers establish a DR and BDR that control LSA flooding on the network. OSPFv2 uses the well-known IPv4 multicast address 224.0.0.5 and the MAC address 0100.5300.0005 to communicate with neighbors, and OSPFv3 uses the well-known IPv6 multicast address FF02:: 5 and the MAC address 0100.5300.0005 to communicate with neighbors.

Question 13

What port does BGP?

Answer 13

TCP port 179

Question 14

BGP peering

Answer 14

BGP peering does not happen dynamically. It has to be configured manually. A BGP peer is a peer that has an active TCP connection to another BGP spearker.

Once peering is established, exchange of the complete BGP routing table occurs with the other peer. After, only incremental updates are sent when a topology change or routing policy change occurs.

Keepalives are sent to maintain the peering in the periods of inactivity between updates. Cisco NX-OS supports the following peer configuration options:

• Individual IPv4 or IPv4 address: BGP establishes a session with the BGP speaker that matches the remote address and AS number.
• IPv4 or IPv6 prefix peers for a single AS number: BGP establishes sessions with BGP speakers that match the prefix and the AS number.
• Dynamic AS number prefix peers: BGP establishes sessions with BGP speakers that match the prefix and an AS number from a list of configured AS numbers.

To establish BGP sessions between peers, BGP must have a router ID, which is sent to BGP peers in the OPEN message when a BGP session is established. If BGP does not have a router ID, it cannot establish any peering sessions with BGP peers. You can configure the router ID. By default, Cisco NX-OS sets the router ID to the IPv4 address of a loopback interface on the router. If no loopback interface is configured on the router, the software chooses the highest IPv4 address configured to a physical interface on the router to represent the BGP router ID. The BGP router ID must be unique to the BGP peers in a network.

Question 15

BGP Path Selection

Answer 15

The best-path algorithm runs each time a path is added or withdrawn for a given network. The best-path algorithm also runs if you change the BGP configuration. BGP selects the best path from the set of valid paths available for a given network. Cisco NX-OS implements the BGP best-path algorithm in the following steps.

1. Comparing pairs of paths
2. Determining the order of comparisons
3. Determining the best-path change suppression

Question 16

BGP Path Selection Step 1 - Comparing Pairs of Paths

Answer 16

This first step in the BGP best-path algorithm compares two paths to determine which path is better. The following sequence describes the basic steps that Cisco NX-OS uses to compare two paths to determine the better path:

1. Valid path. Next-hop reachable? If not, it’s not a valid path.
2. Highest weight.
3. Highest local preference.
4. Locally originated.
5. Shorter AS-path Cisco NX-OS ignores confederation segments and counts AS set as 1.
6. Lower origin. IGP is considered lower than EGP.
7. Lower multi-exit discriminator (MED). In general, MED is only compared if both paths were received from peers in the same AS; otherwise, MED comparison is skipped.
8. Path from external peer is preferred.
9. Paths have different IGP metrics to their next-hop addresses, the path with the lower IGP metric is chosen.
10. If all path parameters in step 1 through step 9 are the same, you can configure the best-path algorithm to compare the router IDs.
11. Path with shorter cluster length is chosen. If no cluster list attribute, cluster length is 0.
12. Path received from peer with lower IP address is chosen. Locally generated paths, have a peer IP address of 0.

Question 17

BGP Path Selection Step 2 - Determining the Order of Comparisons

Answer 17

The second step of the BGP best-path algorithm implementation is to determine the order in which Cisco NX-OS compares the paths:

1. Compares the MED among all paths. Typically, the comparison results in one group being chosen for each neighbor AS. Configuring “bgp bestpath med always” command, forces one group to be chosen.
2. Best path in each group is determined by iterating through all paths in the group. Each path is compared with the temporary best path found so far, and if the new path is better, it becomes the new temporary best path and it is compared to the next path in the group.
3. A set of paths that contain the best path selected from each group in step 2. The overall best path from this set of paths is selected by iterating through them as in step 2.

Question 18

BGP Path Selection Step 3 - Determining the Best-Path Change Suppression

Answer 18

The next part of the implementation is to determine whether Cisco NX-OS will use the new best path or suppress it. The router can continue to use the existing best path if the new one is identical to the old path (if the router ID is the same). Cisco NX-OS continues to use the existing best path to avoid route changes in the network.

You can turn off the suppression feature by configuring the best-path algorithm to compare the router IDs. If you configure this feature, the new best path is always preferred to the existing one.

You cannot suppress the best-path change if any of the following conditions occur:

• The existing best path is no longer valid.
• Either the existing or new best paths were received from internal (or confederation) peers or were locally generated (for example, by redistribution).
• The paths were received from the same peer (the paths have the same router ID).
• The paths have different weights, local preferences, origins, or IGP metrics to their next-hop addresses.
• The paths have different MEDs.

The path selection uses the BGP AS-path attribute. The AS-path attribute includes the list of autonomous system numbers (AS numbers) traversed in the advertised path. If you subdivide your BGP autonomous system into a collection or confederation of autonomous systems, the AS-path contains confederation segments that list these locally defined autonomous systems.

Question 19

Describe the MED comparison process in regards to BGP best path selection.

Answer 19

NX-OS will perform a MED comparison that depends on the AS-path attributes of the two being compared:

a. If a path has no AS-path or the AS-path starts with an AS_SET, the path is internal, and Cisco NX-OS compares the MED to other internal paths.
b. If the AS-path starts with an AS_SEQUENCE, the peer autonomous system is the first AS number in the sequence, and Cisco NX-OS compares the MED to other paths that have the same peer autonomous system.
c. If the AS-path contains only confederation segments or starts with confederation segments followed by an AS_SET, the path is internal and Cisco NX-OS compares the MED to other internal paths.
d. If the AS-path starts with confederation segments followed by an AS_SEQUENCE, the peer autonomous system is the first AS number in the AS_SEQUENCE, and Cisco NX-OS compares the MED to other paths that have the same peer autonomous system.
e. If the nondeterministic MED comparison feature is enabled, the best-path algorithm uses the Cisco IOS style of MED comparison.

Question 20

BFD Configuration Limitations

Answer 20

1. NX-OS supports BFD v1.
2. NX-OS supports IPv4 only.
3. BFD supports single-hop BFD; BFD for BGP supports single-hop EBGP and iBGP peers.
4. BFD depends on Layer 3 adjacency information to discover topology changes, including Layer 2 topology changes. A BFD session on a VLAN interface (SVI) may not be up after the convergence of the Layer 2 topology if no Layer 3 adjacency information is available.
5. For port channels used by BFD, you must enable the Link Aggregation Control Protocol (LACP) on the port channel.
6. HSRP for IPv4 is supported with BFD. HSRP for IPv6 is not supported with BFD.