Routing Protocols Flashcards
1
Q
OSPF Adjacency
A
When OSPF neighbors have synchronised link-state databases.
2
Q
OSPF Hello packets
A
Sent out on each OSPF-enabled interface to discover neighbour routers and for keep-alive
3
Q
What are LSAs and their use?
A
Link-state advertisement, used to build routing tables in an OSPF area.
4
Q
What is the name for OSPFv2 Type 1 LSA?
A
Router LSA
5
Q
Which routers send OSPFv1 Type 1 LSAs?
A
This LSA is sent by every OSPF router in the network.
6
Q
What is the name for OSPFv2 Type 2 LSA?
A
Network LSA
7
Q
What is the name for OSPFv2 Type 3 LSA?
A
Network Summary LSA
8
Q
What is the name for OSPFv2 Type 4 LSA?
A
Autonomous System Boundry Router (ASBR) Summary LSA
9
Q
What is the name for OSPFv2 Type 5 LSA?
A
AS External LSA
10
Q
What is the OSPFv2 Type 6 LSA?
A
Retired - was designed for multicast OSPF
11
Q
What is the name for OSPFv2 Type 7 LSA?
A
Not-so-stubby Area (NSSA) External LSA
12
Q
What are the Opaque LSAs?
A
Type 9 - 11 LSAs.
13
Q
What is LSA Group Pacing?
A
Groups LSAs with similar refresh times and packs into a single OSPF update message.
14
Q
What is an OSPF Area?
A
Logical division of routers and links within an OSPF domain.
15
Q
What is the OSPF Backbone Area?
A
Area with a reserved ID of 0 or 0.0.0.0. which connects to all other areas in the OSPF domain.
16
Q
What is an OSPF ABR?
A
Area Border Router
17
Q
What is an OSPF ASBR?
A
Autonomous System Boundary Router
18
Q
What is a Stub Area?
A
An area that does not allow AS External Type 5 LSAs.
19
Q
What is a Not-so-stubby Area (NSSA)?
A
A stub area that is configured to allow the importing of AS external routes using redistribution.
20
Q
What is an OSPF Virtual Link?
A
Allows an ABR to connect to the backbone area where no physical connection is possible.
21
Q
What is an OSPF Designated Router (DR)
A
Used in a broadcast network to control LSA flooding and represent the network to the rest of the area.
22
Q
OSPF Broadcast network type
A
Multiple routers communicating over a shared medium such as ethernet.
23
Q
OSPF Point-to-point network type
A
Exists only between two routers, such as a single l3 link.
24
Q
How does OSPF Simple password authentication work?
A
Cleartext password sent as part of the OSPFv2 message.
25
How does OSPF MD5 authentication digest work?
An MD5 digest based on the message and the encrypted password is sent with the OSPF update.
26
What is BGP?
Border Gateway Protocol, a dynamic routing protocol that uses a path vector algorithm to exchange routing information between speakers.
27
BGP AS (Autonomous System)
A routing domain with one or more interior gateway protocols (IGPS) and a consistent set of routing policies.
28
What are BGP peers?
BGP peers are speakers that have been configured to communicate and have an active TCP connection with each other.
29
What are the three BGP Peer configuration options supported by NXOS?
IP address and single AS number
Prefix and single AS number
Prefix and multiple AS numbers
30
What is the BGP Router ID?
32-bit value often represented by an IPv4 address that must be unique to the BGP peers on the network.
31
BGP Path Selection Algorithm
Runs each time a path is added or removed for a given network, or if BGP configuration is changed.
Steps:
1 - Valid path (e.g. next-hop is reachable)
2 - Highest weight
3 - Highest local preference
4 - Locally originated
5 - Shortest AS path
6 - Lowest origin type (e.g. IGP over EGP)
7 - Lowest Multi-exit discriminator (MED)
8 - eBGP or iBGP
9 - Lowest IGP metric
10 - Last selection in best-path algorithm
11 - Shortest cluster length
12 - Lowest peer IP address
32
Multiprotocol BGP (MBGP)
Supports multiple address families, e.g. one set of routes for IPv4 unicast, one for IPv4 multicast and one for IPv6 unicast
33
Configuring OSPFv2
```
feature ospf (requires Enterprise Services Package license)
!
router ospf 1
router-id 1.1.1.1
log-adjacency-changes
auto-cost reference-bandwidth 100 Gbps
!
interface vlan 30,40
ip router ospf 1 area 1
!
interface ethernet 2/1
ip router ospf 1 area 0
ip ospf authentication message-digest
ip ospf message-digest-key 1 md5 cisco
```
34
Configuring OSPFv3
```
feature ospfv3
!
router ospfv3 1
router-id 1.1.1.1
log-adjacency-changes
auto-cost ref-bandwidth 100 Gbps (sets the reference bandwidth to calculate the default metrics for an interface, default is 40 Gbps so must be set higher if there are interfaces with bandwidth greater than that)
!
interface vlan 30, 40
ip router ospfv3 1 area 1
!
interface ethernet 2/1
ip router ospfv3 1 area 0
```
35
OSPF and BFD
Detects an outage between two adjacent devices.
Layer 1 mechanisms are unreliable in detecting the loss of a link or neighbouring device, e.g. WAN or switch between peers goes down.
L3 protocols use hello mechanism to detect the loss of a neighbor to acheive fast convergence.
BFD provides sub second failure detection between two adjacent devices.
36
BFD Neighbor Session
OSPF discovers a neighbour.
OSPF sends a request to the local BFD process to initiaite a BFD neighbor session with the OSPF neighbor router.
The BFD neighbor session with the OSPF neighbor router is established.
37
BFD OSPF Convergence
A failure occurs in the network
The BFD neighbor session with the OSPF neighbor router is torn down.
BFD notifies the local OSPF process that the BFD neighbor is no longer reachable.
The local OSPF process tears down the OSPF neighbor relationship.
38
4 steps for configuring BFD
Disable address identical IDS check
Enable BFD feature
Disable ICMP redirects on any interfaces that use BFD
Enable BFD for the required L3 protocol.
39
Configuration example BFD
```
no hardware ip verify address (disable IDS check)
!
feature bfd
!
interface ethernet 1/2-4
no ip redirects (disable ICMP redirects)
!
router ospf 1
bfd
!
interface vlan 10
no ip redirects
no ipv6 redirects
hsrp bfd
```
40
MP-BGP
MP-BGP supports multiple address families carryingg different sets of routes
BGP can carry one set of routes for a variety of protocols (ipv4 unicast, ipv4 multicast, ipv6 unicast, ipv6 multicast,etc)
MP-BGP maintains a separate Routingg Information Base (RIB) for each configured address family
41
MP-BGP Configuration Example
router bgp 62000
neighbor 192.168.1.2 remote-as 22003
address-family ipv4 multicast
address-family ipv6 multicast
42
MP-BGP template example
```
router bgp 61000
template peer-policy BasePolicy
maximum-prefix 20
!
neighbor 192.168.1.1 remote-as 61001
address-family ipv4 unicast
inherit peer-policy BasePolicy
```
43
OSPF hello_interval
How often hello packets are sent, default 10 seconds
44
OSPF dead-interval
If a hello packet from a neighbor is not recieved in this interval, it is removed from the local neighbor table. Default is 40 seconds.
45
What does a router do when an LSA is recieved?
Floods it out of all OSPF enabled interfaces, ensuring all routers in the network have identical information.
46
OSPF Convergence
When all routers in the network have identical link-state databases.
47
LSA MaxAge
Interval after which an LSA is removed from the link-state database if no update is recieved.
48
What does the OSPFv2 Type 1 LSA include?
The state and cost of all links and a list of all OSPFv2 neighbours on the link.
49
Does the OSPFv2 Type 1 LSA trigger an SPF recalculation?
Yes
50
What is the flooding behaviour for OSPFv2 Type 1 LSAs?
Flooded to the local local OSPFv2 Area
51
Which routers send OSPFv2 Type 2 LSAs?
Sent by the designated router (DR)
52
What does the OSPFv2 Type 2 LSA include?
It lists all routers in the multi-access network.
53
Does the OSPFv2 Type 2 LSA trigger an SPF recalculation?
Yes
54
Which routers send OSPFv2 Type 3 LSAs?
Sent by the area border router to an external area.
55
What does the OSPFv2 Type 3 LSA include?
The link cost from the area border router to the local destination.
56
Which routers send OSPFv2 Type 4 LSAs?
Sent by the area border router to an external area.
57
What does the OSPFv2 Type 4 LSA include?
This LSA advertises the link cost to the ASBR only.
58
Which routers send OSPFv2 Type 5 LSAs?
Autonomous System Boundry Routers (ASBR)
59
What does the OPSFv2 Type 5 LSA include?
The link cost to an external autonomous system destination.
60
What is the flooding behaviour for OSPFv2 Type 5 LSAs?
Flooded throughout the autonomous system.
61
Which routers send OSPFv2 Type 7 LSAs?
The Autonomous System Boundry Router (ASBR) in a Not-so-stubby Area (NSSA).
62
What does the OSPFv2 Type 7 LSA include?
The link cost to an external autonomous system destination.
63
What is the flooding behaviour for OSPFv2 Type 7 LSAs?
Flooded only within the local Not-so-stubby Area (NSSA).
64
What is the flooding behaviour for OSPFv2 Type 9 LSAs?
Flooded to local network.
65
What is the flooding behaviour for OSPFv2 Type 10 LSAs?
Flooded to local area.
66
What is the flooding behaviour for OSPFv2 Type 11 LSAs?
Type 11 is flooded to local autonomous system
67
What are the Opaque LSAs used for?
Advertising application-specific information that may be used by OSPF or other applications.
68
What does an Opaque LSA consist of?
Standard LSA header followed by application specific information.
69
What does OSPFv2 use Opaque LSAs for?
Supporting the OSPFv2 graceful restart capability.
70
How often are LSAs flooded to the network?
Every 30 minutes
71
What is the benefit of LSA group pacing?
Controls the flooding of LSAs to reduce high CPU or buffer usage.
72
How do OSPF areas affect LSA flooding?
LSA flooding is contained within an area.
73
What is the format for an OSPF area ID?
32 bit value in numerical or dotted-decimal notation.
74
What areas does an ABR connect to?
Connects to both the backbone area and at least one other area.
75
What link-state databases does an ABR keep?
Maintains a separate link-state database for each area it connects to.
76
What LSAs does an ABR send?
Sends Network Summary (type 3) LSAs from one area to the backbone area.
77
What is an Autonomous System?
A network or collection of networks controlled by a single technical administration entity or organisation.
78
What does an OSPF ASBR connect to?
Connects an OSPF area to another autonomous system.
79
How does a stub area route to external autonomous systems?
Using a default route via the backbone area.
80
What is the requirement for all routers in a stub area?
All routers in the area are stub routers
81
Can an ASBR exist in a stub area?
No
82
Can virtual links be configured in a stub area?
No
83
How do ASBRs advertise AS external routes in a NSSA?
By generating Type 7 LSAs and flooding into the NSSA.
84
Does OSPF flood external routes from other areas into an NSSA?
No
85
How would OSPF flood external routes from an NSSA into other areas?
By configuring the NSSA ABR to convert type 7 LSAs into type 5 LSAs.
86
What is an alternative use for OSPF Virtual Links?
Recovery of a partitioned network, where part of the area cannot reach the designated ABR to backbone area.
87
What is an OSPF BDR?
Backup designated router, takes over if the designated router fails.
88
How does a receiving router validate an OSPF message with MD5 message digest?
By locally calculating the digest of the message and configured password against the digest recieved with the message.
89
What is the purpose of the sequence number included in MD5 authenticated OSPFv2 messages?
To ensure no message is replayed in the network.
90
Does NXOS support IPSec options for OSPFv3 authentication?
No
91
What is included in the routing information exchanged by BGP?
The route prefix for a destination, path of autonomous systems, and additional path attributes.
92
How can BGP path selection be influenced?
Path selection can be influenced by configuring policies to alter the well-known mandatory, well-known discretionary, and optional transitive attributes.
93
What is ECMP?
Equal-cost multipath, multiple 'best paths' that tie for top place in routing metric calculations can be used to load balance traffic.
94
What port does BGP use to create sessions with neighbours?
TCP 179
95
What happens once a TCP connection is established between BGP peers?
They exchange complete BGP routing tables with each other.
96
What would cause an incremental BGP update between peers?
A topology change in the network or a routing policy change.
97
BGP keepalives
Messages exchanged between BGP peers during periods of inactivity between routing updates.
98
BGP Hold time
The maximum time limit betwen receiving consecutive BGP update or keepalive messages from a peer.
99
What is the default BGP Router ID?
The IPv4 address of a loopback interface on the router, or if none are configured the highest IPv4 address assigned to a physical interface.
100
When OSPF starts neighbor negotiations, which 5 settings must match between OSPF neighbours for them to be able to establish adjacencies?
```
Area ID
Authentication
Hello/Dead intervals
Stub flag
MTU
```
101
How is the OSPF DR selected?
The router with the highest priority will be selected as DR. If multiple routers exist with the same priority, the one with the highest router ID will be selected
102
How does OSPFv3 expand on OSPFv2?
Provides support for IPv6 routing prefixes and the large size IPv6 addresses.
103
How are OSPFv3 LSAs expressed compared to OSPFv2 LSAs?
Prefix and prefix length instead of address and mask.
104
What does OSPFv3 use link-local addresses for?
Neighbor discovery and other features.
105
Is BFD supported in both OSPFv2 and OSPFv3?
No, only OSPFv2.
