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Cisco CCNP ENCOR > BGP Theory > Flashcards

BGP Theory Flashcards

1
Q

What is BGP?

A

Border Gateway Protocol - a non-proprietary path vector routing protocol.

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2
Q

What is a BGP Autonomous System?

A

A group of multiple public subnets.

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3
Q

What is meant by a well-known BGP attribute?

A

This means that it is an industry standard according to RFC 4271 and that all routers that run BGP will have these attributes.

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4
Q

What are the two well-known, discretionary BGP path attributes?

A

1.) Local Preference
2.) Atomic Aggregate

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5
Q

What are the three well-known, mandatory BGP path attributes?

A

1.) AS-Path
2.) Next-Hop
3.) Origin

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6
Q

What is meant by a BGP attribute that is transitive?

A

It means that the router will pass the attribute along to another router if received, even if that router does not support the attribute.

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7
Q

What is meant by a BGP attribute that is optional?

A

The attribute is not compliant with RFC 4271 industry standard.

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8
Q

What is an NLRI?

A

Network Layer Reachability Information - in BGP this is a routing update that contains the network prefix, prefix length, and BGP PAs for a given route.

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9
Q

Which PA is responsible for loop prevention?

A

AS-Path - it includes a complete list of all the ASNs that a prefix advertisement has traversed from its source AS.

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10
Q

What is MP-BGP?

A

Multiprotocol BGP - in RFC 2858, Address Families for IPv4, IPv6, Multicast, and Unicast were added making BGP “multi-protocol”.

NOTE that every protocol maintains its own database and configuration for each protocol in BGP, including unicast and multicast.

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11
Q

What characteristics make BGP unique?

A
  • Does not use hello packets
  • Uses TCP (source port 179) for inter-router communication
  • Can form peering adjacencies across multiple hops (multiple networks away)
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12
Q

What makes up an iBGP session?

A
  • Routers in the same AS or same BGP confederation
  • Routes (prefixes) default Administrative Distance is 200
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13
Q

What makes up an eBGP session?

A
  • Routers in different AS
  • Routes (prefixes) default Administrative Distance is 20
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14
Q

What are four key differences between iBGP and eBGP sessions?

A
  • eBGP TTL is set to “0” to prevent multi-hop adjacencies (iBGP is 255)
  • eBGP advertising router modifies the BGP next-hop address to be the source router (iBGP cannot)
  • eBGP advertising router prepends its AS number to the AS-Path attribute variable (iBGP does not)
  • eBGP discards NLRIs that fail the AS-Path loop check
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15
Q

What types of simple messages do BGP routers send to each other?

A

Open, Update, Notification, Keepalive

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16
Q

What are the 6 progressive states of BGP peering?

A

Idle
Connect
Active
OpenSent
OpenConfirm
Established

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17
Q

Briefly describe what is happening in each of the BGP peering states

A

Idle - nothing is going on. Could mean a neighbor relationship has failed and returned to Idle

Connect - a TCP three way handshake has been sent.

Active - the BGP router is actively trying to create a neighbor relationship

OpenSent - an “Open” message has been sent and awaiting a confirmation message

OpenConfirm - a BGP neighbor has sent back an OpenConfirm message

Established - a BGP neighbor relationship has been formed.

NOTE: You will not see OpenSent, Open Confirm, or Established when trying to view BGP neighbor states.

18
Q

What is the Cisco BGP Best Path Algorithm in order of attribute preference?

A
  1. Weight
  2. Local Preference
  3. Locally Originated
  4. Shortest AS-Path
  5. Origin Type (code “i”, “e”, or “?”)
  6. Lowest Multi-Exit Discriminator (MED/Metric)
  7. eBGP over iBGP
  8. Oldest Route
  9. Lower Peer Router-ID (RID)
  10. Lowest Cluster List Length
  11. Lowest Next Hop Address
19
Q

What are the primary characteristics of the Weight Path Attribute?

A
  • Cisco proprietary (Optional Non-transitive)
  • Greater Weight is preferred
  • Value is 0 - 65,535
  • Weight is only set on the router where the NLRI is received
  • Weight is not re-advertised from the router where it was set
20
Q

What are the primary characteristics of the Local Preference Path Attribute?

A
  • Well-Known Discretionary attribute
  • Only advertised within an AS (iBGP)
  • Value is 0 o 4,294,967,295 (Default 100)
  • Higher value preferred
  • Indicates preference for exiting the AS to destination network
21
Q

Why would a prefix with a Local Preference of 100 be less desirable than a prefix with no value (null)?

A

The default local preference is 100 and Null is a value that has not been set yet. So these are considered equal and the Best Path Selection evaluates the next Path Attribute.

22
Q

What are the primary characteristics of the Locally Originated Path Attribute?

A
  • Well-Known, Mandatory Path Attribute

Preference is:
1.) Locally advertised network
2.) Locally aggregated networks
3.) Routes received by BGP peers

23
Q

What are the primary characteristics of the Accumulated Interior Gateway Protocol Path Attribute?

A
  • Optional Non-Transitive PA
  • Only advertised within the AS
  • Is a conceptual path metric based on IGP protocols that have been redistributed into an AS
  • Path with an AIGP value is preferred over no value
24
Q

What are the primary characteristics of the “Shortest AS Path” Path Attribute?

A
  • Well-Known, Mandatory
  • AS Number is prepended along the path
  • fewest number of AS numbers is most desirable
25
Q

What are the primary characteristics of the Origin Type Path Attribute?

A
  • Well-Known Mandatory

Preference:
1.) IGP Origin (marked with “i”)
2.) EGP Origin (marked with “?”)
3.) Incomplete Origin

26
Q

What are the primary characteristics of the Multi-Exit Discriminator (MED) Path Attribute?

A
  • Optional, Non-Transitive
  • 32-bit Value (0 - 4,294,967,295)
  • Set equal to the IGP metric if redistribution occurs
  • Should stay within an AS
  • Routes with NO METRIC are preferred, then lowest metric
27
Q

What are the primary characteristics of the “eBGP over iBGP” Path Attribute?

A

Preference:
1.) eBGP Peers
2.) Confederation member AS peers
3.) iBGP Peers

28
Q

What are the primary characteristics of the Lowest IGP Metric Path Attribute?

A
  • Lowest IGP cost to next BGP hop
29
Q

When redistributing IGP routes into BGP, how can you tell from the BGP table which routes were redistributed? What Path Attribute does this reflect?

A

Redistributed routes will show a “?” at the end of the AS Path, BGP routes will show an “i”. BGP routes will be preferred over the “?” routes.

This is the Origin Path Attribute

30
Q

When looking at a BGP routes table, what does an asterisk (*) in front of the route mean?

A

The route is valid

31
Q

When looking at a BGP routes table, what does a sideways carat (>) in front of the route mean?

A

This is the best route for this network; this route will be inserted to the local routing table.

32
Q

Define ASBR

A

Autonomous System Boundary Router - a type of router that runs multiple interior gateway protocols and serves as a gateway to other autonomous systems operating with interior gateway protocols.

33
Q

What general steps are needed to create route filters for BGP route distribution?

A
  1. Create a Access-list to “permit” or capture the network that needs to be filtered
  2. Create a route-map and ‘match ip address’ of the ACL created
  3. Under BGP config mode, use the redistribute command to target the routing protocol and route-map
34
Q

In the BGP process, what are the 9 steps a route will take from when it is received to when it is advertised back out?

A
  1. Get route from peers
  2. Place route into Adj-RIB-In table
  3. Process Inbound Route Policies (Route-Maps, Dist-lists, etc)
  4. Insert to Loc-RIB table
    4a. BGP Router config network statement
    4b. Network statements checked against the RIB
    4c. Routes are placed into the Loc-RIB table
  5. Check the route for Next-hop and validity
    6a. Identify BGP Best Path
    6b. Insert to Global RIB
  6. Apply Outbound Route Policies
  7. Insert route to Adj-RIB-Out
  8. Advertise to peers
35
Q

In the BGP process, how is a network statement handled before it is placed into the BGP Database (Loc-RIB)?

A

The route is checked against the Global-RIB (routing table) to see if it is valid.

36
Q

In the BGP process, when the network command is issued and a route is identified as valid, what fields are set in the Loc-RIB for a connected network compared to a static route or a route from a routing protocol?

A

Connected network: The next-hop BGP attribute is set to 0.0.0.0, the BGP origin attribute is set to “i” (IGP), and the BGP weight is set to 32,768.

Static route or routing protocol: The next-hop BGP attribute is set to the next-hop IP address in the RIB, the BGP origin attribute is set to “i” (IGP), the BGP weight is set to 32,768, and the MED is set to the IGP metric.

37
Q

When working with address families in BGP, why does the network statement and/or redistribute statement need to be specified in the address-family configuration mode?

A

If using VRFs, the routes would not exist in the Global-RIB which is where the BGP process would check against the route.

Router(config-router)# address-family ipv4 unicast vrf VRF-A

38
Q

What are the three rules of iBGP Route Reflectors?

A
  1. If a route reflector receives an (NLRI) from a non-RR client, it advertises the NLRI to an RR client.
  2. If an RR receives an NLRI from an RR client, it advertises the NLRI to both RR clients and non-RR clients.
  3. If an RR receives a route from an external BGP (eBGP) peer, it advertises the route to both RR clients and non-RR clients.
39
Q

What are BGP confederations and what is special about their autonomous system numbers?

A

BGP confederations are when a large Autonomous system is broken up into smaller private autonomous systems.

They use the private autonomous system numbers as specified in RFC 5065 (AS numbers 64,512 to 65,535 and 4,200,000,000 to 4,294,967,295)

40
Q

What is a Member AS?

A

A private Autonomous System number that is part of a BGP confederation.

41
Q
A

Cisco CCNP ENCOR (22 decks)

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