Ch 11: BGP Flashcards
Which of the following autonomous systems are private? (Choose two.)
a. 64,512–65,535
b. 65,000–65,535
c. 4,200,000,000–4,294,967,294
d. 4,265,000–4,265,535,016
A and C. ASNs 64,512–65,535 are private ASNs within the 16-bit ASN range, and 4,200,000,000–4,294,967,294 are private ASNs within the extended 32-bit range.
Which BGP attribute must be recognized by all BGP implementations and advertised to other autonomous systems?
- Well-known mandatory
- Well-known discretionary
- Optional transitive
- Optional non-transitive
- Well-known mandatory attributes must be recognized by all BGP implementations and included with every prefix advertisement.
T/F: BGP supports dynamic neighbor discovery by both routers.
False.
BGP neighbors are statically defined. There is a feature that supports dynamic discovery by one peer (though it is beyond the scope of this book), but the other router must still statically configure the remote BGP peer.
T/F: A BGP session is always one hop away from a neighbor.
False.
BGP supports multi-hop neighbor adjacency.
T/F: The IPv4 address family must be initialized to establish a BGP session with a peer using IPv4 addressing.
False.
The IPv4 address family is automatically initialized by default on IOS-based devices.
Which command is used to view the BGP neighbors and their hello intervals?
- show bgp neighbors
- show bgp afi safi neighbors
- show bgp afi safi summary
- show afi bgp interface brief
B. The command show bgp afi safi neighbors displays all the neighbors, their capabilities, session timers, and other useful troubleshooting information.
How many tables does BGP use for storing prefixes?
a. One
b. Two
c. Three
d. Four
C.
BGP uses three tables (Adj-RIB-In, Loc-RIB, and Adj-RIB-Out) for storing BGP prefixes.
T/F: BGP advertises all its paths for every prefix so that every neighbor can build its own topology table.
False.
BGP advertises only the path that the local router deems is the best path.
Which BGP command advertises a summary route to prevent link-flap processing by downstream BGP routers?
- aggregate-address network subnet-mask as-set
- aggregate-address network subnet-mask summary-only
- summary-address network subnet-mask
- summary-address network mask subnet-mask
2.
The command aggregate-address network subnet-mask summary-only creates a BGP aggregate and suppresses the component routes.
T/F: The IPv6 address family must be initialized to establish a BGP session with a peer using IPv6 addressing.
True.
The IPv6 address family does not exist by default on IOS-based devices.
What is a BGP AS?
From the perspective of BGP, an autonomous system (AS) is a collection of routers under a single organization’s control, using one or more IGPs to route packets internally within the AS..
T/F: An IGP is required in a BGP AS to route internally.
An IGP is not required within an AS; an AS could use BGP as the only routing protocol.
How many AS numbers are available?
2^32 = 4G
ASNs were originally 2 bytes (16-bit range), which made 65,535 ASNs possible. Due to exhaustion, RFC 4893 expanded the ASN field to accommodate 4 bytes (32-bit range). This allows for 4,294,967,295 unique ASNs, providing quite an increase from the original 65,535 ASNs.
What are the two reserved ASN ranges for private networks?
- 64,512–65,535 are private ASNs in the 16-bit ASN range
- 4,200,000,000–4,294,967,294 are private ASNs within the extended 32-bit range.
What entity is responsible for assigning ASNs?
The Internet Assigned Numbers Authority (IANA) is responsible for assigning all public ASNs to ensure that they are globally unique. IANA requires the following items when requesting a public ASN:
- Proof of a publicly allocated network range
- Proof that Internet connectivity is provided through multiple connections
- Need for a unique routing policy from providers
What are PAs?
BGP uses path attributes (PAs) associated with each network path. The PAs provide BGP with granularity and control of routing policies within BGP. The BGP prefix PAs are classified as follows:
- Well-known mandatory
- Well-known discretionary
- Optional transitive
- Optional non-transitive
T/F: Prefix advertisements must include mandatory well-known attributes but discretionary well-known attributes are optionally included.
True.
well-known attributes must be recognized by all BGP implementations.
Well-known mandatory attributes must be included with every prefix advertisement; well-known discretionary attributes may or may not be included with a prefix advertisement.
What is NLRI?
In BGP, the Network Layer Reachability Information (NLRI) is a routing update that consists of the network prefix, prefix length, and any BGP PAs for the specific route.
What are the two types of optional PAs? Do they need to be recognized by all BGP implementations?
There are four categories of path attributes:
-
Well-known mandatory
- Must be recognized by all BGP routers and must be included in every update message. Routing information errors occur without this attribute.
-
Well-known discretionary
- Can be recognized by all BGP routers; can be included in every update message as needed.
-
Optional transitive
- Transitive attribute between ASs. A BGP router not supporting this attribute can still receive routes with this attribute and advertise them to other peers.
-
Optional non-transitive
- If a BGP router does not support this attribute, it will not advertise routes with this attribute.
Optional attributes do not have to be recognized by all BGP implementations. Optional attributes can be set so that they are transitive and stay with the route advertisement from AS to AS. Other PAs are non-transitive and cannot be shared from AS to AS.
T/F: BGP is a path vector routing protocol.
True.
BGP is a path vector routing protocol and does not contain a complete topology of the network, as link-state routing protocols do. BGP behaves like distance vector protocols, ensuring that a path is loop free.
What is AS_Path?
The BGP attribute AS_Path is a well-known mandatory attribute and includes a complete list of all the ASNs that the prefix advertisement has traversed from its source AS.
AS_Path is used as a loop-prevention mechanism in BGP. If a BGP router receives a prefix advertisement with its AS listed in the AS_Path attribute, it discards the prefix because the router thinks the advertisement forms a loop.
Figure 11-1 shows the loop-prevention mechanism:
- AS 100 advertises the 172.16.1.0/24 prefix to AS 200.
- AS 200 advertises the prefix to AS 400, which then advertises the prefix to AS 300.
- AS 300 advertises the prefix back to AS 100 with an AS_Path of 300 400 200 100. AS 100 sees itself in the AS_Path variable and discards the prefix.
What is MP-BGP? What is the AFI? and the SAFI?
Originally, BGP was intended for routing of IPv4 prefixes between organizations, but RFC 2858 added Multi-Protocol BGP (MP-BGP) capability by adding an extension called the address family identifier (AFI).
An address family identifier (AFI) correlates to a specific network protocol, such as IPv4 or IPv6, and additional granularity is provided through a subsequent address-family identifier (SAFI) such as unicast or multicast.
What are the MP_REACH_NLRI and MP_UNREACH_NLRI attributes used for? Where are they carried?
These attributes are carried inside BGP update messages and are used to carry network reachability information for different address families.
NLRI = Network Layer Reachability Information
What is the difference between Multiprotocol BGP, MP-BGP, and MBGP?
No difference, just different names for the same thing.
Some network engineers refer to Multiprotocol BGP as MP-BGP, and other network engineers use the term MBGP. Both terms refer to the same thing.
T/F: BGP maintains a single database that contains the AF and Sub-AF information for each protocol.
False.
Every address family maintains a separate database and configuration for each protocol (address family + subsequent-address family) in BGP.
This allows for a routing policy in one address family to be different from a routing policy in a different address family, even though the router uses the same BGP session with the other router. BGP includes an AFI and SAFI with every route advertisement to differentiate between the AFI and SAFI databases.
What is the period that BGP sends out Hello packets to discover new neighbors?
Never.
BGP does not use hello packets to discover neighbors, as do IGP protocols, and it cannot discover neighbors dynamically.
By what are BGP neighbors defined?
BGP neighbors are defined by IP address.
What port does BGP use for inter-router communication?
BGP uses TCP port 179 to communicate with other routers.
What mechanism in BGP allow for handling of fragmentation, sequencing, and reliability?
None.
BGP relies on TCP for this. TCP allows for handling of fragmentation, sequencing, and reliability (acknowledgment and retransmission) of communication packets.
What allows BGP to form adjacenies that are multiple hops away? Why can’t IGPs do this?
IGPs follow the physical topology because the sessions are formed with hellos that cannot cross network boundaries (that is, single hop only).
BGP uses TCP, which is capable of crossing network boundaries (that is, multi-hop capable). While BGP can form neighbor adjacencies that are directly connected, it can also form adjacencies that are multiple hops away.
What is a BGP session?
A BGP session refers to the established adjacency between two BGP routers.
T/F: Multi-hop sessions require that the router must use an manually defined static route that is installed in the RIB in order to establish the TCP session with the remote endpoint.
almost right…
Multi-hop sessions require that the router use an underlying route installed in the RIB (static or from any routing protocol) to establish the TCP session with the remote endpoint.
T/F: R3 is located in between R2 and R4. If R2 forms a neighborship with R4 through R3, then R3 will be unaware that R2 and R4 have established a neighborship.
True.
In Figure 11-2, R1 is able to establish a direct BGP session with R2. In addition, R2 is able to establish a BGP session with R4, even though it passes through R3.
R1 and R2 use a directly connected route to locate each other. R2 uses a static route to reach the 10.34.1.0/24 network, and R4 has a static route to reach the 10.23.1.0/24 network.
R3 is unaware that R2 and R4 have established a BGP session even though the packets flow through R3.
T/F: BGP neighbors connected to the same network use the ARP table to locate the IP address of the peer.
True.
BGP neighbors connected to the same network use the ARP table to locate the IP address of the peer.
T/F: A default route is sufficient to establish a multi-hop BGP session.
False.
A default route is not sufficient to establish a multi-hop BGP session.
T/F: BGP can be thought of as a control plane routing protocol or as an application because it allows for the exchange of routes with a peer that is multiple hops away.
True.
BGP can be thought of as a control plane routing protocol or as an application because it allows for the exchange of routes with a peer that is multiple hops away.
BGP routers do not have to be in the data plane (path) to exchange prefixes, but all routers in the data path need to know all the routes that will be forwarded through them.
What are the two types of BGP sessions?
BGP sessions are categorized into two types:
- Internal BGP (iBGP): Sessions established with an iBGP router that are in the same AS or that participate in the same BGP confederation. iBGP prefixes are assigned an administrative distance (AD) of 200 upon installation in the router’s RIB.
- External BGP (eBGP): Sessions established with a BGP router that are in a different AS. eBGP prefixes are assigned an AD of 20 upon installation in the router’s RIB.
What is the AD for an iBGP prefix upon installation in the router’s RIB?
iBGP prefixes are assigned an administrative distance (AD) of 200 upon installation in the router’s RIB.
What is the AD of an eBGP route in the RIB?
eBGP prefixes are assigned an AD of 20 upon installation in the router’s RIB.
What is the TTL on eBGP and iBGP packets and why does this matter?
Time-to-live (TTL) on eBGP packets is set to 1 by default. eBGP packets drop in transit if a multi-hop BGP session is attempted.
TTL on iBGP packets is set to 255, which allows for multi-hop sessions.
T/F: The advertising router appends its ASN to the existing AS_Path variable.
False.
The advertising router prepends its ASN to the existing AS_Path variable.
How does BGP prevent loops?
The receiving router verifies that the AS_Path variable does not contain an ASN that matches the local router’s ASN.
BGP discards the NLRI (Network Layer Reachability Information) if it fails the AS_Path loop prevention check.
T/F: The configurations for eBGP and iBGP sessions are fundamentally the same except that the ASN in the remote-as statement is different from the ASN defined in the BGP process.
True.
- Can R2 form an adjacency directly with R4?
- If yes, will traffic from AS 65100 or AS 65300 be able to reach the other?
- What needs to happen for both eBGP and iBGP to function in this network diagram?
- What is the formula to determine the number of links requred to form a full mesh network?
- Yes. R2 can form an iBGP session directly with R4.
- No. While the adjacency is formed, R3 would not know where to route traffic from AS 65100 or AS 65300 when traffic from either AS reaches R3, because R3 would not have the appropriate route forwarding information for the destination traffic.
- Figure 11-5 shows the eBGP and iBGP sessions that would be needed between the routers to allow connectivity between AS 65100 and AS 65300. Notice that AS 65200 R2 establishes an iBGP session with R4 to overcome the loop-prevention behavior of iBGP learned routes.
- n(n-1) / 2
How many message types does BGP use?
Four.
BGP communication uses four message types, as shown in Table 11-2.
Match these 3 lists.
- Type 1
- Type 2
- Type 3
- Type 4
- NOTIFICATION
- OPEN
- KEEPALIVE
- UPDATE
- Advertises, updates, or withdraws routes
- Sets up and establishes BGP adjacency
- Ensures that BGP neighbors are still alive
- Indicates an error condition to a BGP neighbor
Which BGP message is used to establish a BGP adjacency?
An OPEN message is used to establish a BGP adjacency.
T/F: Both sides negotiate session capabilities before BGP peering is established.
True.
Both sides negotiate session capabilities before BGP peering is established.
What is the BGP FSM? What are the possible states?
- Idle
- Connect
- Active
- OpenSent
- OpenConfirm
- Established
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Figure 11-6 shows the BGP FSM and the states, listed in the order used in establishing a BGP session.
What command will display the active TCP sessions?
The command show tcp brief displays the active TCP sessions between a router.
Notice in the attached figure that the TCP source port is 179 and the destination port is 59884 on R1; the ports are opposite on R2.
This is because the router initiating the request uses a dynamic source port, but the destination port is always 179.
What command will verify the BGP session?
The BGP session is verified with the command show bgp afi safi summary. (show bgp ip4 unicast summary)
Earlier commands like show ip bgp summary came out before MBGP and do not provide a structure for the current multiprotocol capabilities within BGP. Using the AFI and SAFI syntax ensures consistency for the commands, regardless of information exchanged by BGP. This will become more apparent as engineers work with address families like IPv6, VPNv4, and VPNv6.
Example 11-3 shows the IPv4 BGP unicast summary. Notice that the BGP RID and table version are the first components shown. The Up/Down column indicates that the BGP session is up for over 5 minutes.
What is the command that displays the contents of the Adj-RIB-Out table for a neighbor?
The command:
- show bgp afi safi neighbor ip-address advertised-routes
displays the contents of the Adj-RIB-Out table for a neighbor.
Example 11-8 shows the Adj-RIB-Out entries specific to each neighbor. Notice that the next-hop address reflects the local router and will be changed as the route advertises to the peer.
What command can also be used to verify the exchange of NLRIs between nodes?
The show bgp ipv4 unicast summary command can also be used to verify the exchange of NLRIs between nodes, as shown in Example 11-9.
What command shows BGP routes in the global IP routing table (RIB)?
The BGP routes in the global IP routing table (RIB) are displayed with the command show ip route bgp.
Example 11-10 shows these commands in the sample topology. The prefixes are from an eBGP session and have an AD of 20, and no metric is present.
What do afi and safi stand for in BGP?
When the BGP peers exchange the Multiprotocol extension capability, they exchange AFI and SAFI numbers and thus identify what the other BGP speaker is capable of.
IPv6 in BGP is implementated via MBGP, as is MPLS and VPNs through two new attributes:
- MP_UNREACH_NLRI
- MP_REACH_NLRI
The first two values in these two attributes contain the Address Family Identifier (AFI) and the Subsequent Address Family Identifier (SAFI).
For example:
- If BGP is carrying IPv4 traffic, AFI equals 1, SAFI equals 1 for Unicast, or SAFI equals 4 for MPLS.
- If BGP is carrying IPv6 traffic, AFI equals 2, SAFI equals 1 for Unicast, and SAFI equals 2 for multicast.
T/F: Every route in the Loc-RIB table is advertised to a BGP peer.
False:
Not every route in the Loc-RIB table is advertised to a BGP peer.
All routes in the Loc-RIB table use the following process for advertisement to BGP peers.
- Pass a validity check. Verify that the NRLI is valid and that the next-hop address is resolvable in the global RIB. If the NLRI fails, the NLRI remains but does not process further.
- Process outbound neighbor route policies. After processing, if a route was not denied by the outbound policies, the route is maintained in the Adj-RIB-Out table for later reference.
- Advertise the NLRI to BGP peers. If the NLRI’s next-hop BGP PA is 0.0.0.0, then the next-hop address is changed to the IP address of the BGP session.
What command displays the content of the Loc-RIB?
The command show bgp afi safi displays the contents of the BGP database (Loc-RIB) on the router.
Every entry in the BGP Loc-RIB table contains at least one path but could contain multiple paths for the same network prefix.
Example 11-6 displays the BGP table on R1, which contains received routes and locally generated routes.
What command displays all the paths for a specific route and the BGP path attributes for that route?
The command show bgp afi safi network displays all the paths for a specific route and the BGP path attributes for that route.
Example 11-7 shows the paths for the 10.12.1.0/24 network. The output includes the number of paths and which path is the best path.
T/F: The Adj-RIB-Out table is a unique table maintained for each BGP peer.
True.
What command enables a network engineer to view routes advertised to a specific router?
This command will display the contents of the Adj-RIB-Out table for a neighbor.
- show bgp afi safi neighbor ip-address advertised routes
Example 11-8 shows the Adj-RIB-Out entries specific to each neighbor. Notice that the next- hop address reflects the local router and will be changed as the route advertises to the peer.
What command shows the BGP routes in the global IP routing table (RIB)?
The BGP routes in the global IP routing table (RIB) are displayed with the command show ip route bgp.
Example 11-10 shows these commands in the sample topology. The prefixes are from an eBGP session and have an AD of 20, and no metric is present.
Example 11-11 shows the routing table for R1. How was the loopback (192.168.3.3) on R3 learned? How is the loopback (192.168.4.4) on R4 reached? How was the loopback (192.168.5.5) on R5 learned?
Example 11-11 shows the routing table for R1. Notice that R3’s loopback was learned via EIGRP, R4’s loopback is reached using a static route, and R5’s loopback is learned from OSPF.
What is the command to implement dynamic route summarization? What benefit would route summarization have in the attached diagram? In order for R4 to benefit from this where would the summarization happen?
Dynamic route summarization is accomplished with the BGP address family configuration command:
- aggregate-address network subnet-mask [summary-only] [as-set].
In Figure 11-11 on question side of this card, R3 summarizes all the eBGP routes received from AS 65100 and AS 65200 to reduce route computation on R4 during link flaps. In the event of a link flap on the 10.13.1.0/24 network, R3 removes all the AS 65100 routes learned directly from R1 and identifies the same network prefixes via R2 with different path attributes (a longer AS_Path). R3 has to advertise new routes to R4 because of these flaps, which is a waste of CPU cycles because R4 only receives connectivity from R3. If R3 summarized the network prefix range, R4 would execute the best-path algorithm once and not need to run during link flaps of the 10.13.1.0/24 link
Figure 11-12 removes the flapping serial link between R1 and R3 to demonstrate BGP route aggregation and the effects of the commands.
What commands are needed to configure a BGP process with an AS of 65200, AFI of IPv4, and to advertise a summary route of 192.168.0.0/16 while preserving the BGP path information history? i.e. the advertised summary route will include the AS that summarized the route.
Example 11-24 shows R2’s updated BGP configuration for summarizing both networks with the as-set keyword.
Figure 11-13 demonstrates a simple topology with three different ASs and R2 forming an eBGP session with R1 and R3.
What is the range in IPv6 of all link-local addresses?
What are the advertised link-local addresses in this network?
The link-local addresses have been configured from the defined link-local range FE80::/10.
- All of R1’s links are configured to FE80::1
- all of R2’s links are set to FE80::2
- all of R3’s links are configured for FE80::3
A link-local address is an IPv6 unicast address that can be automatically configured on any interface using the link-local prefix FE80::/10 (1111 1110 10) and the interface identifier in the modified EUI-64 format.
