You are configuring a BGP signaled Layer 2 VPN across your MPLS enabled core network. Your PE-2 device connects to two sites within the s VPN
In this scenario, which statement is correct?
Correct Answer:
D
BGP Layer 2 VPNs use BGP to distribute endpoint provisioning information and set up pseudowires between PE devices. BGP uses the Layer 2 VPN (L2VPN) Routing Information Base (RIB) to store endpoint provisioning information, which is updated each time any Layer 2 virtual forwarding instance (VFI) is configured. The prefix and path information is stored in the L2VPN database, which allows BGP to make decisions about the best path.
In BGP Layer 2 VPNs, each site has a unique site ID that identifies it within a VFI. The site ID can be manually configured or automatically assigned by the PE device. By default, the site ID is automatically assigned based on the order that you add the interfaces to the site configuration. The first interface added to a site configuration has a site ID of 1, the second interface added has a site ID of 2, and so on.
Option D is correct because by default on PE-2, the remote site IDs are automatically assigned based on the order that you add the interfaces to the site configuration. Option A is not correct because by default on PE-2, the site’s local ID is automatically assigned a value of 0 and does not need to be configured to match the total number of attached sites. Option B is not correct because you do not need to create a unique Layer 2 VPN routing instance for each site on the PE-2 device. You can create one routing instance for all sites within a VFI. Option C is not correct because you do not need to use separate physical interfaces to connect PE-2 to each site. You can use subinterfaces or service instances on a single physical interface.
Exhibit
Referring to the exhibit, which three statements are correct about route 10 0 0.0/16 when using the default BGP advertisement rules'? (Choose three.)
Correct Answer:
BDE
The problem in this scenario is that R1 and R8 are not receiving each other’s routes because of private AS numbers in the AS path. Private AS numbers are not globally unique and are not advertised to external BGP peers. To solve this problem, you need to do the following:
✑ Configure loops on routers in AS 65412 and advertise-peer-as on routers in AS 64498. This allows R5 and R6 to advertise their own AS number (65412) instead of their peer’s AS number (64498) when sending updates to R7 and R8. This prevents a loop detection issue that would cause R7 and R8 to reject the routes from R5 and R62.
✑ Configure remove-private on advertisements from AS 64497 toward AS 64498 and from AS 64500 toward AS 64499. This removes any private AS numbers from the AS path before sending updates to external BGP peers. This allows R2 and R3 to receive the routes from R1 and R4, respectively3.
Which two statements about IS-IS are correct? (Choose two.)
Correct Answer:
BC
IS-IS is an interior gateway protocol that uses link-state routing to exchange routing information among routers within a single autonomous system. IS-IS uses two types of packets to synchronize link-state databases among routers: Link State Packets (LSPs) and Partial Sequence Number Packets (PSNPs). LSPs contain information about the state and cost of links in the network, and are flooded periodically throughout the network. PSNPs are used to acknowledge receipt of LSPs and request retransmission of missing or corrupted LSPs. PSNPs contain only descriptions of LSPs, such as their sequence numbers and checksums3. IS-IS also uses another type of packet called Complete Sequence Number Packets (CSNPs), which are used to summarize the entire link-state database at regular intervals or when a new adjacency is formed. CSNPs are flooded periodically throughout the network and contain only descriptions of LSPs4. Therefore, PSNPs contain only descriptions of LSPs and CSNPs are flooded periodically. References: 3: https://www.juniper.net/documentation/us/en/software/junos/routing-
policy/topics/concept/routing-policy-is-is-partial-sequence-number-packet-psnp.html 4: https://www.juniper.net/documentation/us/en/software/junos/routing-policy/topics/concept/routing-policy-is-is-complete-sequence-number-packet-csnp.html
Exhibit
Click the Exhibit button-Referring to the exhibit, which two statements are correct about BGP routes on R3 that are learned from the ISP-A neighbor? (Choose two.)
Correct Answer:
AB
BGP is an exterior gateway protocol that uses path vector routing to exchange routing information among autonomous systems. BGP uses various attributes to select the best path to each destination and to propagate routing policies. Some of the common BGP attributes are AS path, next hop, local preference, MED, origin, weight, and community. BGP attributes can be classified into four categories: well-known mandatory, well-known discretionary, optional transitive, and optional nontransitive. Well-known mandatory attributes are attributes that must be present in every BGP update message and must be recognized by every BGP speaker. Well-known discretionary attributes are attributes that may or may not be present in a BGP update message but must be recognized by every BGP speaker. Optional transitive attributes are attributes that may or may not be present in a BGP update message and may or may not be recognized by a BGP speaker. If an optional transitive attribute is not recognized by a BGP speaker, it is passed along to the next BGP speaker. Optional nontransitive attributes are attributes that may or may not be present in a BGP update message and may or may not be recognized by a BGP speaker. If an optional nontransitive attribute is not recognized by a BGP speaker, it is not passed along to the next BGP speaker. In this question, we have four routers (R1, R2, R3, and R4) that are connected in a full mesh topology and running IBGP. R3 receives the 192.168.0.0/16 route from its EBGP neighbor and advertises it to R1 and R4 with different BGP attribute values. We are asked which statements are correct about the BGP routes on R3 that are learned from the ISP-A neighbor. Based on the information given, we can infer that the correct statements are:
✑ By default, the next-hop value for these routes is not changed by ISP-A before being sent to R3. This is because the default behavior of EBGP is to preserve the next-hop attribute of the routes received from another EBGP neighbor. The next- hop attribute indicates the IP address of the router that should be used as the next hop to reach the destination network.
✑ The BGP local-preference value that is used by ISP-A is not advertised to R3. This is because the local-preference attribute is a well-known discretionary attribute that is used to influence the outbound traffic from an autonomous system. The local- preference attribute is only propagated within an autonomous system and is not advertised to external neighbors.
References: : https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol- bgp/13753-25.html : https://www.cisco.com/c/en/us/support/docs/ip/border-gateway- protocol-bgp/13762-40.html : https://www.cisco.com/c/en/us/support/docs/ip/border- gateway-protocol-bgp/13759-37.html
Exhibit
You are attempting to summarize routes from the 203.0.113.128/25 IP block on R8 to AS 64500. You implement the export policy shown in the exhibit and all routes from the routing table stop being advertised.
In this scenario, which two steps would you take to summarize the route in BGP? (Choose two.)
Correct Answer:
CD
To summarize routes from the 203.0.113.128/25 IP block on R8 to AS 64500, you need to do the following:
✑ Add the set routing-options static route 203.0.113.128/25 discard command. This creates a static route for the summary prefix and discards any traffic destined to it. This is necessary because BGP can only advertise routes that are present in the routing table.
✑ Replace exact in the export policy with orlonger. This allows R8 to match and advertise any route that is equal or more specific than the summary prefix. The exact term only matches routes that are exactly equal to the summary prefix, which is not present in the routing table.
