Juniper JN0-664 - Service Provider Professional (JNCIP-SP)

https://www.juniper.net/documentation/us/en/software/junos/bgp/topics/topic-map/load-balancing-bgp-session.html

https://www.juniper.net/documentation/us/en/software/junos/routing-policy/bgp/topics/example/bgp-advertise-peer-as.html

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.

https://www.juniper.net/documentation/us/en/software/junos/ospf/topics/topic-map/configuring-ospfv2-sham-links.html

TLVs are tuples of (Type, Length, Value) that can be advertised in IS-IS packets. TLVs can carry different kinds of information in the Link State Packets (LSPs). IS-IS supports both narrow and wide metrics for link costs. Narrow metrics use a single octet to encode the link cost, while wide metrics use three octets. Narrow metrics have a maximum value of 63, while wide metrics have a maximum value of 16777215. If there are both narrow and wide metric devices in the topology, IS-IS will advertise two TLVs for each link: one with the narrow metric and one with the wide metric. This allows backward compatibility with older devices that only support narrow metrics12.

To determine the correct answer, let's analyze the BGP path selection process and identify which attribute would be considered next in this scenario.

Background on BGP Path Selection

When multiple paths to the same destination are received via BGP, the router uses a step-by-step process to select the best path. The order of attributes considered is as follows (simplified for this scenario):

Highest Local Preference : The path with the highest local preference is preferred.

Shortest AS Path : The path with the shortest AS path length is preferred.

Lowest Origin Code : Paths with an origin code of IGP are preferred over EGP, and EGP is preferred over Incomplete.

Lowest MED (Multi-Exit Discriminator) : If the first three attributes are identical, the path with the lowest MED value is preferred.

eBGP over iBGP : eBGP paths are preferred over iBGP paths.

IGP Metric to Next Hop : The path with the lowest IGP metric to the next-hop router is preferred.

Router ID : If all else is equal, the path from the router with the lowest Router ID is preferred.

Peer IP Address : As a last tiebreaker, the path from the peer with the lowest IP address is preferred.

Scenario Analysis

In this scenario:

You are receiving the 203.0.113.0/24 network via EBGP from two different autonomous systems (AS 64500 and AS 64501).

Both advertisements have identical local-preference values , AS-path lengths , and BGP origin codes .

Given that the first three attributes in the BGP path selection process are identical, the next attribute to consider is the MED (Multi-Exit Discriminator) value.

Analysis of the Options

Option A: Router ID

Incorrect : The Router ID is considered much later in the BGP path selection process, only after other attributes like MED and IGP metric have been evaluated. Since MED is still relevant here, Router ID is not the next attribute to consider.

Option B: MED value

Correct : The MED value is used to influence inbound traffic from neighboring ASes. When local preference, AS path length, and origin code are identical, the path with the lowest MED value is preferred. This makes MED the next attribute to consider in this scenario.

Option C: Peer IP Address

Incorrect : The peer IP address is a tiebreaker used only at the very end of the BGP path selection process, after all other attributes have been evaluated. It is not relevant here because MED has not yet been considered.

Option D: IGP Metric

Incorrect : The IGP metric to the next-hop router is considered after MED. Since MED is still relevant in this scenario, IGP metric is not the next attribute to evaluate.

Final Answer

The correct answer is:

B. MED value

Summary

When local preference, AS path length, and origin code are identical, the MED value is the next attribute considered in the BGP path selection process.

MED is used to influence how traffic enters your AS from neighboring ASes.

 

Option A (Correct):

Complete Sequence Number PDUs (CSNPs) are periodically flooded by the Designated Intermediate System (DIS) on multi-access networks (e.g., Ethernet).

This ensures all routers on the segment synchronize their Link-State Databases (LSDBs).

Route reflection is a BGP feature that allows a router to reflect routes learned from one IBGP peer to another IBGP peer, without requiring a full-mesh IBGP topology. Route reflectors do not change any existing BGP attributes by default when advertising routes, unless explicitly configured to do so. A BGP peer does not require any configuration changes to become a route reflector client, only the route reflector needs to be configured with the client parameter under [edit protocols bgp group group-name neighbor neighbor-address] hierarchy level.