BGP Receiving and Viewing Routes

Question 1

Receiving and Viewing Routes

Answer 1

Not every prefix in the Loc-RIB table is advertised to a BGP peer or installed into the global RIB when received from a BGP peer.

BGP performs the following route-processing steps:

1. Perform a quick validity check.
2. Store the route in Adj-RIB-In and process inbound route policies.
3. Update the Loc-RIB.
4. Verify next-hop reachability.
5. Compute the BGP best path.
6. Install the BGP best path into the global RIB and advertise to peers.
7. Process outbound neighbor route policies.
8. Advertise the NLRI to BGP peers.

Question 2

Perform a quick validity check

Step 1

Answer 2

This is performed on the route to ensure that a routing loop is not occurring.

Checking for its autonomous system number (ASN) in the AS-Path or its router-ID (RID) in the Internal Border Gateway Protocol (IBGP) path attributes (Originator/Cluster-ID).

Question 3

Store the route in Adj-RIB-In and process inbound route policies

Step 2

Answer 3

The NLRI is stored in the Adj-RIB-In table in its original state.

The inbound route policy is applied based on the neighbor the route was received.

Question 4

Update the Loc-RIB

Step 3

Answer 4

The BGP Loc-RIB database is updated with the NLRI after inbound route-policy processing has occurred.

The Adj-RIB-in is cleared to save memory.

Question 5

Verify next-hop reachability

Step 4

Answer 5

Confirm that the next-hop address is resolvable in the global RIB.

If the next-hop address is not resolvable in the RIB, the NLRI remains but does not process further.

Question 6

Compute BGP Best Path

Step 5

Answer 6

Multiple NLRIs (paths) can exist for the same network prefix in the Loc-RIB table.

BGP only advertises the best path to its neighbors.

The router must identify the BGP best path and pass only the best path and its path attributes to Step 6.

Question 7

Install the BGP best path into the global RIB and advertise to peers

Step 6

Answer 7

Install the prefix into the Global RIB using the next-hop IP address from the BGP
Loc-RIB table.

In some occurrences, the route cannot be installed into the Global RIB (a static route that has a lower administrative distance than BGP), which results in a RIB failure.

A RIB failure does not prevent the advertisement of the NLRI to other BGP neighbors.

Question 8

Process outbound neighbor route policies

Step 7

Answer 8

The NLRI is processed through any specific outbound neighbor route policies.

After processing, if the route was not denied by the outbound policies, the route is stored in the Adj-RIB-Out table for later reference.

Question 9

Advertise the NLRI to BGP peers

Step 8

Answer 9

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.

Question 10

Complete BGP Network Advertisement

Answer 10