Bgp

Front 1

ASN - public

Back 1

1 to 64511

Front 2

ASN - Private

Back 2

range of 64512 to 65535

a feature allows the stripping of private AS numbers out of the AS_PATH list before the routes are propagated to a BGP peer.

When a private AS number is allocated to the customer network, the BGP updates from the customer network to ISP will have the private AS number in its AS_PATH list. When the ISP propagates its network information to the global BGP table (Internet), it should not propagate the AS_PATH with the private AS number of the customer to the Internet. To help the ISP remove the private AS number from its AS_PATH list, use the Cisco IOS remove-private-as command.

To remove the private AS number, use the neighbor x.x.x.x remove-private-as router configuration command.

The neighbor x.x.x.x remove-private-as per-neighbor configuration command forces BGP to drop the private AS numbers. You can configure this command for external BGP neighbors. When the outbound update contains a sequence of private AS numbers, this sequence is dropped.

The following conditions apply:

*

You can only use this solution with external BGP (eBGP) peers.

*

If the update has only private AS numbers in the AS_PATH, BGP removes these numbers.

*

If the AS_PATH includes both private and public AS numbers, BGP doesn't remove the private AS numbers. This situation is considered a configuration error.

*

If the AS_PATH contains the AS number of the eBGP neighbor, BGP does not remove the private AS number.

*

If the AS_PATH contains confederations, BGP removes the private AS numbers only if they come after the confederation portion of the AS_PATH.

Front 3

show ip bgp sum

Back 3

provides info about memory consmption

Front 4

neighbor
neighbor-id soft-reconfiguration inbound

Back 4

causes the router to keep a copy of the received Updates from the specified neighbor. (IOS keeps a copy of sent Updates automatically.) With these Updates available, BGP can simply reapply the changed filtering policy to the Update without closing the neighbor connection.

Front 5

soft-reconfiguration inbound --- 1

Back 5

the router must apply a copy of the bgp path b4, and after any local policies are applied.

If soft-reconfiguration inbound is configured, the router stores a copy of all unprocessed updates received by its peers for future use, otherwise this table is discarded after putting the routes in Loc-RIB. BGP soft-reconfiguration inbound can be configured by following command:

neighbor 192.0.2.1 soft-reconfiguration inbound

#show ip bgp neighbors x.x.x.x received-routes

Front 6

soft-reconfiguration inbound --- 2

Back 6

If the soft-reconfiguration inbound is not configured for a neighbor, an error is displayed when we try to view the received-routes. This is just because the Adj-RIB-In table is discarded after processing the routes from it and building the Loc-RIB.

#show ip bgp neighbors 192.0.2.5 received-routes
% Inbound soft reconfiguration not enabled on 192.0.2.5

To view the contents of Loc-RIB table from a specific neighbor, issue the following command:

#show ip bgp neighbors x.x.x.x routes

Front 7

Reset BGP sessions

Back 7

1. Hard reset: dropping and re-establishing TCP session to our peers.
2. Dynamic Soft reset: uses the route-refresh capability to request all NLRI be sent again.

Soft reset using stored information: uses the unmodified and unfiltered stored updates.

Front 8

Reset BGP sessions -- Hard reset

Back 8

Hard Reset
Using a hard reset to clear a BGP session causes cache invalidation and results in a negative impact on network. By clearing the BGP sessions using this method, BGP process drops all the BGP peers and re-establishes the TCP session followed by Open message which makes the neighbor to send all the NLRIs again. This method is not recomended and should only be used if other soft reset methods fail. Hard reset can be done by the following command:

#clear ip bgp 192.0.2.1
*Mar 1 00:54:34.491: %BGP-5-ADJCHANGE: neighbor 192.0.2.1 Down User reset
*Mar 1 00:54:37.083: %BGP-5-ADJCHANGE: neighbor 192.0.2.1 Up

Front 9

Reset BGP sessions -- Dynamic Soft REset

Back 9

Dynamic route refresh features:

* Does not require pre-configuration.
* Does not require additional memory for storing routing update information.

To perform dynamic route refresh to apply inbound policies, use the following command:

#clear ip bgp 192.0.2.1 in
*Mar 1 02:34:04.491: BGP: 192.0.2.1 sending REFRESH_REQ(5) for afi/safi: 1/1
*Mar 1 02:34:04.495: BGP: 192.0.2.1 send message type 5, length (incl. header) 23

or

#clear ip bgp 192.0.2.1 soft in
*Mar 1 02:34:40.471: BGP: 192.0.2.1 sending REFRESH_REQ(5) for afi/safi: 1/1
*Mar 1 02:34:40.475: BGP: 192.0.2.1 send message type 5, length (incl. header) 23

Front 10

Reset GBP sessions --- \Soft Reset

Back 10

To reset the BGP using Soft Reset using Stored Information, the following commands can be used:#clear ip bgp 192.0.2.1 soft in

or

#clear ip bgp 192.0.2.1 in

Front 11

How BGP routes are propagated into BGP table

Back 11

The process:

Adj-RIB-In —> Loc-RIB —> Adj-RIB-Out

Adj-RIB-In: The Adj-RIBs-In contains unprocessed routing information that has been advertised to the local BGP speaker by its peers.

Loc-RIB: The Loc-RIB contains the routes that have been selected by the local BGP speaker's Decision Process.
Adj-RIB-Out: The Adj-RIBs-Out contains the routes for advertisement to specific peers by means of the local speaker's UPDATE messages. This is actually just a pointer back to the record in the Loc-RIB.

Front 12

what is implicit withdraw ?

Back 12

BGP routers and route reflectors (RRs) propagate only their best path over their sessions. The advertisement of a prefix replaces the previous announcement of that prefix. The implicit withdraw can achieve better scaling, but at the cost of path diversity.

Front 13

Path hiding

Back 13

Path hiding can prevent efficient use of BGP multipath, prevent hitless planned maintenance, and can lead to MED oscillations and suboptimal hot-potato routing. Upon nexthop failures, path hiding also inhibits fast and local recovery because the network has to wait for BGP control plane convergence to restore traffic

Front 14

BGP Additional Path feature (add-path) and path diversity

Back 14

The BGP Additional Paths feature provides a way for multiple paths for the same prefix to be advertised without the new paths implicitly replacing the previous paths. Thus, path diversity is achieved instead of path hiding.

Front 15

hot-potato routing

Back 15

In order to minimize internal transport costs, transit ISPs try to forward packets to the closest exit point (according to Interior Gateway Protocol [IGP] cost). This behavior is known as hot-potato routing

passing traffic off to another autonomous system as quickly as possible, thus using their network for wide-area transit.

Front 16

BGP Well-Known Mandatory Attributes

Back 16

AS_PATH
NEXT_HOP
ORIGIN

Front 17

BGP Well-Known Discretionary Attributes

Back 17

LOCAL_PREF
ATOMIC_AGGREGATE

Front 18

BGP Optional Transitive Attributes

Back 18

AGGREGATOR
COMMUNITY

Front 19

BGP Optional Non-Transitive Attributes

Back 19

MULTI_EXIT_DISC (MED)
ORIGINATOR_ID
CLUSTER LIST
MULTIPROTOCOL Reachable NLRI
MULTIPROTOCOL Unreachable NLRI

Front 20

BGP Path Selection Mnemonic

Back 20

N Next hop: reachable?
W WEIGHT (bigger)
L LOCAL_PREF (bigger)
L Locally injected routes
A AS_PATH length (smaller)
O ORIGIN(pref over I,I over E,E over ?)
M MED (smaller)
N Neighbor type(prefer eBGP over iBGP)
I IGP meric to NEXT_HOP (smaller)

Front 21

Dynamic Inbound Soft Reset (route refresh)

Back 21

Requests complete refresh of the ADJ-RIB-OUT

is the most preferred method, it requires the route refresh capability. Simply said, this feature lets your router request its BGP neighbor to send its prefixes again.

Front 22

Soft reset

Back 22

BGP Soft Reset feature provides automatic support for dynamic soft reset of inbound BGP routing table updates that is not dependent upon stored routing table update information. The new method requires no preconfiguration (as with the neighbor soft-reconfiguration command) and requires much less memory than the previous soft reset method for inbound routing table updates

uses extra prefix info stored locally

Front 23

hard reset

Back 23

It kills the TCP session with your BGP neighbor which forces it to restart and as a result you’ll receive all prefixes from your neighbor again. It works, but it’s cruel…

Tears down peering session and deletes prefixes from the peer

Front 24

Enhanced Route Refresh

Back 24

The BGP Enhanced Route Refresh feature provides a way for Border Gateway Protocol (BGP) to find route inconsistencies, and in that unlikely event, to synchronize BGP peers without a hard reset. The feature is enabled by default; there are two optional timers.

Front 25

BGP state - idle 1

Back 25

the initial state of a BGP connection. The BGP speaker is waiting for a start event, generally either the establishment of a TCP connection or the re-establishment of a previous connection. Once the connection is established, BGP moves to the next state.

Front 26

BGP state - Connect 2

Back 26

If the TCP connection completes, BGP will move to the OpenSent stage if the connection does not complete, BGP goes to Active.

Front 27

BGP state - Active 3

Back 27

Active indicates that the BGP speaker is continuing to create a peer relationship with the remote router. If this is successful, the BGP state goes to OpenSent. You'll occasionally see a BGP connection flap between Active and Connect. This indicates an issue with the physical cable itself, or with the configuration.

Front 28

BGP state - OpenSent

Back 28

indicates that the BGP speaker has received an Open message from the peer. BGP will determine whether the peer is in the same AS (iBGP) or a different AS (eBGP) in this state.

Front 29

BGP state - OpenConfirm

Back 29

the BGP speaker is waiting for a keepalive message. If one is received, the state moves to Established, and the neighbor relationship is complete. It is in the Established state that update packets are actually exchanged.