BGP Rule of Synchronization

BGP Synchronization Synchronization is a technique for automatically redistributing routes between the BGP and your IGPs. The main goal of BGP synchronization is to prevent routing inconsistencies. It serves to guarantee that only entries that exist in the IGP's IP routing table are included in the BGP routing tables. The BGP rule of synchronization states that if your autonomous system is passing traffic from another AS to a third AS, BGP should not advertise a route until the entire collection of local AS routers has learned about the route via an IGP routing protocol. BGP will wait until IGP has propagated the route within the AS before advertising it to external neighbor routers. For example, suppose that a BGP router advertised a route to external network 10.10.10.0 without first allowing IGP to flood this route information throughout the local AS. If another BGP router received a packet destined for network 10.10.10.0 without receiving the update, this second router would discard the packet. BGP synchronization can be disabled, but it is only safe to do so when full mesh connectivity exists between all IBGP routers within the AS. After the entries between the tables are synchronized, routes can be redistributed between the protocols without the risk of black holes.

Flex Links - In Switching Environment

Flex links are layer-2 interfaces manually configuredin primary/failover

pairs. The Spanning Tree Protocol normally provides primary/failover 

functionality, but it was designed for the sole purpose of preventing 

loops.Flex links are used to ensure that there are backup links for

primary links.Only one of the links in a flex-link pair will be forwarding

traffic at any time.Flex links are designed for switches where you do 

not wish to run spanning tree, and should be usedonly on switches 

that do not run spanning tree. Should flex links be configured on a 

switch running spanning tree ? The flex links will not participate in 

STP.Flex links are configured on the primary interface by specifying

the backup interface with the switchport backup interface command

interface GigabitEthernet1/0/20

switchport access vlan 10

switchport backup interface Gi1/0/21

!

interface GigabitEthernet1/0/21

switchport access vlan 10

No configuration is necessary on the backup interface.

Neither of the links can be an interface that is a member of an 

EtherChannel.An EtherChannel can be a flex-link backup for another 

port channel.A single physical interface can be a backup to an Ether

Channel as well.The backup link does not need to be the same type 

of interface as the primary. For example, a 100 Mbps interface can

be a backup for a 1 Gbps interface.

Monitoring flex links is done with the show interface switchport backup 

command:

3750# sh int switchport backup

Switch Backup Interface Pairs:

Active Interface Backup Interface State

------------------------------------------------------------------------

GigabitEthernet1/0/20 GigabitEthernet1/0/21 Active Down/Backup Down

Best Regards,

Deepak Arora

BGP Session Establishment

BGP is a normal TCP application, which means that a TCP client 

initiates the session to the TCP server with a SYN packet going 

to the well known port of 179. If the BGP server is configured to 

accept the session, a reply with SYN/ACK comes from port 179, 

going to the high port that is negotiated between them. In the 

case that both BGP peers attempt to establish the connection 

at the sametime, RFC 4271 (A Border Gateway Protocol 4) defines

a “BGP Connection Collision Detection” mechanism, in which 

essentially the session originated from the device with the higher

BGP router-id is maintained, and the secondary session is dropped.

Storm Control

The Storm Control feature protects a LAN from being affected by

unicast, broadcast, or multicast storms that might develop.

The switch implements storm control by counting the number of 

packets of  a specified type received within the one-second 

time interval and compares the measurement with a predefined 

suppression-level threshold. Storm Control can typically enable

the administrator to control traffic by a percentage of total bandwidth 

or the traffic rate at which packets are received. It is important to 

note that when the rate of multicast traffic exceeds a set threshold, 

all incoming traffic (broadcast, multicast, and unicast) is dropped 

until the level drops below the specified threshold level.

Only spanning-tree packets are forwarded in this situation. When 

broadcast and unicast thresholds are exceeded, traffic is blocked for

only the type of traffic that exceeded the threshold.

Storm Control is configured at the interface level with the following

command:

storm-control {broadcast | multicast | unicast} level {level

[level-low] | pps pps [pps-low]}

Best Regards,

Deepak Arora

SPANNING TREE LOOP GUARD FEATURE

As its name implies, Loop Guard is a method for ensuring that STP
loops never occur in a particular topology. Even though STP guards
against such loops as best it can, they could still occur because of
things like unidirectional link failures or switch congestion issues.
Loop Guard prevents loops conservatively by preventing alternate or
root ports from becoming DPs in the topology. If BPDUs are not
received on a non-DP, and Loop Guard is enabled, that port is moved
into the STP loop-inconsistent Blocking state, instead of the Listening /
Learning / Forwarding state.

Loop Guard operates only on ports that are considered point-to-point
by the spanning tree, and it cannot be run in conjunction with Root
Guard on an interface.

To enable Loop Guard, you can use the following global configuration
mode command:
spanning-tree loopguard default

Best Regards,
Deepak Arora