technical phone screen

1. difference between ebgp and ibgp

2. AS number , private 64512-65535 public 1-64511

3. bgp multipath

   1. BGP, typically selects only one best path for each prefix and installs that route in the forwarding table. When BGP multipath is enabled, the device selects multiple equal-cost BGP paths to reach a given destination, and all these paths are installed in the forwarding table. BGP advertises only the active path to its neighbors, unless add-path is in use.

4. igmp port number

5. what is bgp loop protection

6. bgp attributes ? category : well-known mandatory, transitive

7. Path Attributes fall into one of the two categories

8. difference between transitive and no-transitive

9. 

10. 1. Well-known Path Attributes
    2. Optional Path Attributes

    Well-known:Meaning these attributes must be recognized by all the BGP implementations.

    Well- Known BGP Path Attributes fall into two sub-categories known as

    1. Mandatory (Called as Well-known Mandatory)
    2. Discretionary (Called as Well-Known Discretionary)

    Mandatory:This means the attribute must be always included and carried in all BGP update messages to peers. The BGP implementation has to recognize the attribute, accept it and also advertise it to its peers.

    Discretionary:Meaning these are recognized by the BGP implementation but may or may not be sent in a specific Update message. Its up to the discretion of BGP Implementation to send or not to send these attributes in the update messages to the peers.

    Optional:Meaning these attributes may or may not be supported by the BGP implementations.

    Optional BGP Path attributes also fall into two sub-categories

    1. Transitive (Called as Optional Transitive)
    2. Non-transitive (Called as Optional Non-transitive)

    Transitive:BGP process has to accept the path in which it is included and should pass it on to other peers even if these attributes are not supported. Meaning if any optional attribute is not recognized by a BGP implementation, then BGP looks to check if the transitive flag is set. If the transitive flag is set then BGP implementation should accept the attribute and advertise it to its other BGP Peers.

    Non-transitive:If the BGP process does not recognize the attribute then it can ignore the update and not advertise the path to its peers. If the transitive flag is not set then BGP implementation can quietly ignore the attribute, it does not have to accept and advertise this attribute to its other peers.

    To Summarize the BGP Path Attribute Categories

    1. Well-known Mandatory: Recognized and Included in all BGP Update messages.
    2. Well-known Discretionary: Recognized and May or May not include in BGP Update messages
    3. Optional Transitive: Even if Not Supported it Still need to accept and Send in Update Message.
    4. Optional Non-transitive: Can be ignored and not advertise to peers.

11. origin what is this attribute, IGP > EGP > imcompelted

12. bgp message ( open, update,keepalive, notification , what are they for ?)

13. reason why bgp stay in two way status

1. for the open message ,what port number is used to set up the tcp connection ? 179

2. tcp 3 way handshack

3. do you know whitebox ?

   1. ONIE is an Open Compute Project open source initiative contributed by Cumulus Networks that defines an open “install environment” for bare metal network switches

   2. A white box switch may come pre-loaded with minimal software or it may be sold as a bare metal device. The advantage of this approach is that switches can be customized to meet an organization's specific business and networking needs. White box switches are often used in software-defined networking (SDN), an approach to networking in which control is decoupledfrom the physicalinfrastructure.

4. protocol distance metric?

5. if get a route from ospf, static ,ebgp,ibgp what will be the best path ?

6. 192.168.1.0/24(ebgp) ,192.168.1.0/25(ibgp), 192.168.1.0/26(ospf),192.168.1.0/27(rip) , if want to go to the 192.168.1.1 ,which route will be used ?

7. A----B-----C , B get the same prefix from C and D , what will A see the route , one or two ?

      `| D`
   

Community values in these ranges are reserved0 – 65535 [Hex: 0x00000000-0x0000FFFF] and
4294901760 – 4294967295 [Hex: 0xFFFF0000-0xFFFFFFFF]some of the well-known communities fall into these reserved ranges, as below.

1. INTERNET:The internet community is the default community, it has no value. All routes by default to belong to this community and all of the routes in this category are advertised freely.

2. NO_EXPORT:Routes received carrying this value cannot be advertised to EBGP Peers. That is these routes must not be advertised outside the AS. The value is 0xFFFFFF01. If there is a confederation defined and this value is received then the routes cannot be advertised outside the confederation.

3. NO_ADVERTISE:Routes received carrying this value cannot be advertised at all, that is they cannot be advertised to IBGP or EBGP peers. The value is 0xFFFFFF02.

4. LOCAL_AS:Routes received carrying this value cannot be advertised to EBGP peers and peers in other AS within a confederation. The value is 0xFFFFFF03. As per RFC 1997 this attribute is called as NO_EXPORT_SUBCONFD.

Apart from these well-known community attributes, private community attributes can also be defined for certain uses, but these private community attributes will only be significant to the AS that has defined it in context to its BGP peers.