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الموضوع: ملاحظات فى BGP

  1. #1
    عضو الصورة الرمزية frozenEyes
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    ملاحظات فى BGP



    السلام عليكم ورحمه الله وبركاته

    وجدت بعض الملاحظات فى BGP فحبيت انقلها لكم لتعم الاستفادة على الجميع ان شاء الله و نأخذ ثواب نشر العلم فقد كان يسأل بعض

    الاخوة عن هذا البروتوكول وكما قال الحبيب
    رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ مَنْ سُئِلَ عَنْ عِلْمٍ فَكَتَمَهُ أَلْجَمَهُ اللَّهُ بِلِجَامٍ مِنْ نَارٍ يَوْمَ الْقِيَامَةِ .



    BGP uses TCP port 179 for transport. Router with the higher BGP router-id
    initiates BGP session from a random port.


    2. The interface from which the BGP router ID is taken does not have to be
    running BGP. Any valid IP address can be used as BGP router-id, even an
    address that is not locally configured on the router.


    3. The BGP router-id must be the same as the OSPF router-id for redistributing
    the routes from OSPF to BGP or vice versa.


    4. If the 'network …' command is configured with the 'mask' option under the
    BGP process, then an exact match (network/mask) must exist in the IP
    routing table in order to advertise this route into BGP regardless of
    'auto-summary' / 'no auto-summary' command. But the 'network …'
    command configured without the 'mask' assumes the default classful mask
    and if 'auto-summary' is configured then BGP will advertise a classful
    network only if any subnets of the classful network exist in the IP routing
    table. Again if the 'network …' command is configured without the 'mask'
    option and if 'no auto-summary' is configured, then that router must have
    the exact classful network in the IP routing table in order to advertise it in
    BGP.


    5. To accept and attempt BGP connections to the external peers residing on
    networks that are not directly connected, we need to use either 'neighbor
    ebgp-multihop …' or 'neighbor ttl-security …' command. These two
    commands are mutually exclusive. We can use another command 'neighbor
    disable-connected-check' to accomplish the same task if the BGP neighbor is
    one-hop away.


    6. The synchronization rule states that an iBGP learned prefix cannot be
    considered best unless there is a matching IGP route for that BGP prefix. BGP
    only advertises what it considers the best path. This issue can be resolved (1)
    by redistributing BGP routes into the IGP, (2) by creating a full-mesh of IBGP
    routers and disabling the synchronization, or (3) by creating a GRE tunnel.
    When BGP is synchronizing with OSPF, the router ID must match in both
    protocols in order to make it work.


    7. When a prefix is received from an eBGP neighbor, it is advertised to both
    eBGP & iBGP neighbors. When a prefix is received from an iBGP neighbor, it is
    advertised ONLY to eBGP neighbors and not to any iBGP neighbors. To
    advertise iBGP leaned routes to other iBGP peers requires the use of
    route-reflectors or confederations or a full-mesh of iBGP peers.


    8. While sending BGP updates, EBGP peers modify the next-hop value to its own
    IP address. But iBGP peers do not modify it.


    9. The ‘default-information originate’ command, however, requires explicit
    redistribution of the route 0.0.0.0. . Default routes can be injected into BGP in
    one of three ways: (1) using the 'network …' command (default route must
    exist in the local routing table), (2) using the 'default-information originate'
    command (a redistribution statement must also be configured to redistribute
    the default route from the local routing table to the BGP table), and (3) using
    the 'neighbor … default-originate [route-map route-map-name]' command
    (this method does not even check for the existence of a default route in the
    IP routing table). The 'default-information originate' command should not be
    configured with the 'neighbor … default-originate' command on the same
    router.


    10. 'weight' and 'local-preference' are set inbound and they affect outbound
    traffic. But 'as-path' and 'med' are set outbound and they affect inbound
    traffic.


    11. The weights assigned with the 'set weight …’ route-map command overrides
    the weights assigned using the 'neighbor… weight …' command.


    12. Origin code 'i' is default on the BGP routes advertised by 'network ...',
    'aggregate-address ...' (if all subnet has 'i'), and 'neighbor …
    default-originate' commands. And origin code '?' is default on the BGP routes
    advertised by 'redistribute ...', 'aggregate-address ...' (if any single subnet
    has '?', but can be changed using ‘attribute-map’ option),
    'default-information originate', and 'bgp inject-map ...' commands.


    13. When BGP originates a route with the ‘network …’ command, MED is copied
    from the metric of the original route.


    14. BGP MED values are not passed beyond the receiving (neighbor) AS.


    15. Enabling the ‘bgp deterministic-med’ command ensures the comparison of
    the MED variable when choosing routes advertised by different peers in the
    same autonomous system. Enabling the ‘bgp always-compare-med’
    command ensures the comparison of the MED for paths from neighbors in
    different autonomous systems.


    16. The default behavior of BGP routers that run Cisco IOS software is to treat
    routes without the MED attribute as having a MED of 0, making the route
    that lacks the MED variable the most preferred. The 'bgp bestpath med
    missing-as-worst' command can be configured to treat the route that missing
    MED as the least preferred one.


    17. ‘bgp bestpath as-path ignore’ is a hidden command in Cisco IOS which allows
    BGP to not consider the AS path during best path route selection.


    18. There are two ways to create an aggregate address under BGP. The first is to
    create a static route to null interface in the routing table for the aggregate
    address and then advertise it with the ‘network …’ command. The second
    way is to use the ‘aggregate-address …’ command.


    19. By default when aggregation is configured in BGP, the 'atomic-aggregate'
    attribute is attached to the aggregate address if the 'as-set' argument is not
    used in the 'aggregate-address …' command. The 'as-set' argument reveals
    the AS numbers which can prevent a routing loop, and once 'as-set' is
    configured along with the 'aggregate-address …' command, the
    'atomic-aggregate' attribute is automatically removed.


    20. A router reflector and its clients are known collectively as a cluster. If the
    cluster contains a single route reflector, the cluster ID is the router ID of the
    route reflector. If the cluster contains multiple route reflectors, each RR must
    be manually configured with a cluster ID.


    21. A client router in a route reflection cluster can peer with external neighbors,
    but the only internal neighbor it can peer with is a route reflector in its
    cluster or other clients in the cluster. Clients cannot peer with routers outside
    of their own cluster. However, the RR itself can peer with both internal and
    external neighbors outside of the cluster and can reflect their routes to its
    clients.


    22. In case of route reflection, (1) routes from EBGP are advertised to EBGP,
    client, non-client (2) routes from client are advertised to EBGP, client,
    non-client (3) routes from non-client are advertised to EBGP, client.


    23. When the 'no bgp client-to-client reflection' command is configured the RR
    does not reflect routes from one client to another. It does, however, continue
    to reflect routes from clients to peers outside of the cluster, and from peers
    outside of the cluster to clients.


    24. Standard and extended BGP communities are removed from the reflected
    routes unless the 'neighbor ... send-community [both]' is configured on the
    route reflector. The link bandwidth community is removed from reflected
    route if the route-reflector performs IBGP multipath load-sharing for that
    route.


    25. The “neighbor … nexthop-self� on router reflectors only affects the next hop
    of eBGP learned routes because the next hop of reflected routes should not
    be changed. To avoid a common configuration error for reflected routes, the
    “set ip next-hop� command should not be used in a route map to BGP route
    reflector clients.


    26. Unlike route reflector environments in which only the route reflector itself
    has to support route reflection, all routers within a confederation must
    support the confederation functionality.


    27. EBGP routes external to the confederation are preferred over EBGP routes to
    member autonomous systems, which are preferred over iBGP routes.


    28. AS_PATH types are AS_SEQUENCE, AS_CONFED_SEQUENCE, AS_SET, and
    AS_CONFED_SET. AS_SEQUENCE is an ordered set of AS numbers, and
    AS_SET is an unordered set of AS numbers. AS_CONFED_SEQUENCE and
    AS_CONFED_SET are the same as AS_SEQUENCE and AS_SET but are used
    only within BGP confederations.


    29. When 'bgp bestpath med confed' command is configured, the router picks
    the confederation-internal path with the lowest MED and ignores the path
    with the external AS number.


    30. BGP private autonomous system numbers are from 64,512 to 65,535


    31. BGP prefixes can be filtered using (1) 'distribute-list', (2) 'prefix-list', (3)
    'filter-list', (4) 'policy-list', (5) community/extended community lists, (6)
    'route-map' .


    32. For BGP, the ‘distance …’ command sets the administrative distance of the
    External BGP (eBGP) route. This command only affects the routing table and
    not the BGP table.


    33. The 'network … backdoor' command has the same effect as the 'network …'
    command. The EBGP route is treated as a local BGP route, and the
    administrative distance is changed to 200. The difference is that the address
    specified by the network backdoor command is not advertised to EBGP peers.


    34. iBGP routes are not redistributed into an IGP unless you use "bgp
    redistribute-internal" command under BGP routing process.


    35. 'bgp inject-map ... exist-map ...' command injects prefixes in the local BGP
    RIB when a valid parent route exists. Only prefixes that are equal to or more
    specific than the aggregate route (existing prefix) can be injected. exist-map
    (route-map) must contain a 'match ip address prefix-list ...' command
    statement to specify the aggregate prefix and a 'match ip route-source
    prefix-list ...' command statement to specify the route source. If the parent
    route is a default route, we can inject any route out of it.


    36. A BGP neighbor cannot be configured to work with both peer groups and
    peer templates. BGP peer templates and BGP peer groups are mutually
    exclusive.


    37. Peer session template can inherit only one session template directly, but peer
    policy template can inherit multiple policy templates.


    38. When the maximum number (as set by the ‘neighbor … maximum-prefix ...’
    command) of prefixes are reached, the string "PfxRcd" appears in the entry,
    the neighbor goes to shutdown state, and the connection becomes idle.


    39. No penalty is applied to a BGP peer reset when route dampening is enabled.
    Although the reset withdraws the route, no penalty is applied in this instance.


    40. In case of iBGP multipath load sharing, when multiple iBGP paths installed in
    a routing table, a route reflector will advertise only one of the paths (one
    next hop).


    41. For multiple paths to the same destination to be considered as multipaths, all
    attributes including weight, local preference, autonomous system path
    (entire attribute and not just length), origin code, MED, and IGP distance
    must be same. But if 'bgp bestpath as-path multipath-relax' command is
    configured, the AS paths still have to be the same length, but don't have to
    be identical.


    42. Though BGP Multipath allows the installation of multiple BGP paths (for load
    sharing purpose) into the IP routing table for the same prefix, it does not
    affect the bestpath selection. A router still designates one of the paths as the
    best path and advertises this best path to its neighbors.


    43. 'neighbor … dmzlink-bw' command can be used with eBGP and iBGP
    multipath features to enable unequal cost load balancing over multiple links.
    BGP can originate the link bandwidth community only for directly connected
    links to eBGP neighbors.


    44. The 'bgp update-delay ...' command is used to tune the maximum time the
    software will wait after the first neighbor is established until it starts
    calculating best paths and sending out advertisements.


    45. The “neighbor … local-as …� command is valid only if the peer is a true eBGP
    peer. It does not work for two peers in different sub-ASs in a confederation.


    46. In a route-map, a continue clause can be executed, without a successful
    match, if a route map entry does not contain a match clause. But if a match
    clause exists, the continue clause is executed only if a match occurs. If no
    successful matches occur, the continue clause is ignored. The continue
    statement proceeds to the specified route map entry only after configured
    set actions (if any) are performed.


    47. When multiple values are configured in the same community list statement, a
    logical AND condition is created. All community values must match to satisfy
    an AND condition. When multiple values are configured in separate
    community list statements, a logical OR condition is created. The first list that
    matches a condition is processed.


    48. While redistributing OSPF into BGP, by default only OSPF intra-area and
    inter-area routes are redistributed into BGP.


    49. When a BGP router with synchronization enabled has also a OSPF route
    (redistributed from BGP) for a iBGP-learned route, then the OSPF ASBR
    router-id must match the originating BGP router-id in order to synchronize
    BGP route with OSPF route.


    50. An “update group� is a group of peers with a common outbound policy which
    will be converged as if they are in a peer-group.


    وبالتوفيق ان شاء الله ،،،،


  2. #2
    عضوية جديدة
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    رد: ملاحظات فى BGP

    شكرا جزيلا ،،،
    جعله الله في ميزان حسناتك ....

  3. #3
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    رد: ملاحظات فى BGP

    ياليت يترجمها لنا ويشرحها بأسلوبه المتميز

    الأستاذ أحمد عمر



  4. #4
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    شكرا جزيلا أخي العزيز، بالفعل ملاحظات وكلام رهيب ، يعطيك العافية يا وحش.
    CCIE R&S Written exam
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    CCNA
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    Dell Equallogic: Disaster Recovery


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    عضو الصورة الرمزية kamel_s_s
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    جزاك الله خيرا موضوع مفيد جدا

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    رد: ملاحظات فى BGP



    iBGP routes are not redistributed into an IGP unless you use "bgp redistribute-internal" command under BGP routing process.

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