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46 Cards in this Set
- Front
- Back
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What is BGP?
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It is an enhance path vector routing protocol which supports VLSM, CIDR and summarization. It’s the only routing protocol that supports interdomain routing an can connect multiple ASs.
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What are the 3 tables involved in BGP?
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Information is kept in the neighbor table. BGP routes are maintained in a separate BGP table, and the best BGP routes are passed on to the IP routing table.
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What's the difference between iBGP and eBGP?
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eBGP is used externally between ASs, whilst iBGP is used internally within ASs.
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How does BGP avoid loops?
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It does so by tracking routes in terms of AS, and rejecting routes that have already passed through their AS. It does this by adding the AS to the to the path when advertising externally. There is no way to use AS-path to detect loops internally to detect loops, this is why there are some special rules to prevent self-inflicted wounds.
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What is a peer group?
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It is a group of neighbors that share characteristics. They reduce configuration since one configuration effectively configures every router in the peer group. They also ease the overhead on the router since updates are generated once per group, and then send the same update to all members of the group. Because of this all members of the group must be either internal or external.
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Why is it advisable to peer to a loopback address?
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If there are several routes to a router, if an associated link goes down BGP will use one of the other paths to get to the router. So by peering to a loopback address BGP isn't affected by individual links going down. You would set the route to neighbors loopback (neighbor cmd), and set the source to this routers loopback (update-source cmd).
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What are the 4 message types of BGP?
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Update, Open, keepalive and Notification. Notification messages are used to inform the receiving router of errors.
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What are open and keep alive messages for?
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Open messages create sessions between peers (neighbors) on TCP 179, whilst these sessions are maintain by periodic keepalives.
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What do update messages do?
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When neighbourship is first established BGP routers exchange complete routing table between the routers by using them, and subsequently they are only sent when something changes. E.g. a network
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What are the 5 states BGP cycles through as it runs?
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Idle=searching for neighbors, Connect=TCP 3-way handshake complete with neighbor, Open Sent=BGP open sent message has been sent, Open Confirm=Response receive, Established-BGP neighborship established. To
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What cmd can be used to check the BGP state?
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Show ip bgp neighbors. If idle look for next-hop address that cant be reached, if active (if in Open sent or open confirm) means a neighbor is not responding as expected, and if Established all is good
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What does BGP use to determine the best route to a dst when multiple paths exist to a particular dst?
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Instead of having a metric it has a 10 step process for selecting routes. It uses BGP attributes to select the best path which is then put into the IP routing table, and then propagates the path to its neighbors. More specific routes (longer prefix) will always be preferred regardless of BGP attributes.
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What are the first 6 BGP path selection attributes in order of most important first?
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Synchronized (True), Weight (Highest), Local Preference (Highest), Self originated (Highest), AS-path (Shortest), Origin (I), MED (lowest).
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What is the AS-path attribute?
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It is a list of ASs that the route has passed through. The shortest path is preferred.
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What is the Origin attribute?
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It describes how the route was learned. It can be I (IGP), e (EGP) or ? (redistributed)
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What are the main approaches to connecting to the Internet?
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Accept only default routes from all providers, accept some routes and a default route, or accept full routing updates. With less routing info sometimes traffic will take an indirect path, but more routes accepted by the router means higher memory and CPU usage.
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What is the BGP Synchronization requirement?
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It states that before iBGP can propagate a route, the route must be known from another source. This is to say, it must be learned from a IGP. This is to sure that a router really knows how to get to a location it advertises. Is on by default.
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What are the benefits of Synchronization?
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Prevents traffic from being forwarded to unreachable destinations, reduces unnecessary traffic, and ensures consistency within a AS
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Under what circumstances may it be beneficial to turn synchronization off?
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If all routers in the AS are running BGP, all BGP routes inside the AS are meshed, or when the AS is not a transit AS
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What is Multihoming?
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Refers to redundant Internet connectivity. It can be 1 or more redundant links to the same ISP or to different ones. If connected to 2 different ISPs it is undesirable to let your AS become a transit AS between the ISPs.
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What is route Dampening?
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It is a mechanism to minimize the instability caused by route flapping. By counting the number of times the identified route fails and applying a penalty to the route for each flap, BGP can ignore ill-behaved routes and use only the well-behaved.
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Why should you do soft resets instead of hard resets?
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Soft resets have the same effect as hard resets but don't interrupt routing or cause a flap. BGP only exchanges routes when neighbor relationships are established, so if neighbors change policies (eg to implement a new distribution list), resetting is the only way to propagate the changes without cycling the power.
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To enable BGP or identify the AS of this router.
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router(config)# router bgp [AS-number]
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To tell the router the IP of the BGP neighbor and the AS to which it belongs. This allows the router to create a TCP session with the neighboring router and exchange BGP routing information. If the neighbors AS is same as the routers AS it is an iBGP connection, if different an eBGP.
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router(config-router)# neighbor [ip-add | peer-group-name] remote-as [AS-number]
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To create a peer group
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router(config-router)# neighbor [peer-group-name] peer-group
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To define neighbors as members of that group. Routers are listed as members of the peer group, and then policy is associated with the peer group (identify peer group & AS)
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router(config-router)# neighbor [ip-add] peer-group [peer-group-name]
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To disable a peer group. Administratively shutting down a peer is done during maintenance to prevent flapping.
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router(config-router)# neighbor [ip-add | peer-group-name] shutdown
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To determine the networks that are originated by this router. It permits BGP to advertise a network if it is present in the routing table. This cmd doesn't identify the interfaces on which to run BGP, rather it states the networks that are available within the AS.
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router(config-router)# network [network-add] mask [network-mask]
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To aggregate (summarize) routes within the BGP domain. Summary-only=specific routes are suppressed, only summary route propagated, As-set=lists the AS numbers that the more specific routes have traversed
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router(config-router)# aggregate-address [ip-add] [mask] [summary-only] [as-set]
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To change the link source address. To truely allow for redundant paths, the src address must always be what the neighbor expects.
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router(config-router)# neighbor [peer-group-name] update-source [interface]
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To make the router substitute its internal address as the next hop. The BGP next hop is the point of entry into an AS, typically will be that of the edge router. This is useful for routes received from external neighbors, since will advertise an external next-hop IP address. This avoids problems seen on NBMA networks or non-meshed environments, such as X25 or Frame Relay.
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router(config-router)# neighbor [ip-add | peer-group-name] next-hop-self
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To change the default TTL of external connections (is 1 hop)
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router(config-router)# neighbor [ip-add | peer-group-name] ebgp-multihop [hop]
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To setup BGP authentication, which involves agreeing to a key (password) and is accomplished by sending an MD5 hash of the key with every BGP packet.
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router(config-router)# neighbor [ip-add] password [password]
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To turn synchronization off
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router(config-router)# no synchronization
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To set the weight attribute. Selects the exit interface when there are multiple paths to same destination. Higher the better. Is local to the router and not propagated. Default is 0, unless this router sources route, when it is 32,768.
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router(config-router)# neighbor [ip-add | peer-group-name] weight [0 - 65,535]
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To set the Local Preference attribute as a default (can also be set per-prefix). This is propagated to neighbors within the AS (internal neighbors). Higher values preferred, default is 100
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router(config-router)# bgp default local-preference [0 - 4,294,967,255]
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To change the Multi-exit Discriminator (MED), which is advertised to external neighbors to try to influence path selection into an AS. For example to influence ISP to send traffic into your AS on a certain path. Default is 0.
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router(config-router)# default-metric [value]
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To apply a route map to a peer or peer group using the neighbor cmd. Various methods exist to filter or modify routes using BGP and using route maps to change the BGP attributes values is one of them.
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router(config-router)# neighbor [ip-add | peer-group-name] route-map [name] [in | out]
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To disconnect the session between neighbors and reestablish it. An outbound one (out) tells local router to resend its entire BGP table to that neighbor, but does so without tearing down the TCP session between them.
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router(config-router)# clear ip bgp [* | address] soft [in | out]
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To generate an unprocessed copy of a neighbor update. This causes the router to remember 2 copies of an advertisement so you can reply a previous advertisement. You would do before a "soft in" requesting an advertisement
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router(config-router)# neighbor [ip-add] soft-reconfiguration inbound
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To reset the BGP peer associations, clear the routing table and reestablish BGP connections to neighbors. Unlike the soft option does tear down the session.
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router(config-router)# clear ip bgp [* | address]
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Displays the BGP routing table. Also shows RID, attribute values and status. It is a good cmd to verify attribute configuration changes and manage the traffic flow to and from the AS.
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router# show ip bgp
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To display the routing table with the status of all BGP sessions, as well as he number of prefixes learned per session.
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router# show ip bgp summary
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To display information about the TCP connections to neighbors, such as the number of each type of BGP message sent out and received with each neighbor. When the connection is established, the neighbors can exchange updates to exchange routing tables and synchronize their databases.
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router# show ip bgp neighbors
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To display active processes. Is used to identify any process that is using excessive resources.
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router# show process cpu
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To display live info of events as they occur. Dampening=minimizes instability caused by flapping, Keepalives=maintaining TCP session with the peer, Updates= routing updates sent and received.
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router# debug ip bgp [dampening | events | keepalives | updates]
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