Term
| Which protocol does BGP use? |
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Definition
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Term
| What scenarios do we require BGP? |
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Definition
| 1 If we want to send 80 percent traffic on a primary link and 20 percent on the secondary link. |
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Term
| Why is BGP called a Path Vector routing protocol? |
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Definition
| If you issue a "show ip bgp" command on ios, you see that the BGP stores the network as well as the path to reach the particular network. Hence it is called path vector routin protocol. |
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Term
| What are BGP peer groups on CISCO IOS? |
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Definition
| To simplify the configuration of BGP and to reduce the number of updates BGP has to create, we can use peer groups. We can add neighbors to a peer group and then apply all our configurations to the peer group. BGP will prepare the updates for the peer group which requires less CPU resources than preparing them for each neighbor separately. |
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Term
| What are the states in BGP? |
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Definition
1. Idle
2. connect
3. Open Sent
4. Open Confirm
5. Active
6. Established |
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Term
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Definition
| This is the first state where BGP waits for a “start event”. The start event occurs when someone configures a new BGP neighbor or when we reset an established BGP peering. After the start event, BGP will initialize some resources, resets a ConnectRetry timer and initiates a TCP connection to the remote BGP neighbor. It will also start listening for a connection in case the remote BGP neighbor tries to establish a connection. When successful, BGP moves to the Connect state. When it fails, it will remain in the Idle state. |
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Term
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Definition
| BGP is waiting for the TCP three-way handshake to complete. When it is successful, it will continue to the OpenSent state. In case it fails, we continue to the Active state. If the ConnectRetry timer expires then we will remain in this state. The ConnectRetry timer will be reset and BGP will try a new TCP three-way handshake. If anything else happens (for example resetting BGP) then we move back to the Idle state. |
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Term
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Definition
| In this state BGP will be waiting for an Open message from the remote BGP neighbor. The Open message will be checked for errors, if something is wrong (incorrect version numbers, wrong AS number, etc.) then BGP will respond with a Notification message and jumps back to the Idle state. This is also the moment where BGP decides whether we use EBGP or IBGP (since we check the AS number). If everything is OK then BGP starts sending keepalive messages and resets its keepalive timer. At this moment, the hold time is negotiated (lowest value is picked) between the two BGP routers. In case the TCP session fails, BGP will jump back to the Active state. When any other errors occur (expiration of hold timer), BGP will send a notification message with the error code and jumps back to the Idle state. In case someone resets the BGP process, we also jump back to the Idle state. |
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Term
| Explain Open Confirm state. |
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Definition
| BGP waits for a keepalive message from the remote BGP neighbor. When we receive the keepalive, we can move to the established state and the neighbor adjacency will be completed. When this occurs, it will reset the hold timer. If we receive a notification message from the remote BGP neighbor then we fall back to the Idle state. BGP will keep sending keepalive messages. |
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Term
| Explain Established State. |
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Definition
| The BGP neighbor adjacency is complete and the BGP routers will send update packets to exchange routing information. Every time we receive a keepalive or update message, the hold timer will be resetted. In case we receive a notification message we will jump back to the Idle state. |
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Term
| Explain the types of messages in BGP. |
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Definition
The Various message types in BGP are as under.
1. Open
2. Update
3. Keepalive
4. Notification |
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Term
How do we advertise the network that we dont have in the routing table?
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Definition
This can be done with a static route that points to a null interface.
R1(config)#ip route 1.0.0.0 255.0.0.0 null 0
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Term
What is a BGP Auto Summary command?
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Definition
Normally when you advertise a network in BGP you have to type in the exact network and subnet mask that you want to advertise or it won’t be placed in the BGP table.
With auto-summary enabled, you can advertise a classful network and you don’t have to add the mask parameter. BGP will automatically advertise the classful network if you have the classful network or a subnet of this network in your routing table. |
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Term
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Definition
eBGP (external BGP) by default requires two Cisco IOS routers to be directly connected to each other in order to establish a neighbor adjacency. This is because eBGP routers use a TTL of one for their BGP packets. When the BGP neighbor is more than one hop away, the TTL will decrement to 0 and it will be discarded.
When these two routers are not directly connected then we can still make it work but we’ll have to use multihop. This requirement does not apply to internal BGP. |
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Term
| What does BGP use as a loop prevention mechanism? |
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Definition
| BGP uses Autonomous System numbers as a loop prevention mechanism. |
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Term
| Explain the AS number assignment. |
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Definition
0: reserved.
1-64,495: public AS numbers.
64,496 – 64,511 – reserved to use in documentation.
64,512 – 65,534 – private AS numbers.
65,535 – reserved. |
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Term
| What causes two directly connected eBGP connected routers not establish a neighbor adjacency? |
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Definition
- Layer 2 down preventing us from reaching the other side.
- Layer 3 issue: wrong IP address on one of the routers.
- Access-list blocking TCP port 179 (BGP).
- Wrong IP address configured for BGP neighbor router.
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Term
| What are BGP communities? |
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Definition
A community is a group of prefixes that should be treated the same way. For example maybe you have 100 prefixes that require the same local preference or weight. You could match all prefixes using an access-list or prefix-list but using BGP communities is more convenient.
Instead of manually selecting the prefixes, an ISP could instruct its customers to tag prefixes with a certain BGP community. When the customer does this, their prefixes get a certain treatment.
Here are the 4 well known BGP communities:
- Internet: advertise the prefix to all BGP neighbors.
- No-Advertise: don’t advertise the prefix to any BGP neighbors.
- No-Export: don’t advertise the prefix to any eBGP neighbors.
- Local-AS: don’t advertise the prefix outside of the sub-AS (this one is used for BGP confederations).
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Term
| What are Route Reflectors in BGP? |
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Definition
Route reflectors (RR) are one method to get rid of the full-mesh of IBGP peers in your network. The other method is BGP confederations.The route reflector allows all IBGP speakers within your autonomous network to learn about the available routes without introducing loops.
The route reflector can have three type of peerings:
- EBGP neighbor
- IBGP client neighbor
- IBGP non-client neighbor
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Term
| What are the rules for a route reflector to forward a route? |
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Definition
When a route reflector forwards a route, there are a couple of rules:
- A route learned from an EBGP neighbor can be forwarded to another EBGP neighbor, a client and non-client.
- A route learned from a client can be forwarded to another EBGP neighbor, client and non-client.
- A route learned from a non-client can be forwarded to another EBGP neighbor and client, but not to a non-client.
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Term
| What are the two fields that a route reflector adds when it forwards a route? |
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Definition
The two fields that a route reflector adds when it forwards a route are
1. Originator id
2. cluster list
Originator id : The route reflector sets the originator id as the ip adress of the router that originated the route.When an IBGP router receives a route with its own originator ID, it will not accept the route.
Cluster List : Cluster list is the router ID of the route reflector |
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Term
| What is a BGP confederation? |
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Definition
| A BGP confederation divides our AS into sub-ASes to reduce the number of required IBGP peerings. Within a sub-AS we still require full-mesh IBGP but between these sub-ASes we use something that looks like EBGP but behaves like IBGP (called confederation BGP) . |
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Term
| What numbers are assigned to sub-AS and why? |
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Definition
| Since the sub-AS numbers are not seen outside of your network you will often see private AS numbers used for the sub-ASes (64512 – 65535) |
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Term
| What is Multiprotocol BGP? |
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Definition
The normal version of BGP (Border Gateway Protocol) only supported IPv4 unicast prefixes. Nowadays we use MP-BGP (Multiprotocol BGP) which supports different addresses:
- IPv4 unicast
- IPv4 multicast
- IPv6 unicast
- IPv6 multicast
MP-BGP is also used for MPLS VPN where we use MP-BGP to exchange the VPN labels. |
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Term
| How do you remove the private as numbers on the path? |
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Definition
We use the following command.
neighbor 192.168.2.3 remove-private-as all
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Term
| How do you remove the private as numbers and replace them with your own as numbers? |
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Definition
We use the following command
neighbor 192.168.1.2 remove-private-as all replace-as |
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Term
| What is BGP synchronization? |
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Definition
| BGP will not advertise something that it learns from an IBGP neighbor to an EBGP neighbor if the prefix can’t be validated in its IGP. |
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Term
| What are the three ways to refresh your BGP policies? |
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Definition
- Hard reset
- Dynamic Soft Reset (route refresh)
- Soft reset with pre-stored information
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Term
| How do you do hard reset in BGP? |
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Definition
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Term
| What is Dynamic Soft reset? |
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Definition
| This feature lets your router request its BGP neighbor to send its prefixes again. |
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Term
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Definition
On the left side we see a table called adj-RIB-in. This is the unedited routing information from a BGP neighbor. There’s a separate table for each BGP neighbor that you peer with. We apply our inbound BGP policy to this information and the result is a table called the loc-RIB, this is the actual BGP table.
BGP will select the best path from the BGP table and the router will install this in the routing table. Also, the best paths can be advertised to other BGP neighbors. We can apply an outbound BGP policy to outbound updates and when this is done we have a table called adj-RIB-out (per neighbor). The adj-RIB-in table is actually stored in memory for each neighbor, the adj-RIB-out table not. |
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Term
| Explain what does the command soft reconfiguration inbound does? |
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Definition
| The soft-reconfiguration inbound command tells R2 to save the routing information from R1 unmodified in the adj-RIB-in table. It will then apply the inbound BGP policy and store the information in the BGP table. |
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Term
| What is a route refresh method? |
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Definition
| Route refresh is a preferred method to refresh the BGP table when we have BGP policy changes. When you change your BGP policy, you send a message to the BGP neighbor and it will resend the prefixes. |
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Term
| What is implicit withdraw? |
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Definition
| BGP routers only advertise the best path to their neighbors. When a better path is found, it replaces the current path. Advertising a path and replacing it with a new path is called an implicit withdraw. |
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Term
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Definition
Since we only advertise the best path, a lot of other possible paths are unknown to some of the routers. We call this path hiding.
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