NET4009-Module 4- Tshooting BGP

What command is used to verify IPv4 unicast BGP neighbors?

1. show bgp ipv4 unicast summary

2. show bgp ipv4 unicast neighbors

When running the show bgp ipv4 unicast summary what is important about the State/PfxRcd column?

If there is a number in this column (as there is in Example 14-1) , it means you have successfully established a BGP neighbor relationship. If you see Idle or Active, there is a problem in the formation of the neighbor relationship

What are some reasons that a BGP neighbor relationship might not form

1. Interface is down

2. Layer 3 connectivity is broken

3. Path to the neighbor is through the default route

4. neighbor does not have a route to the local router

5. incorrect neighbor statement

6. ACLs

7. BGP packets sourced from the wrong IP address

8. the TTL expired

9. Mismatched authentication

10. misconfigured peer group

11. timers

If you can ping the host why would the state still be in idle for BGP unicast neighbor?

It is using a default route which is not accepted by BGP as a valid route

what command is used to verify the TCP session on the routers?

show tcp brief all

To control the IP address that is used when sending BGP messages, you use what command?

neighbor ip_address update-source interface_type interface_number

What command is used to change the ttl of a bgp packet?

neighbor ip_address ebgp-multihop hops

BGP timers have to match

False

What are some common reasons for missing BGP routes?

1. Missing or bad network mask command

2. Next-hop router not reachable

3. BGP split-horizon rule

4. Better source of information

5. Route filtering

What command is used to show bgp routes/

show ip route bgp

What command is used to advertize routes into BGP?

network ip mask ip_mask

The network ip_add mask ip_mask command must be a perfect match to the network/prefix listed in the routing table.

True

How to deal with next hop router not reachable in BGP?

1. Create a static default route then advertise it into IGP
2. Advertise the next-hop into IGP
3. use “neighbor ip next-hop-self” command

How to troubleshoot route filtering for BGP?

use show ip protocols

How to apply a route filter to a neighbor?

• neighbor ip_address distribute-list access_list_number {in | out} `
• neighbor ip_address prefix-list prefix_list_name {in | out}
• neighbor ip_address route-map map_name {in | out}
• neighbor ip_address filter-list access_list_number {in | out}

The Best-Path Decision-Making Process

The output from this command shows updates to a router’s IP routingtable

debug ip routing

This command can be useful in watching real-time state changes for IPv4BGP peering relationships

debug ip bgp

This command produces more detailed output than the debug ip bgp command. Specifically, you can see the content of IPv4 BGP updates.

debug ip bgp updates

An IPv6 route can have an IPv4 next-hop address

False

How to fix the IPv6 next hop issue when learning a route from an IPv4 address?

create a route map on the advertising router (R2) that changes the next hop to a valid IPv6 address on R2 and attach it to the neighbor statement